New Fuel

JoshNH4H · 2008-01-30 15:49:21

New (new [new]|new| ) Fuel

Ammonia and oxygen? kind of like methane, but obviously not. Would there be a similar isp?

JoshNH4H · 2008-02-07 15:09:07

Another idea. Ammonia contains 2/11 H. It would decompose exothermically in the presence of a catylist to by the reaction:

2NH3=>N2+3H2

Then you burn the hydrogen with oxygen in a regular J-2X/SSME This is the kind of thing that would be useful on mars, or anywhere where boiloff is a problem. The funny thing is, for liquid ammonia, there is a greater hydrogen density than liquid hydrogen.

Terraformer · 2008-02-08 15:42:54

SSTO applications, then?

But I suppose it would be heavier, so no.

GCNRevenger · 2008-02-09 09:26:43

SSTO applications, then?
But I suppose it would be heavier, so no.

Yeah: the most efficient fuels generate the maximum heat energy for the minimum mass of exhaust, and in an Ammonia + Oxygen burner like this, the relatively low energy NH3 -> N2 + H2 decomposition would not create enough heat to make up for the increased mass of adding Nitrogen's "dead weight" to the H2 + O2 -> H2O exhaust products. In other words, you have only a little more energy for a big increase in exhaust mass, which doesn't make up for the improved density of the Ammonia.

JoshNH4H · 2008-02-09 10:02:16

I forgot about that

I just had a thought- what would the Isp of a jet engine be if it had to carry its own o2?

Terraformer · 2008-02-09 10:10:49

The same as a Rocket burning Jet Fuel.

GCNRevenger · 2008-02-09 12:14:39

The engine is merely the machine that converts stored chemical energy into the kinetic energy of the exhaust products. It can't pull extra energy out of the fuels if you are already squeezing 100% of that energy out. The best chemical rockets already operate at ~90% of the theoretical maximum (SSME, Vulcain II).

Terraformer · 2008-02-10 05:05:14

If a Jet was carrying its own O2 the fans would be ditched to save extra weight. They wouldn't do anything to help and would have to be shielded against the extra heat. So you'd end up with a Rocket engine.

JoshNH4H · 2008-02-10 08:54:33

I assume you mean a rocket engine?

I ask because jet engines are safer and more mass produced than rocket engines. Also, the fuel is cheaper and more compact. I think an Isp of 400 would be a decent tradeoff.

JoshNH4H · 2008-03-02 20:24:29

I worked it out. Using Hydrazine as a Hydrogen source could give Isp's of up to 490, easily 460. I assumed
1) The hydrazine was used as a monopropellant first, with an Isp of 225, then burned in an oxygen afterburner
2) Once burned the hydrazine doesn't lose its kinetic energy.

GCNRevenger · 2008-03-02 21:45:59

Nice thought, but I am afraid the Nitrogen present will interfere with the Hydrogen/Oxygen combustion, as well as increase the net mass of exhaust more than it increases the net energy released, reducing Isp. In other words, the mass of the Nitrogen you have to lug offsets the energy released by the Hydrazine decomposition.

Furthermore, the Hydrazine breakdown requires a catalyst for efficient decomposition, which is fine for small thrusters but is not practical to process tens or even hundreds of kilograms per second needed for a good sized rocket engine.

GCNRevenger · 2008-03-02 22:19:21

Or to put it another-another way:

What really determines Isp is how much energy you get from your fuel per mass of exhaust products.

In the case of the Hydrazine/Hydrogen+Oxygen hybrid mix, you are adding one Nitrogen molecule (N2) for every two Hydrogen molecules (H2), for a total increase of 77% mass over Hydrogen/Oxygen only, but the Hydrazine contributes much less than 77% more energy for both propellants. Hydrazine is energetic stuff, but not anywhere near as much as Hydrogen/Oxygen.

Further-furthermore, some of the Nitrogen is likely to react with the Oxygen, creating unwanted Nitrogen Oxides, further increasing exhaust molecular mass (lowering Isp) and creating a clogging/corrosion hazard in the engine's workings.

The Hydrogen really doesn't weight that much. The problem is the Oxygen more than anything, but there aren't any good alternatives that don't involve flesh eating chemicals.

GCNRevenger · 2008-03-03 14:13:40

Or another-another-another way of putting it:

What you have is the average of the Isp of the Hydrazine and Hydrogen/Oxygen propellants, except the latter is a little higher since the Hydrogen is "free."

JoshNH4H · 2008-03-03 15:17:27

This is the reaction:

N2H4 => N2 + 2H2

N2 + 2H2 + O2 => N2 + 2H2O

Total Mass of H2O: 36 (2 18's)
Total Mass of N2: 28

The Isp of just the H2O is the hydrazine's Isp (225) + the H2/LOX Isp (425), or 650 total.

