HPR Primer

Although I spent decades building low and mid power rockets before I got into high power rocketry (HPR), I still remember how confusing some of the HPR concepts were when I first started. A lot of what you learn in your LPR/MPR journey directly translates to HPR, but there are a number of differences that can make HPR a daunting prospect.

My intent with this page was to answer a lot of those common questions about high power rocketry. I will attempt to post a bunch of things I have seen or done and hope that it helps some of the newcomers to HPR in their journey. Hopefully, I can give you a positive and helpful resource to give constructive guidance.

Those of us with experience take it for granted that HPR newbies just magically know what they should do. I will try to write this from the perspective of what it was like when I first put my L1 rocket together and give recommendations from that lens. If you have any questions or suggestions, please use the contact page to provide some feedback or send an email directly to admin@mountainmanrockets.com. Thanks!

Intro

First and foremost, there are no right answers, except for safety. Everyone has a technique and most of them work.

Second, if you haven’t built and flown Low Power Rockets (LPRs) and/or Mid-Power Rockets (MPRs) already, I can’t express strongly enough that you should start there. It may seem like jumping straight to High Power Rockets (HPRs) is the “fast track” to fun in this hobby, but I can assure you it is not. Besides the fact that HPRs are inherently more dangerous than “Model Rockets” (LPRs/MPRs), building and flying LPRs/MPRs is fun, rewarding, instructive and requires far less money and planning. While I am not aware of any rules, regulations or laws prohibiting you from jumping straight into HPR, I am aware that many people qualified to certify rocketeers in high power rocketry won’t do so unless the rocketeer has already shown some level of competence in LPR/MPR construction and flight.

Third, this post is just a primer and far from comprehensive. It should hopefully give you the launch platform to do your own research and learning. It is not meant to be a researched and published book. In fact, there are already some fabulous publications you really should read. The truth is, if everyone read the following books, there would be no need for this thread. These authors have already done an amazing amount of work to lay it all out for you in easy to read books.

Harry Stine’s “Handbook of Model Rocketry”. If you don’t have a copy of this and it isn’t dog-eared with labels and notes, you need to stop reading this post, get a copy and start there. https://www.amazon.com/gp/product/0471472425/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

Mark Canepa’s “Modern High-Power Rocketry 2”. https://www.amazon.com/Modern-High-Power-Rocketry-Mark-Canepa/dp/1412058104/ref=sr_1_6?dchild=1&keywords=high+power+rockets&qid=1629943597&sr=8-6

Mike Westerfield’s “Make: High-Power Rockets: Construction and Certification for Thousands of Feet and Beyond”. https://www.amazon.com/Make-High-Power-Construction-Certification-Thousands/dp/1457182971/ref=sr_1_1?dchild=1&keywords=high+power+rockets&qid=1629943597&sr=8-1

And, for a really nice history of how we got here, Mark Canepa’s “Large and Dangerous Rocket Ships: The History of High-Power Rocketry s Ascent to the Edges of Outer Space”. https://www.amazon.com/Large-Dangerous-Rocket-Ships-High-power/dp/149079655X/ref=sr_1_2?dchild=1&keywords=high+power+rockets&qid=1629943597&sr=8-2

Also, the ambitious folks over at Hot Nozzle Society have put together a slick powerpoint explaining a lot of what you need to know to get started: https://3cc1c301-418a-48d9-bacd-787ff55bfa60.filesusr.com/ugd/f3b99f_f63cbc3631464760b999b397f1e4a93d.pdf

Finally, Safety. This topic will be touched upon in other posts, but it cannot be emphasized enough. The further you delve into HPR and the bigger your projects get, the more dangerous methods, materials and chemicals you will be exposed to. From a build perspective, the materials will cut you, rip your clothes, make a mess and are often flammable. From an exposure perspective, the dust and chemicals involved will cause lung/eye damage, burn your skin, cause sensitization and have long term effects on your health. At the range, HPRs are literally “large and dangerous”. Can you get seriously injured from an Estes Alpha? Sure you can. But it probably won’t kill anyone. An HPR gone awry has the potential to cause serious damage. The main point is that there is no way to remove all the risk, but there are tried and true methods to keeping you and your family healthy and to make it reasonably sure no one gets hurt in your home, workshop or at the field.

What is High Power Rocketry?

I am going to start by answering a few basic questions and also make some recommendations that, I hope, will help you start your HPR journey. Since this is an HPR primer, I am going to assume you have built and flown “low power rockets” (LPRs) and “mid-power rockets” (MPRs). If you have not started out that way, once again, I cannot encourage you enough to stop here, go build up some experience in LPR/MPR building and flying and then come back and dive in. If concepts like Center of Pressure, Center of Gravity, Thrust-to-weight ratios, etc are not currently in your lexicon, you need to learn about them before starting your HPR journey. This is not a race, there are no points for getting certifications faster than anyone else.

Study for the Tripoli and NAR Level 2 exams.

Okay, I get it. You are sitting there thinking I have lost my mind. You came here for an HPR “primer” and I am telling you to jump the gun straight to Level 2? What the heck?

In my opinion, I think the Level 2 test should be a Level 1 test. You can’t actually take the test until you are Level 1 certified, but there is nothing stopping you from downloading the tests right now and studying them until you can get a perfect score on both the tests. Tripoli and NAR post all the questions and answers to their tests on their websites.

So, why should you start there? Frankly, those tests answer a whole lot of basic questions about HPR. They dig deep into the regs and the “5 W’s” of it all. When I studied for those tests and took them, I had a lot of “Aha moments”. If you go and read those tests and understand the answers, you will be much better prepared to start your HPR journey.

