Author Topic: Revisiting the X-15 Spaceplane LR99 NH3/LOX Propulsion  (Read 1237 times)

Offline sdsds

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An interest in propellants easily cryo-cooled at an on-orbit depot led to review of the LR99 NH3/LOX engine used for the later flights of the X-15 spaceplane. There's an excellent summary of this propellant combination by Paul Raveling on stackexchange, an excerpt from which is quoted below.

Vac Isp of 343s, and that might be with (only) a gas-generator cycle and a chamber pressure of 40.82 bar. (Compared to the Raptor FFSC engine chamber pressure of maybe 300 bar.)

Note also combustion products don't include any carbon. (Although Energomash thought they could mix the ammonia with acetylene and do even better; admittedly that would be carbon producing.)

So the historical question here is, "What happened to NH3/LOX in the United States?" Did Reaction Motors / Thiokol just abandon it for no reason, or am I missing something here?
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 2800-2850C, 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 132.4C meaning that its 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,
    Its self-pressurized gas, vapor pressure 10bar at 25C,
    Its not corrosive, explosive and highly flammable.

Yet ammonia is not without drawbacks:

    It is toxic when inhaled and must be handled with respect. Its highly toxic for marine life. Still its 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.

Offline Emmettvonbrown

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Re: Revisiting the X-15 Spaceplane LR99 NH3/LOX Propulsion
« Reply #1 on: 12/18/2023 03:56 pm »
Random thoughts
- Where did he got 343 seconds ?  XLR-99 was 276 seconds. Which was plenty enough for X-15A-2 to hit 2 km/s
- Ammonia on X-15 was somewhat an accident. All the other contemporary engines ( around 1955-56: Thor, Atlas, Titan) went for RP-1 as more practical (not a cryogen)
- RP-1 simply did better. RD-0124 has an isp of 359 seconds. Block D RD-58 is close enough.
- Until SpaceX (maybe) rocket carbon emissions were extremely marginal as space transportation was marginal, compared to airlines or military aircraft
- Ammonia is second best - for EP arcjet thrusters. Except hydrogen is twice as good, isp- wise
- Overall ammonia fell victim of RP-1 (same energy, more practical) and hydrazine (half storable propellants & hypergolic, and not a cryogen either). It was squeezed between them.

- Note that I'm great fan of ammonia fuel - for cars and trucks. Far easier to handle than hydrogen, and unlike gasoline and methanol - no carbon emissions. Although Nox may be an issue to be taken very seriously.

- Ammonia and methanol are (IMHO) the only two fuels that could replace gasoline in IC cars engines with very little mods. Hydrogen is a storage nightmare. Wish I could convert my car to run on NH3, but  E85 and Teslas seems to have won the day.

- Interestingly enough, methanol and ammonia have the same issue: only half the energy of gasoline. That's the real problem, and even more for aircraft.

- That's why we don't have ammonia Airbus or Boeing: range would take a colossal penalty. Unless some hydrogen could be cracked, in flight, from ammonia: to get a fuel with a bit more energy in the turbofans.  To me this concept - aparently explored by REL, of Skylon fame - is the only viable solution for air transportation with no carbon emissions. Not enough SAF, and hydrogen - shudders.

- Back in 1963 the US Army had one smart plan to get ride of oil and gasoline. Use portable nuclear reactors to split nitrogen (from air) and hydrogen (from water) and recombine that into a) hydrogen b) ammonia and c) methanol. That was called the Mobile Energy Depot but proved unpractical with 1966 tech levels. A pity, imagine in 1973 when the first oil shock stroke: the Army (and nuclear ammonia) could have saved the day.

- they aparently flew a DHC Caribou transport and a UH-1 Huey helicopter on ammonia, but performance was dismal.
« Last Edit: 04/15/2024 05:31 am by Emmettvonbrown »

Random thoughts
- Where did he got 343 seconds ?  XLR-99 was 276 seconds. Which was plenty enough for X-15A-2 to hit 2 km/s

The XLR-99 was also designed in the mid 1950s, so it's performances isn't really useful as a reference point in the modern day.

343s is a theoretical isp number. So someone at some point  (like circa 1958) did some math which may or may not have been right, and he's repeating the result.

That being said, we have points of comparison which support that result. Hydrazine's chemical formula is N2H4, and Methane's is CH4. Both of those have demonstrated specific impulses in the 360s. So given that, it would be rather strange if Ammonia, NH3, couldn't even manage 300 seconds.

- Ammonia on X-15 was somewhat an accident. All the other contemporary engines ( around 1955-56: Thor, Atlas, Titan) went for RP-1 as more practical (not a cryogen)

It was not an accident. Unlike all those other programs you mentioned (plus the Jupiter missile), the X-15 needed to be reusable.

Kerosene had the "coking" issue, where (basically) soot produced when burning it would gunk up an engine's high speed turbine and fuel channels while it was operating. That, as it turns out, was bad. It's the reason that the Redstone missile used ethanol.
By the mid-50s, the advent of RP-1 may had solved the coking issue enough for missiles to use Kerosene. But it didn't make the coking issue disappear entirely. And the engineers running the X-15 program, reasoning that having to stop and clean carbon out of the engine every few flights would have made the operational phase of the X-15 program a lot more painful, chose Ammonia instead.

Now obviously we now have reusable Falcon 9 flying, which makes that concern seem silly. And as early as late 60s, NASA seemed pretty comfortable with the idea of reusing the H-1 or F-1, so perhaps the coking concerns on X-15 were overblown to begin with. But it wasn't an accident; there was a logic to it.

One final note; technically, liquid anhydrous ammonia isn't a cryogenic liquid. In almost every way operationally, using liquid ammonia is like using a cryogen. But the actual definition of cryogenic is to have a boiling point bellow -153 degrees C, and liquid ammonia's is above that. Basically all semantics for our purposes, but worth noting I think.

EDIT: After writing this post, I decided to google a bit, and found a NACA report on the performance of Ammonia/LOX. Guess what year? Yep, 1958! It's nice to get something spot on every once in a while.

According to this (fair warning, I've only barely skimmed it), at stoichiometic mixture ratios, 344s is possible. And actually, combustion chamber temperature at that ratio doesn't seem too insane. Which is nice, because it means building an engine with that mixture ratio is probably feasible, but the low temperature is also probably the main reason the isp is so much worse than Hydrazine or Methane.

I've attached the document, if anyone wants to read it themselves.
« Last Edit: 04/10/2024 08:33 pm by JEF_300 »
Wait, ∆V? This site will accept the ∆ symbol? How many times have I written out the word "delta" for no reason?


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