Both Flight 90 and Flight 91 of the North American X-15 crossed the Kármán line, reaching altitudes of 106.01 and 107.96 km respectively. Both flights were piloted by Joseph A. Walker, who became in 1963 the "United States' seventh man in space" and "qualified him as an astronaut under the rules of the U.S. Air Force and the Fédération Aéronautique Internationale (FAI)"

From X-15:

By November 1960, Reaction Motors was able to deliver the XLR99 rocket engine, generating 57,000 pounds-force (250 kN) of thrust. The remaining 175 flights of the X-15 used XLR99 engines, in a single engine configuration. The XLR99 used anhydrous ammonia and liquid oxygen as propellant, and hydrogen peroxide to drive the high-speed turbopump that delivered propellants to the engine. It could burn 15,000 pounds (6,804 kg) of propellant in 80 seconds.

I hadn't heard of anhydrous ammonia as a fuel before reading this. It needs to be either pressurized or cryogenic (about -33C) to remain as a liquid, unlike organic fuels like alcohols or the heavier hydrocarbons. What were the various reasonings behind developing an ammonia burning engine in this case? Has ammonia been used again after the X-15? Or before for that matter!

enter image description here

above: Test pilot and astronaut Joseph A. Walker

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above: Reaction Motors XLR99 rocket engine from here.

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above X-15 from here.

  • 1
    $\begingroup$ @MartinSchröder Thanks, but I specified plural reasons on purpose. Changing it to singular conflicts with the author's intention. As you can see in the answer there were indeed several, or actually more than one set of several! $\endgroup$
    – uhoh
    Commented Aug 29, 2017 at 12:12

5 Answers 5


According to Clark's "Ignition!", German rocket scientists in WW2 had done the math on ammonia, and JPL had burned it with RFNA and WFNA oxidizers in 1949-1951.

Regarding the XLR99, Clark says:

But something more potent than alcohol was needed for the X-15 rocket-driven supersonic research plane. Hydrazine was the first choice, but it sometimes exploded when used for regenerative cooling, and in 1949, when the program was conceived, there wasn't enough of it around, anyway. Bob Truax of the Navy, along with Winternitz of Reaction Motors, which was to develop the 50,000 pounds thrust motor, settled on ammonia as a reasonably satisfactory second best. The oxygen-ammonia combination had been fired by JPL, but RMI really worked it out in the early 50's. The great stability of the ammonia molecule made it a tough customer to burn and from the beginning they were plagued with rough running and combustion instability. All sorts of additives to the fuel were tried in the hope of alleviating the condition, among them methylamine and acetylene. Twenty-two percent of the latter gave smooth combustion, but was dangerously unstable, and the mixture wasn't used long. The combustion problems were eventually cured by improving the injector design, but it was a long and noisy process.

At that point in time, state of the art in big rockets was the Redstone, which was burning 75/25 ethyl alcohol/water with LOX in an engine which was largely copied from the V-2; watering the fuel down was necessary to moderate combustion temperature. While regeneratively cooled, the design of the cooling tubes was not as complexly efficient at that time as it would be in later engines.

Kerosene would "coke up" (polymerize) in regeneratively cooled engines, potentially catastrophically clogging coolant tubes. That problem would eventually be solved by the development of the RP-1 kerosene specification in the mid-50s.

So during the development of the XLR99, ammonia had a useful niche -- it gave better performance than 75% alcohol, was more suitable for regenerative cooling than cheap kerosene, and was safer than the hydrazine fuels.

  • 1
    $\begingroup$ It looks like you've hit almost all aspects nicely. Are the problems with kerosene related to reusability, or would the problem happen so fast it could interfere with a single ~80 second burn? Also, you've inspired the question Why would red fuming nitric acid become white fuming nitric acid if left out at low temperature? $\endgroup$
    – uhoh
    Commented Aug 22, 2017 at 15:14
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    $\begingroup$ Per the RP-1 article, burn-through on a single firing is a concern -- as soon as you get some coking, you get less coolant flow, thus more heat, thus more coking, in a rapidly accelerating cycle. $\endgroup$ Commented Aug 22, 2017 at 15:16
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    $\begingroup$ IANAchemist so won't answer on that SE, but I assume the NTO evaporates out. $\endgroup$ Commented Aug 22, 2017 at 15:17
  • $\begingroup$ oic - in the small tubes where it is first being used as a coolant. $\endgroup$
    – uhoh
    Commented Aug 22, 2017 at 15:22
  • 2
    $\begingroup$ Yeah -- coking in the nozzle is less of a concern for expendable engines. $\endgroup$ Commented Aug 22, 2017 at 15:41

The book Aerofax Datagraph 2 / North American X-15/X-15A-2 by Ben Guenther, Jay Miller, and Terry Panopalis has some more info on the history of the propellant choice (page 27):

Eventually it was determined that the two most important requirements from a safety standpoint concerned the propellant combination, and the means of achieving combustion safety during starting and shut-down. Seven propellant combinations were explored in depth, these eventually being narrowed to liquid oxygen as the oxidizer and anhydrous ammonia as the fuel. The choice was based primarily on the fact that Reaction Motors had significant experience with liquid oxygen/ammonia propellant systems, and also on the fact that this propellant combination had much less critical starting characteristics. Additionally, the liquid oxygen/ammmonia combination was an ideal coolant for the regenerative cooling of the proposed engine's thrust chamber.

