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Alternative Rocket Fuels
SpaceX debuts Falcon 9 rocket at Cape Canaveral, FL, Feb. 19.
Rocket Fuel
Rockets have re-entered the popular imagination again. With NASA designing the SLS and SpaceX flying the first reusable rocket, the Falcon 9 Full Thrust, and soon flying the Falcon Heavy. I just wanted to take a moment and think about the advanced fuels that could be powering these incredible machines.
NASA has an unreasonable and passionate love for Liquid Hydrogen and Liquid Oxygen fuels. Every new rocket they have designed since the end of the Apollo program has been powered by this fuel mixture. Even to the overall detriment of the program they where attempting – Yes, I am looking at you Delta-Clipper.
Why is Liquid Hydrogen a poor fuel choice? Several reasons:
- It’s cold. Very cold. -252c or -423F, it takes a lot of energy to keep something that cold.
- Storage. There is no such thing as a Hydrogen tight container or tank. Hydrogen is an element known to react with everything, it permeates any container its stored in. Severely limiting how long it can be stored. This effect is sometimes called boil off and can be as high as 1% per day. During the Shuttle Program, its estimated that only 25% of the hydrogen purchased actually made it into the external fuel tank for use in flight.
- Expense. Because of Hydrogen’s propensity to permeate any solid object, it means that fuel lines, storage tanks, and fittings have to be regularly inspected and replaced. As Hydrogen embrittlement could cause such devices to suddenly and catastrophically fail. Making launch complex operations very expensive.
- Low energy density. Liquid Hydrogen only contains 8.5 MJ/L (megajoules/Liter) of energy in comparison LNG (Methane) which has 22 MJ/L and Jet A (Kerosene) that has 33 MJ/L. This means that a fuel tank can be much smaller if it’s storing Kerosene or Methane rather than Liquid Hydrogen.
- Because of its size and insulation requirements, a tank designed to store liquid hydrogen is much heavier than a similar tank for Liquid Methane or Kerosene.
So, what alternatives are there? Most commonly there is RP-1 (Rocket Propellant 1, a highly refined kerosene similar to jet fuel) and Liquid Oxygen. This is the fuel used by SpaceX’s current Falcon series of rockets, as well as the Atlas V, Antares and Soyuz, as well as many others historically – most famously the first stage of the Saturn V, which launched men to the moon, and is the most powerful rocket ever built and is the heaviest man-made object ever to fly.
Methane
Probably the alternative rocket fuel getting the most development love is Liquid Methane. It has several advantages over Hydrogen:
- Liquid in the same thermal range as Oxygen, meaning that both the oxidizer and fuel thank can share the same thermal envelope, lowering costs, saving weight, and simplifying the design.
- Liquid Methane is a commonly used commercial product. Engineering talent familiar with designing for it is common and therefore easier to recruit and retain.
- Energy Density, as noted above, Liquid Methane has almost 3x the energy of liquid hydrogen. Making fuel tanks smaller, lighter and more aerodynamic.
- Because the hydrogen atom is locked into a long-term relationship with a carbon atom, methane is less chemically reactive to surfaces which it’s in contact, making storage and transporting much simpler.
Liquid Methane powered rockets have been the product of design studies and research in Russia since the 1990s, the Soyuz 5 rocket, expected to fly for the first time in 2022, will replace the Soyuz 2 which is currently used as a carrier rocket for co-operative space program between Russia and Europe flying from French Guinea. SpaceX is also developing a methane-powered engine for its Mars program, called Raptor.
Hypergolic Fuels
The primary advantage of a hypergolic rocket is simplicity. You don’t need a complex ignition system to start the rocket, since the fuel and oxidizer ignite on contact, simply mix them in the combustion chamber (at the proper ratios) and you’re off to the races. This also makes it ideal for upper stage engines that may need several burns to get a payload into the proper orbit or trajectory. The Briz stage used in the Russian Proton rocket, for example, can be restarted 8 times up to 24 hours after launch.
Some of the advantages of Hypergolic fuels:
- Fuel and forget, because hypergolic fuels are stable at near room temperature a rocket can be fuelled for months without adverse consequences. Before the USAF became enamored with solid rockets for its ICBMs there where Hypergolic Rockets, like the Titan II, which for many years was the backbone of the American nuclear deterrence. (in fairness the 1980 accident in Damascus Ark, which a nuclear armed missile was destroyed by fire after a dropped socket wrench punctured the side of the missile – releasing fuel. Would scare the daylights out of the joint chiefs and civilian command)
- Fuel is stable at room temperature means that no special insulation needs to be included in the tanks design, reducing cost and weight.