The Isp of the Nitrogen is the Isp of Hydrazine (225).

The total Isp is an average weighted by total mass. The ratio between them is 28:36, or 7:9. So the average Isp is about 464, burning the Hydrogen at RS-68 like Isp.

Austin Stanley · 2008-03-03 16:39:52

I'll just point out that while the decomposition of Hydrazine in theory just gives you H2 and N2, in practice that turns out not to be entirely the case. The reaction happens VERY fast and (depending upon the conditions) you may end up with a significant percentage of Ammonia (NH4) in your exhaust as well.

What GCRN says is right as well. A similar concept applies to the reason you don't add LOX to a NTR rocket burning LH2, as this similarly only decreases the ISP of your engine. Though it might be useful in situations where you were already burning that fuel and need some additional thrust (assuming it could be added cheaply and easily).

JoshNH4H · 2008-03-04 15:41:45

Oops, mis calculated- real world isp 350's, max ~375.

JoshNH4H · 2008-03-04 15:57:23

There's this- It's better than Kerosene. So maybe it could be kind of a lower stage thing.

Secondly, I have a very simple rocket engine design that will work for any two bipropellants, preferably non-hypergolic, because that defeats the purpose of an engine.

So you start with a SMALL hydrazine flame, caused by hydrazine contact with a catylist. This doesn't generate the thrust. Then, the two bipropellants are brought together within the N2H4 flame. They ignite, and go into the combustion chamber, which is either traditional (de laval nozzle) or aerospike engine.

It's kind of a tripropellant. I use hydrazine beecause it is decent Isp for a monopropellant, dense, and non-cryogneic.

GCNRevenger · 2008-03-05 08:06:42

Perhaps better than Kerosene on paper, but the trouble of a three-propellant engine and fuel system likely outweighs the benefit. Particularly with the low density of Hydrogen.

This second idea is basically using the Hydrazine as an ignition source, right?

JoshNH4H · 2008-03-05 08:26:42

yes, so it's only technically a tripropellant. 99%+ of the thrust will be from the bipropellants. I just figured that this could be a simple, universal, efficient engine. Also, I guess H2O2 or N2O would work, but to use it as a constant source you would need less of it.

Terraformer · 2008-03-07 13:02:01

but there aren't any good alternatives that don't involve flesh eating chemicals.

I suppose you are talking about the exhaust?

I'll check out H2O2.

GCNRevenger · 2008-03-07 17:48:24

Either exhaust or propellant. Hydrogen peroxide, at high enough concentrations, will consume human flesh quite readily. It also has the nasty habit of blowing up if it touches the wrong kind of metal.

Terraformer · 2008-03-08 04:43:05

Oh, I know about that. But since it's the proppelent, not the exhaust, I kind of discount that. The exhaust is just H2O and O2 (used in a H2 afterburner?) There's plenty of substances that you wouldn't want touching your skin that are used in industry.

About the metal thingy, that's kind of the point. It acts as a catalyst to decompose the Peroxide for the exhaust, and don't you want maximum thrust for minimum weight in a rocket? So would it work?

What's all this about ISP anway? I thought it was thrust, not ISP, that counts when you're trying to get out of a Gravity Well. Example: Shuttle SRBs. Terrible ISP, but excellent thrust.

GCNRevenger · 2008-03-08 16:48:54

Actually no, Isp is the most important factor, with thrust and propellant density coming in second and third generally speaking.

Isp, the aerospace engineers' symbol for Specific Impulse, is basically the efficiency of a fuel by mass. Thrust determines how quickly you reach a certain velocity, but not how much fuel you need to burn to get there.

The more efficient the fuel is, the more payload you can have. Further, this is an exponential relationship, so small improvements in Isp yield large benefits to performance.

Because of Earth's fairly high gravity, it is quite hard to launch anything into orbit at all, so rockets generally need high Isp as well as decent thrust in as compact a package as is practical.

Recall that you have to reach orbital velocity to place anything into space, and the only reason why you need high thrust is to get off the ground and avoid crashing until you've reached this speed.

Terraformer · 2008-03-09 06:12:37

So Isp and thrust are most important when trying to reach orbit? Eg. Ion thrusers can't reach orbit but have10x the Isp of a rocket?

GCNRevenger · 2008-03-09 11:32:34

I would say that Isp is considerably more important than thrust... Isp determines the payload, the amount of fuel needed (roughly), and hence the size of the rocket. Except for the first minute or two of the ascent, the amount of thrust you need is matched to the rocket after Isp determines how big it is.

And yeah, thats why nobody is going to use ion thrusters for launch.

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Re: New Fuel

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