Here are the Tripoli and NAR study guides (current as of January 2022):

NAR Level 2 Study Guide

Tripoli Advanced Study Guide 2021

What kit should I buy?

Probably the single, most common question asked is “what kit should I use for my Level 1 or Level 2?”

This question has been asked so many times, it probably should have its own forum.

You are going to get two basic answers to this question. The first is “Don’t use a kit. Build from scratch.” The second is “You need to use [fill in kit name here]”.

The truth is, both answers are right. You need to self-assess here. How experienced are you? Have you been building rockets for 40 years and have decades of experience scratch building LPRs and MPRs? If so, scratch building may be for you. Are you relatively new to the hobby or are a BAR that, when you last built a rocket, thought that a D motor WAS high powered? Maybe a kit would be the right answer.

If you go down the scratch build path, there are lots of good threads on various rocketry forums about choosing a design and building it.

If you decide to buy a kit, I would tell you that every seller I know of has great kits for Level 1 and Level 2 and they tend to specifically market those kits for those purposes.

I am not going to tell you which kit to buy. However, I can give you some recommendations on what to look for. The traditional common wisdom for certification flights are to go “low and slow” (we’ll talk about motors in a bit). I am going to assume that you are planning to use that guidance for your cert flights. If you want to throw caution to the wind and fly your cert rockets “high and fast”, this post may not be for you. I wish you luck.

There are lot of tried-and-true Level 1 kits out there. Many have been around for years and years and have repeatable success in Level 1 flights. Success here is simplicity. Look for a 38mm motor mount, heavy duty cardboard tubes, strong plywood fins and a well-made nosecone. Personally, I would stick with the 3FNC/4FNC (3 or 4 fins-and-a-nosecone) design. The more exotic you get, the more likely something will go wrong or break. You can, of course, go straight to fiberglass rockets for a Level 1 rocket, but I personally recommend starting off with a nice heavy duty cardboard/plywood kit. It will be very similar to what you are used to in LPR/MPR and probably will come with instructions. Most fiberglass kits are just a bag of parts and usually leave it up the builder to figure out things like parachutes, recovery systems, etc.

John Coker has a great video about building a Level 1 rocket – a great place to start on the topic of what you should buy/build:

John Coker – Level 1 build video

It is entirely possible to build one rocket for your Level 1 flight and use the same rocket for your Level 2 flight. I don’t recommend it, but it can be done. There are some situations where this is desirable, though. For example, maybe you live very far from a club that can certify you and you can’t attend many launches. However, even then, the experience you gain by building multiple cert rockets will never be wasted. Plus, there is no rush to certify. There are no “points” for doing it faster. This really is a hobby where the journey is often more fun than the destination. When I got my Level 1 cert, I actually had 3 rockets built and ready to go. Same for my Level 2. I love building rockets. I couldn’t imagine using one rocket for two certs, so I built 6!

Maybe a better way to think about a Level 1 kit is to choose a rocket that can be used with both G motors and Level 1 motors. Ideally, your Level 1 cert flight won’t be that rocket’s maiden voyage. Build a good rocket, go and fly it a couple/few times on a G motor and then, when you fly that cert flight on an H Motor, it will just be another successful flight instead of a nerve-wracking experience.

Here are some examples of great Level 1 rockets by some of the popular manufacturers:

Apogee Rockets:

Binder Design:

Binder Design 3″ Kits

LOC Precision:

MAC Performance:

Madcow Rocketry:

These are just examples – everyone has their own personal favorite Level 1 rocket. Just pick one, build it well, make sure it is safe and have fun launching it!

What motor should I use?

Right after kit selection, the second most common question is “What motor should I use for my certification flight?”. While you will get some people to answer that question with a specific motor recommendation, you will also get a lot of answers from frustrated people essentially saying “that is the whole point of certifying – you need to figure it out yourself!”.

What they mean is that, if you are going to build and fly HPRs, you need to learn about motors. Unless you were heavily into scratch building LPRs and MPRs, you could really get away with nothing more than a basic understanding of motors before. After all, every LPR and MPR kit I know of gives you a list of “recommended motors”. I know that before I got into HPR, I had a spreadsheet with all my LPR/MPRs listed on the left and all the recommend motors in columns to the right. I knew a B motor was bigger than an A motor and I understood the last number meant how long the delay was, but I really didn’t understand what the motor designations meant. What was the difference between a B4-4 and a B6-4? I couldn’t have told you with any degree of certainty or understanding.

That won’t fly in HPR. Understanding how motors work and what the designations mean is vital to a safe and successful flight.

One reason this is so important is that no two HPR rockets are the same. This is a big difference from LPR. For example, any two Estes Alphas are going to be so similar (unless you modify or add to the kit), you can reasonably say they are identical enough to give specific motor recommendations and those recommendations will be good 99% of the time. In HPR, you almost can’t get the same parts in two kits of the same rocket! I can almost guarantee that if you go buy two kits of a common HPR rocket and measure and weigh the parts in each bag, there will be a tangible difference in the dimensions and weights. And then the build process itself will add another significant difference in measurements to the point that one person’s Zephyr will be very different from another person’s Zephyr. As the rockets get bigger and more complex, the differences between rocket kits from the same manufacturer and the differences in build techniques will ensure that you will never be able to just grab a list of “recommended motors” – you will need to learn to figure out how to determine the best motor for YOUR rocket.