- High energy density of the fuel means that tanks can be smaller.
These fuels are the most exotic and controversial, as historically they’re carcinogenic and generally toxic. Several years ago, the US Army had a research program called CINCH (Competitive Impulse, Non-Carcinogenic Hypergol) the fuel molecule they came up with is called Dimethylaminoethylazide or DMAZ to its friends. Its chemical formula is C4H10N4, making it a very energetic and reactive chemical. While not a carcinogenic, it would still kill you. (because it’s so reactive, it’ll ignite with oils on your skin – and be a nightmare should you inhale it.)
Because it’s a fuel still in research and never used as a propellant in any launch system, information about it is kinda rare. I recently found a summary of a research paper that claimed that DMAZ with Hydrogen Peroxide gave the best specific impulse of the oxidizer mixtures tested of 360 seconds. If correct this is very impressive. By comparison, sea level impulse of a methane engine is about 330 seconds, and Hydrogen is about 380.
Gelled Fuels
Increase the energy density of a fuel, by increasing the density of the fuel. This is still a highly speculative area of research, and publicly available information on it is very thin. The idea is to have a propellant that behaves like a solid in storage, but a liquid when deployed. There is also a safety advantage that the gelled propellant wouldn’t form an explosive cloud, should the rocket structure fail. There is a lot of research in this area going on in France and Germany. Prototype rockets are rumored to have impulses above 660 seconds, that if true would be groundbreaking technology critical to making single stage to orbit boosters possible.
Our recent discussions of the shuttle program, its shortcomings, and alternatives, made me think of writing this. Hopefully, we could discuss alternative launcher technologies that haven’t really – pardon the pun – taken off.
Published in Technology
Very cool summary, thank you!
I thought there was also a push to use paraffin (yes, candle wax/oil) as rocket fuel?
No Paraffin is an obsolete term for a form of lamp oil – Kerosene. I think the term was used longer in Britain, and was used on the British launcher with High Test Peroxide as an oxidizer.
No. I mean candle wax. See this story from Stanford. Here is a NASA article on it as well. Aviation Week reports on a 2012 test firing.
Oh! I get it now its a hybrid engine.
Its an unusual choice for a fuel. Spaceship one uses a similar rocket engine using HTPB rubber as a fuel.
I was a little lost, because a lot of the documentation on the “Black Arrow” which was a British launcher – that had 1 successful test flight in the 1960s. Called its fuel Paraffin as well (even though they meant kerosene) with Hydrogen Peroxide as an oxidizer.
Ah, Rocket Science!
If you want more details on the nitty-gritty of rocket propulsion, I recommend Ignition!, by genuine rocket scientist John D. Clark. Everything you wanted to know about rocket propulsion, told in an intensely amusing style.
https://www.dropbox.com/s/r53k5ftwbpti4gk/ebook_Ignition_An_informal_history_of_liquid_rocket_propellants_John_D_Clark.pdf?dl=0
I started it and was immediately hooked! Downloaded, printed, and I aim to read it in the coming days!
I read it too! It’s a fun read.
Thanks I’ve been looking for this book. It was recommended on another site.
Putting aside the propaganda generated by K Street in support of “Big Hydrogen”, what are the advantages to using liquid hydrogen?
I’ve been firing hybrid rocket motors in hobby rockets since 1998. The oxidizer is N2O, which we get from hot rod shops, denatured with some butylmercaptan (stinky stuff) so it doesn’t get used as a recreational drug. Industrial-grade N2O isn’t a hazmat; it is basically the same as CO-2, so it’s no more dangerous than a soda machine. The first fuel grains we used in sport rocketry were PVC and polyurethane, basically thick pipe. Folks are now 3D printing fuel grains to give them interesting geometries in the combustion chamber. Also popular are cast rubber grains, generally something that is cured at room temperature, packed into a liner tube with a mandrel in the center, or with a core drilled after it is solid. The beauty of hybrids is the safety of the system: since you don’t have the intimate mixture of a fuel and an oxidizer until you fill the motor with N2O, which happens by remote control, you’re never dealing with a potential explosive (although conventional solid propellants don’t explode…that’s another rant).
