Energomash Develops Revolutionary (?) Ammonia/Acetylene Rocket Engine

[Galactic Penguin SST]

According to a RIA Novosti article (http://en.rian.ru/russia/20120505/173241862.html), the Russian engineers are trying to develop such an engine from the RD-161 (keroLox). I found it very strange to compare the ammonia/acetylene combo (what's the I_sp?) with hydrogen engines, as I doubt that the specific impulse would be high. And why would they try to develop such an engine for launch vehicles?

Thoughts?

GPS

Edit: Some Russian discussions here: http://www.novosti-kosmonavtiki.ru/phpBB2/viewtopic.php?t=12447

[ciscosdad]

Acetylene is theoretically an energetic propellent, but the liquid is unstable (explosive). Perhaps the ammonia mixture (presumably a solution) is stable.
The ammonia may potentially mitigate the coking issues that may come from the acetylene's low hydrogen content. Interesting idea.

[mmeijeri]

Maybe the idea is that the acetylene contributes energy while the ammonia helps Isp because of its lower molecular weight? Liquid hydrogen is obviously too cold to mix with acetylene and liquid ammonia would be the next best thing. Sort of a premixed fuel tripropellant rocket.

[Nathan]

My chemistry is rubbish but I assume that ammonia/acetylene is the fuel in some appropriate mix but there is still an oxygen oxidizer. Is that correct?
Would the exhaust products be safe?

[AlexCam]

My chemistry is rubbish but I assume that ammonia/acetylene is the fuel in some appropriate mix but there is still an oxygen oxidizer. Is that correct?
Would the exhaust products be safe?

Yes, atsetam is a mixture of NH3 and C2H2. It replaces RP-1 (kerosene) as the fuel for the RD-161, where RP-1 can be approximated as C12H24 (of course its a mixture of all kind of hydrocarbons) and will still require LOX (so yes, in some sense this is a tripropellant, because ammonia and acetylene do not combine massively to a new molecule when mixed).

As you can see, depending on the right mixture, atsetam can be carry more hydrogen in its total mass than RP-1. I think the article's 30% efficiency gain compares to the RD-161, a kerosene engine.

The cost gain is massive, but only on the fuel side. Fuel, however, isn't the cost factor for rockets, engines, structures, software, integration etc. are. Atsetam engines would only really help, if there is a new approach to launching rockets (the "little reliability, cheap, throw away rocket" approach, which has been advocated for decades, but never put into reality).

[MP99]

The cost gain is massive, but only on the fuel side. Fuel, however, isn't the cost factor for rockets, engines, structures, software, integration etc. are. Atsetam engines would only really help, if there is a new approach to launching rockets (the "little reliability, cheap, throw away rocket" approach, which has been advocated for decades, but never put into reality).

Are you saying this fuel combo isn't going to be completely safe?

Otherwise, I don't see why that would follow. ISTM an RP1/LOX rocket could convert to atsetam and lift a bigger payload. All else being equal, that should reduce cost per pound.

cheers, Martin

[DarkenedOne]

This fuel seems like a good alternative to RP-1/LOX because it eliminates the cryogenic concerns of LOX.  However I do not understand why they say it can replace hydrogen.  Hydrogen has a significantly higher ISP that is why it is used.  It is also only used on fairly large rockets where the higher insulation required has a lower effect on the tanks.

[mmeijeri]

This fuel seems like a good alternative to RP-1/LOX because it eliminates the cryogenic concerns of LOX.

As noted above acetylene and ammonia are both fuels, you would still need an oxidiser, which would be LOX in the engine under discussion.

[Nathan]

My chemistry is rubbish but I assume that ammonia/acetylene is the fuel in some appropriate mix but there is still an oxygen oxidizer. Is that correct?
Would the exhaust products be safe?

Yes, atsetam is a mixture of NH3 and C2H2. It replaces RP-1 (kerosene) as the fuel for the RD-161, where RP-1 can be approximated as C12H24 (of course its a mixture of all kind of hydrocarbons) and will still require LOX (so yes, in some sense this is a tripropellant, because ammonia and acetylene do not combine massively to a new molecule when mixed).

As you can see, depending on the right mixture, atsetam can be carry more hydrogen in its total mass than RP-1. I think the article's 30% efficiency gain compares to the RD-161, a kerosene engine.

The cost gain is massive, but only on the fuel side. Fuel, however, isn't the cost factor for rockets, engines, structures, software, integration etc. are. Atsetam engines would only really help, if there is a new approach to launching rockets (the "little reliability, cheap, throw away rocket" approach, which has been advocated for decades, but never put into reality).