The books listed above can teach you about the motor designations, but here are two links for an excellent description of the motor terminology and what they mean:

https://themodelrocket.com/model-rocket-engine-sizes-and-classifications/

https://www.thrustcurve.org/info/motorstats.html

Here is another recommendation that may be controversial: If you really want to understand how your motors work, I can’t recommend Dr. Terry McCreary’s book Research in Rocketry: Experimental Composite Propellant highly enough: http://www.compositepropellantbook.com/. You may never decide to make your own propellants, but if you really want to understand how your motors are made and the science behind them, this book is a great way to learn. Personally, I have not yet made my own propellants, but I have read Terry’s book cover-to-cover many times.

Here is a chart of the rocket motor letter designations:

Okay, so if you need to figure out the motors yourself, how do you go about doing that?

Okay, so if you need to figure out the motors yourself, how do you go about doing that?

In 2022, the best way to do that is use a rocket simulator. Maybe somebody will come up with a better method in the future, but, for now, a simulator is your best friend.

It all starts before you build a single piece of the rocket. The first thing you should do is measure every single part (dimensions and weight) and build a simulator file. This is an important step and you can’t skip it. It is a LOT more difficult to build an accurate sim file after you have started building because you will no longer be able to get the discreet dimensions and weight of each piece.

Hopefully, during your LPR/MPR journey, you ran across the two main rocket simulators out there – RockSim and OpenRocket. RockSim costs money, OpenRocket is free to use. You can choose either one, they both work fine and the methods used to build your rocket are very similar.

Apogee has good videos on how to build a rocket in RockSim. Most of the stuff they cover in the videos will work pretty much the same way in OpenRocket.

https://www.apogeerockets.com/RockSim/Rocksim_Video_Tutorials

One question you may ask is – can I use a RockSim/Openrocket file someone else has made? or Can I use the one the manufacturer has on their website? In all likelihood, any sim file you get on the web will NOT match the parts you get in your kit. Sometimes these files, even the “official” ones you get from the manufacturer, may be wildly inaccurate. Vendors change their suppliers all the time, so the plywood in the 5 year old sim file may be different than the plywood you got. The fiberglass tube may be a completely different weight. They may have made design changes such lengthening or shortening body tubes or changed a nosecone from conical to ogive. At the very least, if you are going to use a sim file you did not personally make, you should weigh every part and make sure the weights match what is in the sim file.

Once you have a good sim file, update it, update it and update it throughout the build. All that epoxy, paint, hardware, shock cords, parachutes, etc you are adding will change the sim significantly. It is not uncommon to start with 5 pounds of parts end up with a rocket that weighs 9 pounds without a motor.

Once you have a final sim file and have an accurate “dry weight” (that is, the weight when the rocket is completely flight ready, except for the motor), head over to https://www.thrustcurve.org/, open an account and enter your rocket info to log your rocket. Choose “Match Rocket” and then choose the motor types and manufacturers you want to check out and Thrustcurve will spit out a list of motors that are most likely to work with the parameters you have entered. I like to then download the spreadsheet of information and use that chart to setup my motor list. I would recommend NOT using the Thrustcurve list as your final decision making device for motors. As John Coker (maker of Thrustcurve) has noted many time, Thrustcurve is not a simulator. It is merely a tool to narrow down your motor choices using size and weight algorithms. Once you have culled the list down to the motors you most want to try out, you then go back to your sim of choice and “launch” each motor on your simulated rocket. This is how you obtain a good, accurate list of flight characteristics for each motor. I then like to download this chart, put the data in a spreadsheet on the cloud and use that to choose the motors based on the field I plan to fly at.

This is the process I use to choose motors for every rocket I fly. If you follow this basic procedure, you will not need to ask “which motor should I use for my Level 1 cert?”, but, more importantly, you will understand WHY you chose a particular motor. With all the data you will now have at your fingertips, when you get to the field and your certifying official or the RSO asks you things like: What is the thrust-to-weight ratio (for most clubs, that ratio should be at least 5:1 or higher)? What is the expected altitude for the flight? What is the weight on the pad and the recovery weight? What is the max speed you expect for the flight? What is the expected speed at the end of the rail? Etc,. You will be ready to answer all that and more!

As of January 2022, there are essentially 3 main motor manufacturers that sell commercial high power motors in the United States. Aerotech, Cesaroni and Loki. Your choice of motor should also include whether you want to use a single use motor or reload (discussion to follow) and whether there is a vendor at your local club that sells a particular brand of motors. Many high power motors require a HAZMAT shipping fee, so buying local can often save you a lot of money.

Motor Retention

High power motors are physically bigger than the black powder motors used in LPR and most of the MPR motors as well. In MPR, you may have already used “positive” motor retention that is more robust than an “engine hook” or “friction fit”. For HPR, positive motor retention is absolutely necessary and should be checked by the RSO as part of the safety check before launch. Although a “friction fit” is allowed by the NAR safety checklist, Tripoli uses the verbiage that “positive retention” is required. Your RSO may not accept a friction fit as “positive retention” for a large HPR motor.

As a quick review, motor retention is not for the “up” part of the flight, it is to prevent the motor from falling out of the back of the rocket when an ejection charge goes off in the rocket. All modern HPR motors have “thrust rings” built in to the back of the motor, so you should not have to worry about keeping the motor in place during the “boost” phase. The ejection charges in HPR are much larger than the ones in LPR black powder motors and will easily throw a motor out the back if not properly retained. In LPR, an ejected motor results in a relatively small and light piece of cardboard tubing fluttering down from altitude. In HPR, an ejected motor most often results in a heavy aluminum casing coming down at terminal velocity, which can cause serious injury and/or damage. Also, if the motor is ejected, the pressure in the body tube may have been lowered to the point that your recovery system was not ejected. Now you have the motor and the rocket coming in ballistic, yikes!