Youtube video of a hybrid flight with an onboard camera looking down, and an altimeter that deployed a drogue chute at apogee (right around 3000 feet) and a big main chute at 1000 feet. That’s how we keep them from drifting into the next county. The rocket is a scale model of a Tomahawk sounding rocket, 6 inch diameter fiberglass tubing. This was at the NAR National Sport Launch at the Warbird Museum in Geneseo, NY. The NSL is coming back to Geneseo in 2018 and I am developing nefarious plans. I also hasten to add that I have lost 100 pounds since these pictures were taken.
High specific impulse. Low pollution; the combustion products are H2O. And if you happen to be on the moon and extracting water ice from the regolith, you can use electrolysis powered by your solar array to make fuel and oxidizer for your rocket.
Thank you. I actually met Dr. Clark once, when I was a teenager, at a meeting of the Baker Street Irregulars. He wrote an essay speculating that Nero Wolfe was the illegitimate child of Holmes and Irene Adler. Right up there with Asimov (who greatly admired him) as a science writer who could make you feel the thrill of science.
One of the coolest things about hybrid motors is the fact that if something is solid, can burn, and gives up an acceptable number of BTUs in the process, you can use it for fuel. I know of two groups that have drilled a hole in a piece of hard salami and burned it as fuel with GOX in a hybrid rocket motor. One of them sliced up the fuel grain and ate it afterwards.
You’re correct in stating the advantages of hydrogen, but I think the real reason NASA uses it is expense.
For all the whining about NASA’s low budget – as compared to the Apollo era – NASA gets almost as much money as all the other space programs in the world COMBINED. They’re also the only space agency the will use hydrogen on a fist stage of a rocket system. I think they do it just to spend the money – to justify to congress – see we spent all that money developing a rocket…
Prime example of this is the DC-X or Delta Clipper. There were several technical challenges presented by using hydrogen in this project – primarily having a carbon composite tank that could survive the low temperatures of liquid hydrogen. All of those challenges could have been avoided by switching to RP-1.
I know he’s a little controversial in these parts, but here’s a speech by Robert Zubrin, critical of NASA’s budget and results:
Ok, I stand corrected on that point of other launch systems. I was unaware of the fuels used in Japan, and miss-remembered Energia. (I thought it was entirely RP-1).
I disagree with you on single stage to orbit. IF its possible at all – I think the exact opposite – I think its more likely achievable with fuels other than hydrogen. (Primarily thinking of methane) Also thinking that the engine(s) would have to be an aerospike design – or possibly also an air breathing rocket like SABRE from Reaction Engines in the UK. Clearly a fairly fundamental breakthrough will have to be made in launcher design to make such a thing possible.
Iam not a fan of hydrogen use in first stages, but I think it makes an better fuel for upper stages, as an example the Saturn V.
I’ll give your post a read, it looks very well researched.
Another great book, that I’ve found today, is called “To Reach the High Frontier”.
Very in depth look at post war and cold war ICBM and launcher development programs.
anonymous
I dont think that SSTO is possible with our current understanding of technology. Just like staging was once described as cheating the rocket equation. We’re going to have to find a new cheat. I appreciate fuels that have a high energy density, I want the rocket stage to be as small as possible – for weight, cost and aerodynamic reasoning. The downside of higher density, is that the fuel will produce heavier exhaust for a lower ISP, but a higher thrust.
Back in the early 60s the soviets had a project called PR-90 Gnom. http://www.astronautix.com/p/pr-90.html
It was a small ballistic missile. Solid fuel, but it incorporated a trick into the rocket equation. It was an Air Augmented Rocket. Giving it an ISP of 550(!) Unfortunately the soviets cancelled the project. (or perhaps fortunately considering it was a ballistic missile) I think in order for SSTO to work, we’ll have to develop an engine like this. A highly energy dense fuel (although for safety reasons, I prefer not a solid) Maybe methane/lox or DMAZ/Hydrogen Peroxide driving either a single stage to orbit machine, like Skylon, or boosting an orbiter like Energia/buran.
We’re also on the cusp of an entirely new material world. With metallic foams, aerogel insulation, carbon/ceramic composites and whole host of new materials bursting to come of age. I think we could radically cut the weight of everything. I think rocket stages designed with these materials could easily be 25% lighter than current models.
Last week I had the fun of touring the forging house where most (all?) the rings used to make rocket housings in the USA. Very, very cool.
Scott Manley has more information about rocket fuels, with the example of the Delta IV Heavy: