Monopropellant Logic

[fenderzilla]

The most commonly used Monopropellant is Hydrazine. The chemical formula for Hydrazine is H4N2. Ammonia's chemical formula is NH3, so ammonia and hydrazine have the same chemical ingredients, albeit different amounts of each. That means that with enough of one, you could make some of the other (with some space elements left over). where do you find ammonia? well, urine. that means it's feasable to power a rocket from kerbal pee.

[K^2]

Hydrazine is, basically, synthesized from amonia. So you're on the right track there. Typically, you'd have a few steps involving chlorine and some intermediate products, but the net reaction is amonia + oxygen -> hydrazine + water.

Now, I don't know how practical it'd be to try and extract amonia from urine for this, but there are many asteroids and comets out there with amonia ice. You could probably mine these for amonia you need to turn into monoprop in deep space.

[Camacha]

Taking the water to form oxygen and hydrogen is probably an easier way of making (rocket) fuel out of urine. Considering it is about 95% water, your volumes will be up a bit too. The down side is you would be venting water into space, which you might want to retain aboard your ship.

[Nuke]

Hydrazine is, basically, synthesized from amonia. So you're on the right track there. Typically, you'd have a few steps involving chlorine and some intermediate products, but the net reaction is amonia + oxygen -> hydrazine + water.

Now, I don't know how practical it'd be to try and extract amonia from urine for this, but there are many asteroids and comets out there with amonia ice. You could probably mine these for amonia you need to turn into monoprop in deep space.

if hydrazine is burned in an arcjet engine you get 1600 seconds of isp. so i like the idea of mining comets. they could become refueling stations for spacecraft. you could probibly even break off very large chunks of ammonia ice and move them into a more useful orbital trajectory.

[K^2]

The only thing I see with 1,600s for arcjet is H2 powered. Hydrazine will give you much better TWR, but at a cost of lower I_SP. The highest specific impulse I could find for Hydrazine is 600s. That's still way, way better than just using it as a monoprop, which would give you up to about 240s.

[Nuke]

well i was going by the poorly sited articles on wikipedia instead of reading actual academic papers like i should have. thats still slightly better than lox/lh2 though, and hydrazine is a hell of a lot easier to store.

Edited February 16, 2014 by Nuke

[Camacha]

thats still slightly better than lox/lh2 though, and hydrazine is a hell of a lot easier to store.

Not too sure about that. LH2 boils off, that is a downside for sure. Hydrazine, on the other hand, is quite dangerous and toxic even without it being reacted as a fuel.

I wonder whether you could get away with compressed hydrogen. None of the downsides that are traditionally associated with it when using it to get to orbit, only more space needed. In the case of monopropellant that might not be a real problem due to the smaller quantities, and in space a larger vessel is not a problem. Only weight really counts.

[K^2]

I wonder whether you could get away with compressed hydrogen. None of the downsides that are traditionally associated with it when using it to get to orbit, only more space needed. In the case of monopropellant that might not be a real problem due to the smaller quantities, and in space a larger vessel is not a problem. Only weight really counts.

Exactly. The weight is the problem. Weight of the tanks you used to keep H2 compressed. And pressure doesn't even matter. Say you have a spherical tank, which is best geometry for weight efficiency. If you double the size of the tank, you can store the same amount of H2 gas at 1/8th of the pressure. But you've also increased cross-section area by factor of 4, while increasing cross-section circumference by factor of 2. So net decrease in thickness is factor of 4. But you've also increased surface area by the same factor, four. So the total mass of the tank remains exactly the same!

Regardless of pressure, given amount of H2 gas at fixed temperature requires a certain mass of the tank. (This works for any gas, by the way.) And it's a lot. If your tank is made of steel, for every 1kg of H2 at 300K, you'll need 1.8kg of steel in the tank!

So the only practical way to store H2 in a rocket of any kind, where weight matters, is to store it as LH2.

[Camacha]

That is the traditional approach. NASA is of course looking quite seriously into using inflatable habitats. I can imagine that similar technology could be employed to haul a volume of gas around, although I am not getting the numbers to work favorably on that one in some quick calculations.

[K^2]

Again. It doesn't matter. You can take H2 at high pressure and small volume, and then you are hauling lots of tank because the walls are so thick. Or can take H2 at low pressure and huge volume, and then you carry exactly the same weight of tank because the walls have sugh a huge surface area. Either way, you need 1.8kg of tank per 1kg of H2 if you make the tank out of steel. You can do better with composites. But you will still have hard time beating 1:1 ratio. Compared to cryo H2, where in large volume, your tank weighs less than a tenth of your fuel. Now, do you want to bring 2kg of stuff for every 1kg of H2 you make use of, or 1.1kg of stuff for the same 1kg of H2? The choice is pretty simple. You bring cryo H2 at a fraction of a weight cost.

[Camacha]

Again. It doesn't matter.

Yeah, that is what the numbers were telling me. Looking at it from a somewhat simplistic point of view, I guess that means it comes down to mission duration. With a boil off rate of about 1% a day, you will lose in the neighborhood 50% of your LH2 in roughly 70 days. After that, it will take about 70 more days to get down to 25% of your original amount. The longer the mission, the more interesting compressed gas becomes. Taking your numbers as a guideline, it is about two thirds of a year in where compressed hydrogen starts becoming the more economical option.

Of course, that is a bit of an oversimplified scenario which assumes compressed hydrogen is not lost at all during the same period of time, that the boiled off hydrogen can not be recovered or replenished, that you do not burn anything and that you are unwilling to consider other fuel type options

[Guest]

I imagine that it could be a good solution to launch with a LH2 tank, then pump the boiling off hydrogen into a large baloon. In space, you can have a nonrigid tank without too many problems.

[K^2]

I imagine that it could be a good solution to launch with a LH2 tank, then pump the boiling off hydrogen into a large baloon. In space, you can have a nonrigid tank without too many problems.

I had that same thought for a few seconds. The problem is that the non-rigid tank would still have to be at least thick enough to contain H2 at some pressure, and it can't have a better tensile strength than a good composite tank.

If you want to look at it another way, a non-rigid gas-filled balloon is at least as heavy as a rigid balloon of the same size. And it has the same exact mass before it is inflated. So in either scenario, you need to bring enough mass with you to contain gaseous H2. And that's way more extra mass than it'd take in cryo.

Regarding the boil-off, for a long duration mission, it will probably make more sense to bring a refrigeration and condensation unit with you, making your cryo tanks closed circuit. We already do that at many facilities that make use of large quantities of liquid helium. (E.g. J-Lab.) Electricity is much cheaper in space than propellant is, and with large enough tanks, the extra weight of the condensation unit is going to be very small. (Boil-off rate scales as area, while total mass scales as volume. So with 2x larger tanks, condensation unit is going to be 2x smaller fraction of your weight.)

[Camacha]

If you want to look at it another way, a non-rigid gas-filled balloon is at least as heavy as a rigid balloon of the same size. And it has the same exact mass before it is inflated. So in either scenario, you need to bring enough mass with you to contain gaseous H2.

That was pretty much what I was thinking earlier. I also considered a hybrid solution, where you capture the boil off for later use in a relatively small tank. Closed circuit seems more interesting though, even though electricity is less available and cheap when going further out. Also, I guess recapturing the boil off is more interesting when talking about main propulsion, and less when using it as monopropellant because of the smaller tanks.

Edited February 17, 2014 by Camacha

[K^2]

Of course. But for monoprop, if we are looking at an arcjet, I'd definitely go with hydrazine. (You can't use H2 at all as conventional monoprop.) While you lose quite a bit of I_SP, what you gain is a lot of extra TWR and power efficiency. Now, TWR probably isn't a huge deal in a lot of cases, but power efficiency is going to be important even on longer missions.

In my mind, H2 only makes sense with LOX as a biprop, or as propellant for an NTR.

[RuBisCO]

Hydrazine is just such a pain to handle, imagine nerve gas as a rocket fuel, oh wait we don't need to image that its hydrazine! Hydrogen peroxide may have pathetic specific impulse, but we are talking about attitude control here not main propulsion, and hydrogen peroxide is much easier to handle and easier to produce in situ then hydrazine which needs a source of nitrogen, not just water.

[Nuke]

apparently you can use straight ammonia in an arcjet, and you get about 800s of isp @ 2n.

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

and this engine has been flown in space.

[Rakaydos]

What if you store the hydrogen at such low pressure, it actually has less gas weight than the oxy/nitro air it displaces? Hydrogen is a lifting gas, after all... let the fuel lift itself to the edge of the atmosphere, then burn some of it to put the rest into orbit.

[K^2]

apparently you can use straight ammonia in an arcjet, and you get about 800s of isp @ 2n.

You can, but your energy efficiency is going to be even worse than with hydrogen. ÃŽâ€H of ammonia is -80kJ/mol. So at 800s, you'll be losing about 600W per 1N of thrust just on ammonia decomposition. H2 is going to stay as H2, so ÃŽâ€H is zero. Hydrazine, on the other hand, gives you ÃŽâ€H of +50kJ/mol. So at 600s, which you get with a hydrazine arcjet, you'll be saving about 270W per 1N of thrust.

Since an arcjet can be up to about 50% energy-efficient to begin with, we're looking at about 5.6kW/N with hydrazine, and about 8.4kW/N with ammonia. In most practical applications, this increase in power requirement is going to negate any benefits of higher I_SP.

Finally, one more advantage of hydrazine is having a backup. An arcjet with no power can be built to function as monoprop on hydrazine. That won't work on hydrogen or ammonia.