“Positive” retention techniques in HPR vary and need not be expensive. Generally, you need to have screwed something on that covers the lip of the motor or use some kind of snap ring retainer. There are many ways to accomplish this. The easiest method is to purchase a purpose built retainer system that is adhered to the end of the motor tube (usually using JB Weld, since the retainer bodies are generally made from aluminum). Aero Pack and Giant Leap Rocketry make popular commercial retainers and are very easy to use, albeit expensive. Your retainer system could be as simple as a piece of all-thread sticking out the back centering ring and then use washers and hex nuts to hold the motor in place. Another inexpensive method is to put a “tee nut” in the rear centering ring near the motor tube and then use a screw to hold a “Z-clip” shaped piece of metal over the lip of the motor.

Here is an example of using all-thread and a washer to provide positive retention on a 38mm motor:

Here is an example of using an Aero Pack 54mm retainer and one of their tailcone shaped threaded caps:

Here is an example of retaining a 75mm motor using tee nuts and “Z-clips”:

Reloadable vs. Single Use motors

While it is possible you used reloadable motors in MPR, it is likely your rocket journey to this point has only included the use of single use motors that you throw away after the flight. It is entirely possible to only fly single use motors in HPR. Aerotech makes single use motors in all common motor diameters and offers a variety of motor sizes, especially in the 29mm and 38mm diameters. As of January 2022, I am not aware of any single use motors sold commercially by Cesaroni or Loki, so if you choose those manufacturers, you will need to learn how to assemble reloads.

Everyone has their own opinion on single use vs reload, but I personally recommend you do your Level 1 cert flight on a single use motor. It takes a lot of the complexity out of the flight and that is a good thing for a cert flight.

Most people flying HPR eventually acquire and use some reloadable motors. The upside of single use motors is that they are easy to use and ready to go out of the box. The main downside to single use motors is that they are relatively expensive compared to similar reloadable motors. Single use Aerotech motors tend to be anywhere from 20%-30% more expensive compared to similar Aerotech reloads.
Aerotech provides a great example since they offer the 54mm J250 in both single-use (Disposable Motor System – DMS) and reloadable (Reloadable Motor System- RMS). They retail for:

J250W (DMS) – $109.99
J250FJ (RMS) – $93.99

As you can see, the single-use version costs 14% more at retail. Further, the DMS motors are almost never discounted by vendors, while you can often find the reloads at significant discounts. At the time of this writing, the best price I could find online for the J250FJ was $76.49, which represents a 30% savings over the DMS version. (I must caveat here that there are some significant differences in these motors since they are made of different propellant types. For the most part, they will achieve very similar altitudes in the same rocket, but their “thrust curves” are different and that would definitely influence the flight, especially the velocity at the end of the rail. If you are using a rocket very close to the maximum weight rating for these motors, they are not interchangeable. For lighter rockets, however, the flights should be comparable).

The two very significant downsides to reloadable motors is that they are complex (relatively speaking) and require an expensive upfront cost of motor hardware. The upside to reloads are that they will save money over time and they open up a vast catalog of available motor sizes.

Another difference between single use and reloadable motors is weight. The single use motors tend to be lighter since they use a lighter casing material than the larger, bulky/heavy aluminum case and closures of a reloadable system. This weight difference can have a significant effect on the performance of your rocket and may alter the center of gravity enough that you would have to make some design changes.

Again, let’s compare the two J250 motors from Aerotech (the same caveat noted above applies here):
J250W (DMS) – 708g
J250FJ (RMS) – 907g

The RMS version is 28% heavier than the DMS version.

Reloadable motor hardware can be confusing at first. I remember having no idea what I needed to buy to start off. Basically, motor hardware consists of 3 basic parts: an aluminum “case” which is a long tube in which the motor propellant “grains” go into, some kind of aft closure that holds a nozzle in the back of the motor, and some kind of forward closure that holds a delay element and your ejection charge (larger motors do not use motor ejection charges, so the forward closures on big motors usually are “plugged” and do not have a well for a black powder charge). Another benefit of using plugged forward closures is that they usually have threaded receptacle that can be used as a hardpoint for various reasons like motor retention in a minimum diameter rocket or as a recovery attachment point in some rockets.

The motor hardware is sold in different diameters and lengths to accommodate different motor sizes. Loki and Aerotech designate the case sizes by “total impulse” the case is expected hold. Cesaroni makes it a little simpler by designating their cases by the number of standard size propellant grains a case can hold.

Your best bet is start off with a smaller diameter (38mm is a good start) “kit” from one of the manufacturers. This will give you all the parts you need in a set (see photo below).

Once you have the hardware, the actual “motor” is sold as a reload kit from a variety of vendors. The reload kits come with the actual propellant “grains” and a variety of disposable parts like o-rings, depending on the motor. There are a number of videos on the web on how to assemble the motors from each of the manufacturers.

Adjustable delay. Another difference from the low and mid-power motors you may be used to is that HPR motors that can provide a motor ejection charge (most Level 1 motors and some Level 2 motors) will have a delay you can adjust. When you bought low power Estes motors, the ejection charge delay was set (i.e. – a B4-4 motor has a 4 second delay after burnout). With high power delays, you “trim” the delay grains to make the delay shorter. All motor manufacturers have a recommended tool to use to shorten the delays. Generally, though, these tools drill a hole in one end of the delay grain and, for every 1/32 of an inch you drill out, the delay is shortened by 1 second. If you use the manufacturers’ delay tools, the depth of cut is automatically set for you depending on how you set up the tool. In practice, this is an easy and accurate way to dial-in the proper delay time for your flight. To determine the optimal delay time, you should refer back to your sim program of choice.