FWIW, this book also has a good writeup on the developmental history of the XLR-99 and specifics on its design.

According to this pdf of a powerpoint presentation the choice was made because one of the chief designers at Reaction Motors Dr. Paul F. Winternitz was an advocate of NH3/LOX engines.

Rationale for propellant choice:

The directive to use NH 3 came from Dr. Paul F. Winternitz, a propellant scientist from Austria.

  • Dr. Winternitz had to find a fuel that would be stable, would be easy to keep, show a good volumetric energy density, density, would work in in the the temp range and allow conclusions for a later H2 fuel system.
  • NH 3 /LOX fit the bill and it worked!
  • Later, when gravimetric energy density was more important than volumetric energy density (for the Shuttle) H 2 was preferred
  • Good heat transfer properties

The presentation is a bit sketchy (and incorrectly states that an earlier Reaction Motors built engine, the XLR-10 was NH3 fueled), but includes this wonderful schematic of the XLR-99.

enter image description here

Interestingly, this presentation was given to the NH3 Fuel Association whose goals are

to promote NH3 as an affordable, sustainable, carbon-free fuel for stationary power, transportation, and energy storage applications, thereby reducing dependence on fossil fuels and enabling the transition to a low-carbon economy.

  • $\begingroup$ I'm not sure "Why was Paul F. Winternitz an advocate of NH3/LOX engines?" should be broken out as a separate question. I've run across suggestions of the ratio of densities to LOX is near unity (better for pumping?) and lack of carbon improved reusability (it is an airplane after all), but so far nothing conclusive on those. $\endgroup$
    – uhoh
    Commented Aug 22, 2017 at 14:40
  • 1
    $\begingroup$ Yeah, I was double checking and it looks like the presentation was wrong about the XLR10. When I get home I'll check some print references I have. $\endgroup$ Commented Aug 22, 2017 at 14:45
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    $\begingroup$ It's possible that RM had fired an ammonia version of the XLR10 at some point as an experiment, as rocket engines tended to be less extremely optimized for a single fuel at that time. I note that later in that same PDF it shows XLR10 as an alcohol/LOX engine. $\endgroup$ Commented Aug 22, 2017 at 15:07
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    $\begingroup$ There's a reason the shuttle used it for supplemental cooling, and the ISS uses it now! $\endgroup$ Commented Aug 22, 2017 at 15:58
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    $\begingroup$ It's also notable that the NH3 Fuel Association is a 21st century organization and will host the 14th annual NH3 Fuel Conference (Practical, carbon-free fuel) at the 2017 meeting of American Institute of Chemical Engineers. $\endgroup$
    – uhoh
    Commented Aug 23, 2017 at 2:41

Chiming in as a long-time member of the NH3 Fuel Association.

First, a small correction. "Cryogenic" refers to gases that liquefy below -150 degrees C. As you state, ammonia liquefies at -33 degrees ... which is fairly close to ambient conditions in the industry. "Refrigerated" would be a more accurate term.

Second, you ask why ammonia was used as the rocket fuel for the X-15. I'm no engine expert, and others have addressed this here with more knowledge than I can, but I understand that the "coking" issue was vital: hydrocarbon fuels formed soot that affected performance, but ammonia contains no carbon and thus formed no soot.

Third, you ask if ammonia was used before or after the X-15. Yes and yes.

In the past ... Belgium, municipal buses in 1940s; Norway, demonstration truck in 1930s; Louisiana, public trolley car in 1870s (ammonia steam engine, working fluid not fuel). See https://nh3fuelassociation.org/introduction/.

Now underway: dozens of projects for carbon-free power and fuel, from island energy projects to national grid-scale import/export projects. See my website if you'd like to search for more information on projects around the world.

  • $\begingroup$ This is an interesting summary of NH3 use. "Supplementary answers" are sometimes OK in stackexchange and I think this is one of them, but in general, SE answers need to address at least some part of the question directly. For example, use before/after was meant to apply to use as a rocket propellant or other space-related activities, not buses and trolleys. If the X-15 did not carry on-board refrigeration equipment for the ammonia, calling it refrigerated would be problematic as well, but it is a very good point. I'm not sure we can call cooled, densified kerosene cryogenic either. Thanks! $\endgroup$
    – uhoh
    Commented Sep 2, 2017 at 2:15

PS to previous comment about refrigeration in the X-15. It had none, keeping weight absolutely minimal precluded it. Temperature was far more of an issue for the LOX than for the NH3. During captive carry to launch the LOX was replenished from a supply in the B-52 carrier aircraft; otherwise some degree of boil-off occurred. A special case of venting LOX was sending it through the XLR-99 to precool the engine just before launch.