Thank you for a very informative answer.

Anyone got any thoughts on safety and ISP?

[sanman]

http://www.space-travel.com/reports/Russia_Develops_Revolutionary_Ammonia_Rocket_Engine_999.html

So existing engines designed to run on RP-1 wouldn't require much modification? Is this a case of a fuel mixture being designed to work with an engine, instead of the other way around?

[butters]

So this fuel mixture is still a gas at STP, right?

So why not use straight ammonia, which has a greater mass fraction of hydrogen and a lower condensation pressure than acetylene? No coking issues, either. Worked for X-15...

[mmeijeri]

So this fuel mixture is still a gas at STP, right?

They'd likely cool it below ~-33C to keep the ammonia liquid.

So why not use straight ammonia, which has a greater mass fraction of hydrogen and a lower condensation pressure than acetylene? No coking issues, either. Worked for X-15...

I imagine the idea is to have higher Isp than ammonia (and kerosene).

[douglas100]

Yes, that's probably the idea. But the X-15's engine had quite a low Isp, around 276. Of course it didn't have a high expansion nozzle that an upper stage engine would have had, and had a low chamber pressure--about 40 bar.

Another point of comparison might be the RD-119 LOX/hydrazine engine for the original Cosmos launcher upper stage. With a chamber pressure of 78 bar and a high expansion nozzle its Isp was 352. It was first used successfully in 1962. With a better hydrogen/nitrogen ratio in ammonia than in hydrazine, I imagine the ammonia/acetylene combination would do better than that.

[suncity]

a bit OT... but I've been asking myself what kind of performance would have been possible with the X.15 by keeping the airframe "as is" and just replacing the fuel / oxidezer /engine to optimize ISP.
The top speed reached was 7200 km/hr (with external tanks if IIRC). Of course heating would have been a problem... probably they could have used the extra performance to reach an higher apogee.   

[Proponent]

Anyone got any thoughts on safety and ISP?

Couldn't somebody who has ProPEP, etc., installed run it on a few ammonia-acetylene combinations?

The USAF Research Lab paper attached gives a specific impulse of 327 s for oxygen-acetylene (see PDF page 11), though it doesn't specify the conditions.  The second paper (which looks like a good course on chemical propulsion, by the way), gives about 260 s for oxygen-ammonia at low pressure (see the bar chart on PDF page 162).  I recall higher numbers, like 270-280 s for the oxygen-ammonia system used on the X-15, consistent with what douglas100 mentioned.  The densities of acetylene (aka ethyne) and ammonia are about 620 and 826 kg/m³, respectively, against 820 kg/m³ or so for kerosene.  So, a 50-50 mixture of acetylene and ammonia would probably have a slightly higher specific impulse than kerosene, but also a somewhat lower density.  Whether that mixture would be stable or not, I don't know.

On the whole my impression is that it might be an improvement on kerosene for some purposes, but it doesn't look to me like this is a wonderfuel.

[mboeller]

I wonder a little bit what's so exotic about Ammonia. The combination of Ammonia and acetylene should be something new but at least russia has already developed 2 rocket engines with ammonia-fuel:

http://www.astronautix.com/engines/rd302.htm

http://www.astronautix.com/engines/rd303.htm

[ciscosdad]

I don't think ammonia is especially exotic. It has some unpleasant handling issues so is not popular. If I recall correctly, it can be hard to ignite too.
Acetylene is the exotic prop here. I don't recall it being used before, although its chemical relatives (methyl acetylene or propyne) have attracted a lot of interest. I'd be interested to know why ammonia was chosen as a diluent, but I would guess its high hydrogen content would be the key. I wonder if methane is a possibility? There have been proposals to use acetylene as an additive to other hydrocarbons.

[mmeijeri]

I'd be interested to know why ammonia was chosen as a diluent, but I would guess its high hydrogen content would be the key.

That would be the obvious reason, the same reason that ammonia is a realistic possibility for an NTR. But apparently there are stability-related reasons too, because acetylene alone is too unstable.

[kook59]

    The concentration according to the Russian forum link appears to be 30% by weight acetylene in ammonia. This appears to give about 10s greater isp than RP-1.

[sanman]

This NH3-C2H2 mixture may be cheaper than LH2, but what cost advantage would it provide over RP-1? They seem to be just comparing it to LH2 only to portray it in a favorable light.

It's not light enough to be useful as an upper-stage fuel, so there's no point in comparing it to LH2.

Since RP-1 would be cheaper, then atsetam's only advantage over it is in Isp, which seems quite marginal.