This is one of my space related projects. The status of this document is work in progress.

Are Amateur Orbital Rockets Possible?

Please note that this document was originally written in 1995 with only minor updates moving forward. As such, it is getting pretty dated.

Table of Contents

• Disclaimer
• Introduction
• Fuel/Oxidizer Selection
• Tanks
• Engines
• Ignitors
• Pressurization/A>
• Thrust Vector Control
• Navigation
• Telemetry
• Launch Facilities
• References

Disclaimer

Even the best designed rockets sometimes explode, and when they do, the high speed metal fragments that are ejected from the explosion can be thought of as shrapnel that can cause grievous harm if any pieces hit anyone. Since this a paper about about big amateur rockets, explosions and the resulting shrapnel are extremely likely. If you decide to try out anything suggested in this document, you do so at your own risk, and with absolutely no liability to me. If this is unacceptable to you, do not attempt anything described herein. Since I value my life, you can rest assured that I will not be anywhere near any of the equipment described in this document when it is being tested.

Introduction

This document is basically a bunch of notes to myself about the issues involved in designing a fairly big amateur rocket that is capable inserting a small payload into low earth orbit. Think of this document as a notes in a design journal and you have the right basic idea. You should definitely not read this document as a paper written by anybody authoritative -- I'm strictly an armchair `wanna be' rocket scientist at the moment. My training is in electrical engineering and computer science, not aerospace engineering. Some of the content in this document is undoubtedly wrong and I would appreciate if people more knowledgeable than me who read it could send me some E-mail pointing out any mistakes.

I am very impressed with what the amateur rocket community has been able to build and launch to date. The recent launch of a 14000 pound thrust rocket to an altitude of approximately 50 miles by the Reaction Rocket Society and written up by David Crisalli [Crisalli97] is one such recent example. As impressive as the amateur rocket community's accomplishments have been, building a rocket that is capable of launching even the smallest of payloads into orbit has been beyond the financial and technical means of the amateur rocket community. This paper explores some ideas on how to expand the envelope of amateur rockets to include the insertion of small payloads into orbit.

The amateur rocket community has been attracted to solid propellant rockets due to their innate simplicity and relative low cost. While a solid rocket is technically capable of putting a payload into orbit, it has to be pretty big multi-stage solid rocket to do so. The large size and multiple stages introduce cost, safety, and control issues that, to date, have not been surmounted by the amateur rocket community.

Liquid fuel engines are quite a bit more complicated to manufacture and test, but they have improved performance, safety and control capabilities over and above solid propellant rockets. Are liquid fueled rocket engines feasible to the amateur community? Some early experimentation with amateur liquid-fueled rocket engines is written up by Leroy Krzycki in a small book entitled How to Design, Build, and Test Small Liquid-Fueled Rocket Engines [Krzycki67]. This book describes at a reasonable level of detail the issues involved with designing, building, and testing small (10-20 pound thrust) liquid-fueled amateur rocket engines. Can such small engines be scaled up by the amateur community to be capable of putting a small payload into low earth orbit?

Fuel/Oxidizer Selection

The ideal fuel/oxidizer combination for amateur rockets would have low toxicity, easy handling characteristics, low cost, and high performance (high Isp.) Alas, there is no such perfect fuel/oxidizer combination. The selection of a fuel/oxidizer combination is an exercise in compromises. So what characteristics are most important to amateur community? Well, I would propose that the characteristics should be ordered by low fume toxicity first, reasonable handling/cost/availability second, and reasonable performance last. By low fume toxicity, I mean that the fumes given off by the substance are not too poisonous. By this criterion, gasoline is considered reasonable and hydrazine is not. In the discussion below, I will simply leave the high fume toxicity fuels and oxidizers off the list; this shortens the oxidizer and fuel lists down considerably.