Here are some examples of reloadable motor hardware. The motor on top is an Aerotech 38mm case which uses threaded closures on each end. The two forward closures pictured show how you can use one for an ejection charge or one that is threaded to attach to some kind of hardpoint inside the rocket. These types of reloadable motors use a disposable nozzle that comes with the reload kit. On the bottom is a 38mm Loki motor case. It has a re-usable graphite nozzle and uses snap-rings to hold the closures in place

Adhesives

For your Level 1 cardboard and plywood rocket, darn near any adhesive will work. You could build a Level 1 cardboard rocket with good old wood glue and probably be fine. Many people have done it.

Eventually, though, if you stick with HPR, you will get to a point where wood glue is no longer sufficient and you must dip your toes into epoxy. John Coker has a great epoxy primer, so there is no need to talk about the basics here, just watch his video and come on back:

https://www.youtube.com/watch?v=1Oq6koK7wcw

Okay, so now you know the basics. I am not going to tell you what brand to buy, but I will point out the some characteristics you should know. First, for Level 1 and a lot of Level 2 rockets, you can head over to your favorite Walmart and pick up whatever two-part epoxy they have on hand and it will be more than good enough for what you need to do. As a general rule for most off-the-shelf epoxies, the longer the cure time, the stronger the bond (there is a whole other discussion about the strength of the epoxy vs the strength of the bond, but that is beyond the scope of this discussion).

As you get into bigger projects and especially fiberglass rockets, you will probably want to consider an epoxy “system”. John talks about it in the video linked above, but, realistically, there are some good reasons to move away from off-the-shelf epoxy. System epoxies tend to come in bigger quantities and often have a lower cost per ounce than the stuff you can find in stores. When you get to the point where you need a quart of epoxy to finish a project, the off-the-shelf stuff gets expensive very quickly! System epoxies tend to be much stronger than the stuff you will find in stores. Finally, system epoxies tend to be much more versatile. You can dial in the viscosities using fillers and you can choose different epoxies based on their properties. JB Weld is probably the only name brand I will mention here since it is almost universally accepted as the most easily obtained proper choice for bonding metal, so when you need to bond some metal parts to your rocket, JB Weld should probably be your go-to.

Viscosity plays a big part in epoxy choice. Out-of-the-box (i.e. – not using any fillers), some epoxies tend to be better at some applications due to their viscosity. A nice thick paste epoxy makes great fillets while an almost water-like laminating epoxy is great for whenever you need to pour or inject epoxy somewhere or “paint” it on to a surface.

Another consideration using epoxy is the cure temperature. Most epoxies do not cure well (or, in some cases, at all) in cold or even cool temperatures. Especially when you move to using epoxy systems, check the manufacturer’s recommended temperature ranges for a proper cure. If your build environment does not comply with the recommended temperatures, it is possible to use or build a “cure oven”. The use of a cure oven does present a fire hazard, so it is beyond the scope of this thread, but you can find plenty of information with a simple Google search. If you are lucky to have a spare room with it’s own heat control, you can just set the room temp to about 80 deg F and put your parts in there while they cure. Space heaters and heat guns can be used, but, once again, beware of fire hazard and, in the case of heat guns especially, make sure you don’t make your parts too hot.

Recovery

For the most part, parachutes are parachutes and they remain the primary method of recovery for HPR. However, compared to LPR/MPR parachutes, HPR requires a bit of planning. For your Level 1 cert flight, you will probably be just fine using the parachute that came with your kit. In the long run, however, you will probably need to invest in a variety of parachutes. Unlike LPR/MPR, most HPR builders do not permanently install parachutes in a rocket. Most rocketeers will acquire a catalog of different parachutes and choose the right one at the field based on the weight/size of the rocket, the field size, weather conditions, etc. Everyone has their favorite parachute vendors. I will not tell you which vendor to go with, but I will recommend you stick with one manufacturer for most of the sizes you need. This is because comparing similar sized parachutes from different manufacturers is an apples-to-oranges comparison. Different manufacturers use different material, geometries, shroud line configurations, etc. In my opinion, you want to be able to choose your parachutes based on comparable factors. If you use parachutes from the same manufacturer, then you can generally narrow your selection criteria at the field to the size of the parachute. If you have a bag of parachutes from 5 different manufacturers, it will make it a lot more difficult to choose the right one.

For HPR, you need to ditch the elastic shock cords. They won’t work here. At a minimum, you should probably use some kind of nylon webbing and your kit likely came with a nylon webbing cord. This is fine for your Level 1 flight, but, as you get into bigger rockets, you will have to supply your own cords and harnesses. Nylon webbing is great, strong and relatively cheap, but it is not usually fireproof, so those ejection charges are probably going to ruin it eventually. Nylon also has a little “give” and stretch in them, so they don’t have as violent of an opening shock. Kevlar cords are stronger, lighter and fireproof. However, they are expensive and have almost no elasticity, so they really stop your rocket on a dime. An economical way to get a bunch of different shock cords is to stitch your own. Using something as simple as a Speedy Stitcher, a bunch of nylon webbing from an online store, and some waxed Kevlar thread, you can have a variety of shock cords made up in no time.

Nomex/fireproof protectors. In a Level 1 rocket, you may be able to get by with dumping a load of dog barf in the body tube to protect your parachutes like you did in LPR/MPR, but it is much more effective to use Nomex blankets and parachute protectors. A number of vendors sell them – purchase them in sizes the vendor recommends for the diameter of your rocket. Attach the blanket to your shock cord somewhere (I just zip tie them to the shock cord a few inches away from the attach point for your parachute) and “burrito roll” the parachute in the blanket. If you are using nylon shock cords, you can extend their life by protecting them with Nomex shock cord protectors. Some parachute vendors sell protective nomex pouches and deployment bags to protect your parachutes more effectively.

To determine an estimate of the descent rate of your rocket using a particular parachute, you can use a descent rate calculator. The sim programs have descent rate calculators built in, but they require you to be pretty precise about the parachute data when set up the sim file and can be a pain to change if you want to test out a variety of parachute configurations. Various website tools are available to make it easier. These tools must be taken with a very large grain of salt –their results are a very rough estimate of the parachute’s performance. They are very useful, but you should adjust your parachute decisions based on the weather, field conditions, and rocket size/shape.

Here are a some good discussions on properly choosing your parachute size:

https://apogeerockets.com/education/downloads/Newsletter149.pdf
http://www.aeroconsystems.com/tips/tips002.htm
http://www.rocketmime.com/rockets/descent.html
http://www.nakka-rocketry.net/parasys.html
http://www.nakka-rocketry.net/paracon.html

Here are some online tools I am aware of:

https://fruitychutes.com/help_for_parachutes/parachute-descent-rate-calculator.htm (This is my favorite overall online calculator that allows you to choose actual parachutes from a variety of manufacturers)
https://descentratecalculator.onlinetesting.net/
https://www.rocketreviews.com/descent-rate-calculator.html
https://www.translatorscafe.com/unit-converter/en-US/calculator/parachute-size/
http://www.ahpra.org/descent.htm
https://www.rocket-simulator.com/descent.php
https://wiki.freecadweb.org/Rocket_Parachute_Size_Calculator
https://www.eoss.org/wbaltrak/descent_calculator

Fiberglass & Composites

At some point in your rocketry progression, you will probably think about building a rocket made of fiberglass or other composite (such as carbon fiber). This transition requires a number of changes and considerations in your build process.

Safety should be the first topic addressed when you move to working with composites. Fiberglass dust is very harmful to your lungs and eyes, it will make your skin itch and it is hard on your power tools and machines. Additionally, carbon fiber dust is electrically conductive, so it can really mess up circuit boards and power tools. Fiberglass edges are sharp and often have splinters that will easily embed in your skin and clothing. When working with composites, you should wear a mask/respirator, use eye protection, wear appropriate gloves and clothing, and plan for dust mitigation. Your work area should not be in your living area. Building cardboard/plywood rockets at your kitchen table may be okay (probably not to your significant other…), but you need to move the fiberglass out to a safe workshop space.

When you get new fiberglass parts, you should wash them to remove any mold release and then sand them with 220 grit sandpaper. In theory, you just need to sand the parts that will come in contact with epoxy or paint, but it is often easier to just go ahead and sand all the surfaces at one time. As the parts get bigger, a random orbital palm sander is very helpful.

While you could get away with adhesives such as wood glue in your cardboard and plywood rockets, composites require epoxy (or, a suitable adhesive. Since epoxy is generally the easiest adhesive to obtain that is appropriate, I’ll assume you are going to use epoxy). At this point, it will probably make sense to move to an epoxy “system” such as West System or Aeropoxy.

You will want to consider the temperature of your work space and where parts cure. Epoxy systems do not cure well in cold (or even cool) temperatures. If you are building rockets in Arizona in the summer, your un-air conditioned garage is the perfect curing oven. If you are building rockets in Montana in the winter, your un-heated garage is exactly the last place you want to let your parts cure. A heater or hair dryer pointed at your parts can greatly assist in this part of your build, but care should be taken to prevent fire hazards.

Dual Deployment

Dual Deployment is probably not something you will encounter/attempt for a Level 1 certification, but it is a fundamental HPR concept, so it should be discussed in an HPR primer.

What is dual deployment? Essentially, it is the practice of deploying a small parachute (a “drogue”) or streamer at the rocket’s apogee and then deploying a large, “main” parachute at a low altitude (typically around 500 feet above the ground). The intent is to ensure your high flying HPR doesn’t float a mile or two away from the launch area. The descent rate of your drogue should be fast (Usually around 60-70 FPS (40-50 MPH)), but the drogue should provide enough lift to ensure the section carrying your main parachute (usually the “payload bay”) is above or even with the rest of the rocket parts so that when your main parachute deploys, it doesn’t go slamming into the other falling rocket parts and cause a malfunction. Your main parachute should be sized to provide a descent rate of about 20 FPS (13 MPH) or less.

There are many ways to accomplish dual deployment, but the most common way is to use electronics (an “altimeter” or “flight computer” with “deployment channels”) inside an “electronics bay”/”avionics bay” (often abbreviated E-bay or AV-bay) and use black powder (most commonly, FFFFg black powder) charges to create deployment “events”. The E-bay normally consists of a coupler with a “switch band” on it that sits between the booster section and payload bay. The coupler is capped on the ends with bulkheads that contain and protect the electronics, while also holding all the necessary hardware to deploy the black powder charges.

A typical dual deployment configuration is shown here:

Let’s back up and break all that down. The first step is usually building the E-bay. For this you need the coupler and then bulkheads. If your E-bay came as part of a kit, the bulkheads may be “stepped” so that an inner circle fits inside the coupler and an outer lip keeps the bulkhead from going all the way into the coupler. The most common way to build out the internal part of the E-bay is to use two lengths of all-thread rod that are a couple inches longer than the E-bay. Two holes are drilled in each bulkhead that line up with the holes on the other bulkhead and the all-thread is passed through the holes on one BH, through the coupler and through the holes on the other BH. The all-thread is secured on both ends with nuts. It is common to secure one side of the E-bay with a fixed installation (using lock nuts, Loctite, epoxy, etc) and then use wing nuts on the other side. This allows you to take the E-Bay apart when you need to and access the electronics.

The electronics usually sit on some kind of sled. The sled can be made of plywood, fiberglass, or a convenient 3D printed plastic. Any decent sheet material can be used, but metal and carbon fiber (or any other conductive material) sleds should be avoided so that you don’t short out your electronics. A simple and cheap technique is to use a piece of plywood and epoxy some metal tubing to the bottom of it that is larger than the diameter of the all-thread and then the sled can be slid onto and off the all-thread as needed. The electronics boards are usually fixed to the sled using stand-offs and screws, most commonly, 4-40 hardware is used. You can get altimeter hardware from a number of online vendors. The sled will also usually house the battery you need for the electronics and deployment charges. As your electronics needs get more complex, you may end up with multiple computers and multiple batteries, but the simplest dual deployment electronics configuration is a single computer and a single battery. You can find all sorts of housings for your batteries, but the simplest method is to drill holes in your sled and zip-tie the battery to the sled. You should secure it both length-wise and width-wise so it doesn’t slip out in flight or during an event. If possible, I like to secure my batteries perpendicular to flight path so acceleration has less of a chance of causing the battery connection to fail.

Choosing electronics. I am not going to recommend a specific computer, you can look around on rocketry forums for all kinds of recommendations, but I will explain the basics. Generally, the more economical computers you can buy will determine the altitude using sensitive barometric sensors, so they will need access to the air outside the rocket in order to operate accurately. Therefore, you will need pressure port holes in the switch band to allow pressure equalization. Generally, you should follow the advice from your altimeter manufacturer on sizing and number of vent holes, but Vern Knowles has some easy to use charts for vent hole sizing here: http://www.vernk.com/AltimeterPortSizing.htm and Off We Go Rocketry has an easy to use spreadsheet calculator here: https://offwegorocketry.com/pages.php?page=Resources&osCsid=8231e526d8acf6c84158bac30a50fef0. Some more expensive computers use accelerometers, but let’s keep it simple for this discussion and assume the computer you are using has a barometric sensor. The computer will sense when the launch occurs and then begin tracking the ascent. When the rocket reaches apogee, the computer sends an electric charge to an electronic match/igniter that is embedded in the black powder charge. This causes the section holding the drogue (typically the booster) to separate and pulls out the drogue parachute. When the rocket descends to the preset altitude for the main parachute, a second electric charge is sent to another igniter which deploys a second black powder charge. This charge causes the section holding the main parachute (typically the payload bay and the nosecone) to separate and deploy the main.

A “switch” is commonly wired to the computer in either a dedicated port on the board or in series with the battery cable so that the rocketeer can turn the computer on and off from outside of the rocket. Both NAR and Tripoli safety codes stipulate that dual deployments electronics may not be armed until the rocket is safely on the launch pad. This switch can be as simple as two wires protruding from a hole in the switch band that are twisted together and taped to the outside of the rocket (“twist and tape” is safer than “twist and tuck”, where the wires are shoved back in the hole, because tucking the wires back in the E-bay normally requires rocket disassembly to disarm the electronics in the event of a launch failure/abort). The switch could also be one of many dedicated switch types that are installed either on the sled or on the switch band. The switch should be a “maintained” switch rather than a “momentary” switch. The switch should be installed in a way that protects it from changing position in flight and possibly turning off the electronics. For example, if a slide switch is used, it should be oriented perpendicular to the flight path to prevent acceleration from causing the switch to slide into the off position during flight. Finally, the switch could be wireless. There are magnetic switches and WiFi switches, for example.

The Ebay must be connected to the payload bay in a way that prevents it from separating in flight, but also be able to come apart on the ground to access the Ebay. You can use a variety of methods, but plastic rivets are commonly used in cardboard tubing and metal screws (with or without a PEM nut backing in the coupler) can be used for composite airframes. Likewise, the nosecone must be secured to the payload bay in such a way that it will not separate when the drogue charge goes off, but does separate when the main charge goes off. This is normally accomplished with “shear pins”. The simplest way to implement shear pins is to get 2-56 sized nylon screws and drill holes in the nosecone to attach it to the coupler or the nosecone shoulder to the airframe.

Ground testing. Before flying your rocket for the first time, you must determine the size of your black powder charges and ground test them. You can search for “black powder charge calculator” on the internet and find a number of online calculators. Once you have an idea of the size of the charge, you need to setup a live test to make sure that charge will separate the pieces with enough force to “eject the laundry”. This ground testing needs to be done in a safe area and someplace you can legally conduct it. If you have no place to conduct a ground test, you can take your rocket to a club launch and ground test it there. When you ground test, you should completely setup your rocket as if it was going to fly, including a dummy motor case (with no propellants). Here is a video of a ground test on a 6” Wildman Darkstar Ultimate: https://www.youtube.com/watch?v=-_SoQ4K_Dzg

The photos below depict a simple dual deploy setup:

The computer ( a Missile Works RRC3, in this case) is attached to the sled using nylon standoffs and nylon 4-40 screws
A switch (in this case, a screw type) is installed on the side of the sled and connected to the computer’s dedicated switch port. The switch is accessed through a hole in the switch band.
The bulkheads are made of aluminum and are “stepped” to fit on the ends of the Ebay coupler tube
The sled sits on ¼”-20 all-thread. The all-thread is secured on the forward end with locking nuts and epoxy. On the aft end, the bulkhead is held on with wing nuts during flight.
A 9-volt lithium battery is zip tied in two directions to the back of the sled and the leads are connected to the computer’s battery terminals.
The “charge wells” on the bulkheads are made of ½” PVC caps. To use them, the appropriate black powder charge is poured in, the end of the e-match is inserted into the black powder and then well is packed tightly with dog barf and taped securely with masking tape. This ensures that the head of the e-match maintains contact with the black powder and that the black powder remains tightly packed.
The e-match wires are fed through a hole in the bulkhead and the hole is sealed with a cover that is foam lined to keep the hot ejection gases out of the bay containing the sensitive electronics. The leads of each e-match wire are then connected to the appropriate terminals on the computer. This computer helpfully labels the terminals as “drogue” and “main”.