On Bob White’s FAI world altitude flight he asked for and received permission to reorder a few of the final items in the prelaunch checklist, in order to gain a few extra seconds of LOX top off from the B-52’s LOX tank, which was unique to the two B-52s used to carry the X-15. White also had an over performing XLR99 on that mission, and the Dryden rocket shop advised him of that in advance.

As a docent at the Aerospace Museum of California I often refer indirectly to the resulting performance. With an 82 second rocket burn, White set an FAI altitude record at 314,750 feet, about 30,000 feet above the planned apogee. That was within the X-15’s overall history of altitude overshoots and undershoots, the max that comes to mind was about 40,000 feet.

Another metric is that when the XLR-99 lit the pilot immediately has 2G of forward acceleration. That grew to 4G at burnout — it gave rise to a famous quote by X-15 pilot Milt Thompsin: “The X-15 is the only aircraft I ever flew where I was glad when the engine quit.”

Some other metrics from that record flight of White’s was that it covered about 315 miles horizontally, 110 miles vertically, and he touched down 10 minutes 20.7 seconds after launch.


As mentioned in one of the previous answers and according to this source the directive to use anhydrous ammonia came from Dr. Paul F. Winternitz, a propellant scientist from Austria and chief of R&D department of Reaction Motors Inc. (RMI). According to his statement It was chosen because of a practical reasons:

’’I have worked with quite a few propellant systems and found NH3/LOX to be among the easier ones to work with. We did encounter a few challenges along the way, but none were related to the fuel itself’’'

Previous works of rocket engineer named Robertson Youngquist on engine regenerative cooling with anhydrous ammonia laid the foundation on it’s use in RMI. Following advantages of anhydrous ammonia were attractive to propulsion engineers:

  • High hydrogen content, 17.65%, paired with decent liquid density of 0.682kg/l at BP (pre-chilling further increases density) gives 70% higher hydrogen content than LH2 itself,
  • Good performance: sea level Isp 293s, vacuum specific impulse 343s. These values are still attractive,
  • Low average molar mass of gasses (around 19.8 g/mol) for fuel rich mixtures due to dissociation of ammonia to hydrogen and nitrogen at higher temperatures,
  • Combustion temperature is around 2800-2850°C, lower than in other combinations under the same conditions due to high content of water in exhaust and dissociation of ammonia,
  • Zero-emission combustion, water and nitrogen are only combustion products. No COx, SOx, NOx and chlorine compounds. Ammonia is not a greenhouse gas.
  • Medium liquefying temperature of -33.3 °C, low freezing point of -77.73 °C and high critical temperature of 132.4°C meaning that it’s liquid in wide range of temperatures,
  • High latent heat of vaporization 1.37 MJ/kg and high heat capacity 4.7KJ/kg*K (higher than water) meaning that its excellent medium for regenerative cooling,
  • It’s self-pressurized gas, vapor pressure 10bar at 25°C,
  • It’s not corrosive, explosive and highly flammable.

Yet ammonia is not without drawbacks:

  • It is toxic when inhaled and must be handled with respect. It’s highly toxic for marine life. Still it’s less dangerous than hydrazine and its derivatives. Also there was a long experience in manufacturing, transporting and storage with excellent safety record,
  • Ignition is troublesome, hard starts and combustion instabilities are often issues. RMI engineers managed to overcome those issues in LR-99 with clever injector design. However reliable re-ignition of the engine would be a challenge for deep space missions despite attractive vacuum specific impulse,
  • Low overall O&F density, lower than Kerolox or NTO/Hydrazine, but still better than Hydrolox.

In rocketry anhydrous ammonia was intended for use in:

  • Douglas D-558-3 american manned rocketplane flown in 1954, a counterpart of X-15 intended for Navy. It employed another RMI engine XLR-30-RM-2. Abounded in favor of X-15, enter image description here

  • Mini shuttle was conceived in 1972 as maned rocketoplane which would use of the self components like LR-99 engine but was never authorized.

  • In 2012 Russian rocket engine manufacturer Energomash announced works on new rocket fueled by mixture of acetylene and ammonia. New atsetam engine should be based ond RD-161 and rocket was planned for launch in 2017-2018. But seems like it faced funding cuts.

  • 2
    $\begingroup$ "WOW" quite a thorough and interesting answer, thank you very much! $\endgroup$
    – uhoh
    Commented Nov 8, 2020 at 21:42
  • $\begingroup$ Technically, ammonia is a greenhouse gas, but it's not an important one due to being short-lived in the atmosphere: earthscience.stackexchange.com/questions/18354/… $\endgroup$
    – Pitto
    Commented Nov 9, 2020 at 3:06

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