The list of reasonable fume toxicity oxidizers is quite short:

Hydrogen Peroxide (H₂O₂)

Hydrogen peroxide is probably a pretty marginal oxidizer for the amateur rocket community. Hydrogen peroxide can be made to react with itself (2H₂O₂ => 2H₂O + 2O₂ + energy). Thus, hydrogen peroxide can be used as a monopropellant (like hydrazine.) I am not really all that familiar with the steps that are needed to prevent hydrogen peroxide from reacting with itself. If the steps are not too onerous, it is possible to use it as an oxidizer in a bi-propellant system as well. Hydrogen peroxide is not readily available in any significant quantity or purity for the amateur rocket community. The hydrogen peroxide sold by the corner drug store is heavily diluted with water. This forces the amateur into developing the technology to manufacture or purify hydrogen peroxide. This can make the cost per liter of purified hydrogen peroxide quite expensive.

Nitrous Oxide (N₂O)

Nitrous oxide is a liquid at ambient temperature and several atmospheres of pressure. This makes nitrous oxide a little more difficult to handle than hydrogen peroxide, but it is still reasonable. Like hydrogen peroxide, nitrous oxide is a bit difficult to obtain. It is used by the dental community for mild anesthesia. The abuse of nitrous oxide at parties has caused some restrictions on obtaining bottles of gaseous nitrous oxide. The cost per liter of purified nitrous oxide can be quite expensive.

Oxygen (O₂)

Oxygen is both inexpensive and readily available in both gaseous and liquid form due to its use by the welding industry. Gaseous oxygen can be used in the early development of an amateur rocket engine. Use of liquid oxygen (LOX) is necessary for a flight weight vehicle. LOX is more difficult to handle than the other ambient temperature oxidizers mentioned above. The difficulty with LOX is that it can cause spontaneous combustion when it comes in contact with various substances -- like finger print grease! Thus, handling LOX requires a level of cleanliness that can be difficult to obtain in the amateur environment.

Hydrogen (H₂)

Hydrogen is not readily available, but it is both easy and inexpensive to electrolyze from water. Liquid hydrogen (LH2) is an extremely cold cryogenic fluid, that requires very careful handling. A pipe of LH2 exposed to air will cause the air around the pipe to liquidize. Thus, pipes carrying liquid hydrogen tend to vacuum jacketed. Another disadvantage of LH2 is that it is not very dense, thereby necessitating fairly large tanks in comparison to the oxidizer tank. LH2 has some truly awesome performance characteristics that can offset its handling disadvantages. Frankly, LH2 is probably too difficult to handle for the amateur rocket community.

Methane (CH₄)

Methane is basically the primary component in natural gas. While liquidized natural gas (LNG) is not that readily available, it is possible to create LNG by simply hooking a tube up to natural gas tube and running the the gas through a heat exchanger consisting of a pipe immersed in liquid nitrogen. Given that both butane and propane are readily available and easier to liquidize, it is probably easier to use either of those fuels instead of LNG. LNG appears to have a higher performance than either butane or methane.

Propane (C₃H₈)

Like butane, propane is readily available and quite inexpensive due to extensive use in the recreational vehicle industry. Propane has an interesting characteristic in that it freezes at a temperature that is below that the boiling point of LOX. This can really help simplify the tank design issues when dealing with LOX.

Butane (C₄H₁₀)

Butane is both readily available and relatively inexpensive due to its use for rural home heating. Butane is a liquid under a pressure of several atmospheres. Butane has one disadvantage in comparison to propane in that it is a frozen solid at the same temperature that LOX boils.

Gasoline (C₈H₁₈)

Gasoline is extremely available and fairly low cost due to its use by cars and trucks. It is a liquid at both ambient temperature and pressure. The handling characteristics of gasoline are quite well understood.

Kerosene (C_?H_?)

Kerosene is not quite as available as gasoline, but it can be obtained and it basically has similar performance characteristics as gasoline.

Methyl Alcohol (CH₃OH)

Methyl alcohol is another viable alternative to gasoline and kerosene. Methyl alcohol is not as available as either kerosene or gasoline.

Ethyl Alcohol (CH₃CH₂OH)

Actually, ethyl alcohol (i.e.grain alcohol, booze, etc.) is not very expensive when you remove the taxes. If you are willing to go through the hassle of convincing the appropriate authorities that you are going to burn it instead of drink it, you can get a waiver on paying the taxes. Be prepared to explain why some other fuel will not suffice though.