Clubs & Ground Support

When I was a kid in the 70’s, my school had a rocket club. A couple times a month after school, we would go to the field behind the school with whatever rockets we had managed to build. The school didn’t support this with a budget, so each kid brought what they had and the teacher who chaperoned the club for us lugged a 12V car battery out for us to use. We had an eclectic collection of Estes bases, rods and launch controllers and a bunch of homemade “ground support” creations. It was ugly, crazy, unorganized… and it was awesome and fueled our dreams of being astronauts. As far as I know, the school is still there and I wouldn’t be at all surprised to find some of our old rockets are still up on the roof (I like to dream they are still there, anyway).

Fast forward to a club launch today, and it is a whole different world. While all clubs are going to have different setups, different equipment, etc, there are some things you can be reasonably sure about on your first visit to a club launch. Since this is an HPR primer, I am going to assume you are attending a club launch that has an HPR waiver.

Rails & Buttons

Many clubs have started to move away from allowing the use of launch rods for any high powered rockets and most mid-powered rockets. I have even heard of some clubs that have banned launch rods all-together, although I think that is a bit extreme. So, you may have already experienced the use of “launch rails” and “rail buttons”. Launch rails are long metal poles with a “square T-slot profile”. Usually, that profile is on all 4 sides of the square pole. A rail button is a round plastic cylinder with a slot running circumferentially about half way up the cylinder. The cylinder is hollow through the core allowing you to place a screw in it and secure it to the airframe. In theory, rail buttons could be made from any material that is suitably strong and has a low coefficient of friction, but most often rail buttons are made of plastic, with Delrin being the most desired material. I have seen rail buttons made out of metal, but some clubs don’t allow metal rail buttons/guides because they can damage the rails. At least two buttons are needed, placed longitudinally on a straight line along the body tube. Generally, one button is placed near the aft of the rocket and one is place somewhere near the center of gravity. In some cases, more than 2 buttons are desirable or required. The buttons slide into the T-slot on the launch rail and this takes the place of a launch rod in order to provide positive flight control until the rocket gains enough velocity for the fins to provide corrective pressure.

Rails and buttons come in various sizes, but the most common size for high powered rockets is the “1010” size. 1010 rails are 1” x 1” and the slot is ¼” wide. The corresponding 1010 buttons are designed to fit in the 1010 rail slots. Another common size for larger rockets are the “1515” rails and buttons. 1515 rails are 1.5” on a side and have slots that are about 5/16” wide. All clubs that I am aware of will have 1010 rails and most will have some 1515 rails, especially on the cells that will accommodate larger rockets. There are some other sizes you may encounter. At the smaller end, there are Mini-rails/mini-buttons and Micro-rails/micro-buttons. These rails and buttons are excellent for use on low and mid powered rockets. Some clubs are starting to provide rails in these sizes, but not all. Mini rails are 20mm x 20mm (0.787” x 0.787) and have a 5.26mm (0.207”) slot. Micro buttons use a 10mm x 10mm rail. Many users have found the micro buttons to bind in the rail, so care should be taken using those very small buttons. Mini buttons should be small enough for just about any small rocket. On the other end of the scale are Unistrut rails and buttons. You may never encounter these very large buttons, but some Level 3 rockets use them. Many clubs that have large hydraulic lift rail pads for big rockets only provide Unistrut rails on them. Unistrut rails are generally 1-5/8” square and have a 1” wide slot.

Also available are rail “lugs”. These are linear lugs that fit into the launch rails. These must be lined up precisely on your rocket airframe and care should be taken when loading the rocket on the rail since any rotational movement puts a large amount of stress on the lug and can tear it off the airframe.

For Level 1 and 2 rockets, you will be well-served with 1010 buttons.

You can install rail buttons in many ways. The easiest method is to just drill a hole in your airframe and screw the buttons on directly. You can put a little CA glue or epoxy on the screw threads to give it a little extra hold. This will work just fine. A more secure method is to screw the rail buttons into the centering rings or some kind of backing like a plywood square. Many people prefer to install some kind of nut in the airframe to be able to remove the rail buttons. One reason is that, over time, rail buttons can wear down and flatten out. They are, after all, plastic buttons traveling in a metal slot. Another reason to make them removable is to be able to switch between different sizes. I like to place an 8-32 PEM or Rotaloc nut in my Level 1 and 2 airframes so I can easily switch between 1010 and 1515 buttons. You could also choose to install multiple size rail buttons on opposite sides of your rocket.

The question about when you need 1010 vs 1515 buttons has been asked many times and there is no clear answer. I don’t think you will probably reach the limits of the 1010 button capacity in Level 1 and 2, however, there is another good reason to install both sizes on your rocket. Due to the popularity of the 1010 button size, those pads are often very crowded at a club launch. There is often a line for these pads on a nice day. If you can utilize one of the lesser used 1515 rails, you may save yourself a long wait in a queue. Further, I think it is a bit of “community spirit” when you do that because you are helping your fellow flyers out by not adding to the congestion at the 1010 pads.

In the photo below, from left to right,

Micro Button
Mini Button
Airfoil 1010, 1010 button with insert, 1010 button with weld nut, 1010 lug
1515 button with weld nut, 1515 airfoil, 1515 lug
Unistrut button

HPR Primer

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