When it comes to choosing a oxidizer/fuel combination, the primary decision seems to be whether to tackle cryogenic temperatures or not:

Cryogenics

If the answer to the cryogenic question is `yes', liquid oxygen (LOX) is the obvious oxidizer choice. Once LOX has been chosen, the next choice is whether to deal with cryogenic temperature fuels. This choice is not nearly as simple:

Cryogenic Fuel

Propane seems to be the best choice for a cryogenic fuel -- it will simplify tank design issues because it can be kept at the same temperature as the LOX. The drawback with propane is that it is not as dense as hydrocarbon fuels (i.e. gasoline, kerosene, methyl alcohol, etc.) The lower density means that the fuel tank will be larger and heavier; this could offset the advantages of keeping the cryogenic fuel and oxidizer at the same temperature.

Non-cryogenic Fuel

If a non-cryogenic fuel is chosen, gasoline will probably edge out the other hydrocarbon fuels based on cost and availability considerations.

It is interesting to note that most commercial launch vehicles seem to go for the LOX oxidizer and either hydrocarbon or liquid hydrogen fuel. The hydrocarbon fuel choice seems to be based on density issues and the liquid hydrogen fuel seems to be based on the excellent performance that liquid hydrogen can provide. The LOX/Propane combination seems to be largely unexplored.

No Cryogenics

If the decision concerning cryogenics is "no", the oxidizer choice is going to be between hydrogen peroxide and nitrous oxide. It is looking like nitrous oxide is easier to obtain than hydrogen peroxide, so that is probably the oxidizer of choice. When it comes to ambient temperature fuels, gasoline still looks like it edges out the others in terms of price and availability.

So what are the fuel/oxidizer combinations? The following seem to be reasonable combinations that are suitable for the amateur rocket community:

LOX/Propane

LOX/Propane seems to be a pretty good choice for when both fuels and oxidizers are cryogenic. This pair is going to require figuring out how to cool down the propane from ambient temperature to cryogenic temperatures. This should be no more difficult that running a tube through a vat of liquid nitrogen.

LOX/Gasoline

Kerosene can be substituted for gasoline. This pair only requires mastering the handling of LOX. In addition, keeping the LOX from freezing the fuel by accident needs to be addressed.

Hydrogen Peroxide/Gasoline

This is one of two ambient temperature fuel/oxidizer combinations. Again, kerosene can be substituted for gasoline. This pair requires mastering how to generate hydrogen peroxide. Hydrogen peroxide does have a higher performance than nitrous oxide.

Nitrous Oxide/Gasoline

As usual kerosene can be substituted for gasoline. The rocket will need to be bigger and heavier to offset the lower performance of nitrous oxide.

Tanks

Both the fuel and the oxidizer need to be stored in tanks. Each tank has to be able to withstand the entire weight of all of the stages above it plus the fluid in the tank (i.e. fuel or oxidizer) while the rocket is accelerating at multiple G's of acceleration. Furthermore, the LOX tanks will need to have enough insulation to prevent undue `boiling' in the LOX. Given the number of other mass compromises that are going to be necessary to build an amateur orbital rocket, tank mass will have to pretty comparable with rockets made by the aerospace community.

The lightest tanks are made out of composites, which are also expensive to manufacture. Aluminum is probably the only relatively affordable tank material for the amateur rocket community. Even so, an orbital rocket built out of aluminum is a lot of aluminum and minimizing the amount of aluminum required will significantly reduce the entire cost of the rocket.

The only real `trick' I know of to reduce tank mass is to keep it pressurized. This technique is used by the Centaur upper stage. The idea here can be demonstrated using an almost empty plastic 2 liter bottle of carbonated soda. When the cap is off, the bottle is extremely flexible. When you put the cap on and shake the bottle, it becomes pressurized and much more rigid. By pressurizing a rocket tank, far less aluminum is needed.

Pressurization is typically accomplished by storing some helium in some high strength titanium spheres. An amateur rocket may have to compromise by using a somewhat larger helium storage tank at a correspondingly lower pressure.

One disadvantage of pressurized tanks is that if pressurization is ever lost while the tanks has weight on it, the tank tends to crumple up. I think the amateur rocket community can live with this constraint.

{Talk about flat end-plates vs. spherical end-plates.}

{Talk about inter-stage adaptors.}

In general, the fluid with the highest density is put in the lower tank to lower the vehicle center of gravity. Since oxidizers are almost always denser than the corresponding fuel, this means that the oxidizer thank is usually on the bottom.

A feed line needs to be run from the upper tank to the lower tank. There are three basic places to run the feed line:

Down the center

xxxx

On the outside

xxx

{Talk about the fuel line placement.}

Assembling a tank out of aluminum is no easy task. The ideal solution would be to find a source of extruded aluminum tubing that is the right length and thickness and just weld two end caps on either end. If no extruded aluminum is available, sheet aluminum must be purchased, cut, shaped and welded into a tube of the appropriate diameter. The more welding there is, the more chances there are to make a mistake that will compromise tank integrity.

{Talk about tank slosh.}

{Talk about tank assembly.}

{Talk about tank testing.}

{Talk about tank insulation.}

Engines

Developing a liquid rocket engine is no small task. Developing a cheap liquid rocket engine makes the task even harder still.

There is a wonderful book called entitled How to Design, Build and Test Small Rocket Engines [Krzycki67] that is basically a `must read' for any one contemplating the development of an inexpensive liquid rocket engine. Through the hard work of Daniel Risacher and some others, this book is now available on the web. The overall strategy espoused by the book is:

• Be safe! Operate the engine remotely and from behind a blast protection wall.
• Start small with engines in the 10 to 25 pound thrust range.
• Start with gaseous oxygen before attempting to work with liquid oxygen. Kerosene or gasoline seemed to be the initial fuel choice.
• Manufacture the initial engines out of copper and use water cooling.

While their strategy makes sense as far as it goes, it does not result in flight ready liquid rocket engines.

Many years later, I read an abstract in NASA Technical Briefs that suggested an interesting way of constructing rocket engines. The abstract proposed constructing an engine out of a stack of machined metal sheets. Each sheet in the stack would be individually machined to have holes for the nozzle or combustion chamber along with holes for regenerative cooling. Basically the whole assembly would be bolted together. It occurs to me that this might be the base technology for building cheap liquid rocket engines.

{Talk about titanium plates versus aluminum plates.}

{Lots more goes here.}

Ignitors

Unlike hypergolic fuels, when you mix liquid oxygen and kerosene, they do not spontaneously ignite. Thus, some form of ignitor is needed. A fairly reasonable choice for an ignitor is to use have port in the injector plate through which the exhaust gases of an Estes model rocket engine can shot. Reliable electric ignition of a model rocket engine no longer seems to be a problem.

{Put a picture here.}

Pressurization

Liquid rocket engines need some sort of pump to take the low pressure fuel and oxidizer, pressurize them, and force them into the relatively high pressure combustion chamber. In general, the higher the combustion chamber pressure, the higher the overall rocket engine performance. The aerospace companies tend to use turbo pumps for rocket engines. While turbo pumps are extremely powerful for their overall mass, they are difficult to design, manufacture, and test. In one space propulsion class I audited at MIT, we estimated a full 10% of the energy of a liquid rocket engine is used to run the turbo pumps to pressurize the fuel. An amateur rocket that uses a liquid rocket engine is going to need to expend a similar amount of energy for pressurization and be inexpensive as well.

A pump based on pistons is a serious possibility as is shown in the diagram below:

The basic idea is similar to how a piston is driven back and forth on a steam locomotive. Hot gases from the combustion chamber are alternately routed from one side of the piston to the other via valves. Other valves are alternately opened and closed to vent the combustion cases when they are no longer needed. A mechanical linkage is used to drive another piston back and forth to pressurize the fluid (i.e. fuel or oxidizer.) Again, valves are alternately opened and closed to route the fluid into and out of the pressurization cylinders.

Since there is a pressure drop across the injector as the fluid is injected into the combustion chamber, it is necessary to construct the pump so that it delivers the fluid at a higher pressure than the combustion chamber. Since the force on the mechanical linkage is the direct product of the pressure times the piston area, it is possible to write the following balance equation:

	Pcc Acc = Pf Af
								

where

Pcc

is the combustion chamber pressure measured in Pascals (i.e. Nt/M2),

Acc

is the area of the piston being driven by the combustion chamber gases measured in square meters,

Pf

is the fluid (i.e. fuel or oxidizer) delivery pressure to the combustion chamber injector measured in Pascals, and

Af

is the area of the fluid piston measured in square meters.

Since Pf > Pcc, it follows that Af < Acc.

The development of a workable piston pump for amateur liquid rocket motors will have to overcome many difficulties, some of which are listed below:

Dealing with Hot Combustion Gases

Since the gases that are coming out of the combustion chamber are quite hot, it can be quite challenging to build a set of pistons and valves that can operate at such high temperatures. My suggestion is to cool he combustion chamber gases down some with a simple `heat exchanger' until they are at a more workable temperature. All I am thinking of when I say `heat exchanger' is a tube with hot exhaust gases on one side and liquid oxygen running past it on the other side. Once the temperatures are in the same region as automobile engine exhaust gases, they should be workable by amateurs again.

High Pressure Differential Pistons

Since building a piston that can withstand several the tens of atmospheres of pressure differential can be quite challenging, it may be desirable to build multi-stage pumps to reduce the pressure differential across each piston.

Induced Combustion Instability Due to Valve Operation

Each time a valve is opened or closed, there will a corresponding increase or decrease in the down stream pressure. These pressure variations can cause combustion instabilities in the combustion chamber. Obviously, if the combustion instabilities are bad enough, they can cause the engine to explode. A possible solution to this problem is to build the pumps so that they have parallel stages that operate out of phase with one another.

{More issues here}

...

{More about pumps goes here.}

Thrust Vector Control

If there is any hope of getting into orbit, it is going to be necessary to steer the rocket into orbit. Rocket steering is accomplished by thrust vector control.

The most efficient form of thrust vector control is to put the entire rocket engine on gimbals and move it to and fro. Building gimbals that withstand the large forces needed to get a rocket going are not easy. Furthermore, if the rocket engine gimbals, it is necessary to have flexible fuel and oxidizer lines. A flexible fuel hose can be made out of rubber. A flexible oxidizer line is a little more challenging because of the cryogenic temperatures involved.

One possible solution to the thrust vector control problem is to use use the exhaust gases from the pumps. Using valves the pump exhaust gases can be directed and vented to push the rocket to and fro. It is unclear to me whether there are enough gases available to generate enough side thrust to actually steer the rocket.

Another possible solution is to uses clusters of three or more off axis rocket engines for thrust vector control. If the engines are 2-3 degrees off axis, a small amount of throttling in the engines can be used to create asymmetric thrust to steer the rocket. This requires that the rocket engines have some throttle capability.

Navigation

{Talk about using GPS instead of inertial navigation.}

{Navigation section goes here.}

Telemetry

{Talk about using the Iridium constellation as telemetry option.}

{Telemetry section goes here.}

Launch Facilities

{Launch facilities section goes here.}

Summary

{Summary goes here.}

References

[Crisalli97]

700 Miles North... ...and 50 Miles Up? by David E. Crisalli in High Powered Rocketry, Vol. 12 No. 8, (Oct./Nov Issue) 1997 published by Tripoli Rocketry Association, P.O. Box 280 Bessemer, AL USA 35021 Phone: 1-205-424-8357 ISSN: 1070-5244.

[Krzycki67]

How to Design, Build, and Test Small Liquid-Fuel Rocket Engines by Leroy J. Krzycki, 1967. ISBN: 9600-1980-4. URL: http://www2.jps.net/~gramlich/projects/rocket/index.html.

[Lide96]

CRC Handbook of Chemistry and Physics, 77th Edition 1996-1997 David R. Lide, Editor-in-Chief, 1996 published by CRC Press, Inc., 9000 Corporate Blvd., Boca Raton, FL 33431. ISBN: 0-8493-0477-6 Library of Congress Card Number: 13-11056

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Copyright (c) 1995, 1998, 2001 Wayne Gramlich. All rights reserved.