Rocket Science

[Crazy Eddie]

Been playing around with the Tsilkovsky Rocket Equation in Microsoft Excel, tinkering with numbers and figures. I think I might have either discovered something interesting or completely screwed up somewhere...

So I have a hypothetical spacecraft with a mass of about 3 tons (including 100kg worth of astronaut). It has propellant tanks with a capacity of about 4000 liters.

I may or may not be getting the fuel mixtures right, but in trying to figure out which type of fuel would be most efficient, I tried to run the equations using normal propellants and figuring their relative densities compared to ISPs for a typical engine.

And I came up with:
LOX/LH2: 2.8 km/s (ISP = 450)
LOX/RP1: 2.79km/s (ISP=370)
Hydrazine: 2.386kms (ISP=290)

And it hit me, looking at an article about slush hydrogen and metallic hydrogen that the only real advantage either system has is the fact that hydrogen has the highest ISP but also the lowest density. So just for curiosity, I asked "What if you could produce a hypergolic or monopropellant fuel with an extremely high density?"

What I found was this, If you increase the density of Hydrazine by just 30% (1.35kg/L instead of 1.02) then the same engine now has a delta-v of 2.8km/s. If you increase the density further, to 1.5kg/L, the hydrazine engine, even with its pathetic 290 seconds, actually outperforms a cryogenic engine.

This seems to suggest, at least to me, that what we really need in the future of rocketry isn't some new breakthrough with nuclear propulsion, a new McGuffin (helium-3 or life on Mars) or even HLVs to "inspire" people. Maybe all we've really been missing is the right propellant? In the same way that the automotive industry never would have taken off if cars had to be powered by coal-fired steam engines, commercial space exploration may never completely take off (pun slightly intended) with LH2/LOX. SpaceX has already gone to LOX and Kerosine, and that leads me to think the real revolution will come from the abandonment of liquid oxygen altogether.

That seems to be the conclusion, anyway. Either a higher density monopropellant that's at least competitive with hydrazine in terms of performance, OR a higher density oxidizer that's denser and easier to work with than liquid oxygen. In the latter case, just replacing liquid oxygen with something as dense as hydrazine would result in a spacecraft that outperforms the cryogentic fuels by a good 200m/s.

tl;dr: it looks like we don't actually need better or more advanced engines. We need denser propellants that can be stored in a smaller volume.

 

[Santaman]

I doubt there is any fuel or fuel mixture that hasn't been tested or researched into, maybe you can also factor in engine nozzle efficiency since the usual bell nozzle isn't an optimal shape, might be that a better engine IS what you are looking for afteral.
https://en.wikipedia.org/wiki/Aerospike_engine
Or maybe a staged combustion cycle engine might also help you?
https://en.wikipedia.org/wiki/Staged_combustion_cycle_(rocket)

I'm a pirate, not a rocket scientist but here are my two dubloons

 

[Asbo Zaprudder]

Been playing around with the Tsilkovsky Rocket Equation in Microsoft Excel, tinkering with numbers and figures. I think I might have either discovered something interesting or completely screwed up somewhere...

Tsiolkovsky

tl;dr: it looks like we don't actually need better or more advanced engines. We need denser propellants that can be stored in a smaller volume.

Assuming no shedding of dry mass, the equation can be written as:

Delta-V = exhaust velocity * ln(1 + propellant mass/dry mass)

Obviously, denser propellants would reduce the required dry mass (due to smaller tanks) and increase the Delta-V as does increasing the exhaust velocity. Hypergolic propellants such as hydrazine might fit the bill but they are usually extremely toxic and environmentally unfriendly. However, the natural logarithm damps the effect of reducing the dry mass somewhat. Increasing the exhaust velocity would seem more effective.

 

[YellowSubmarine]

Or decreasing the dry mass by using lighter materials.

 

[Crazy Eddie]

^ Yeah, that's conventional wisdom in a nutshell. But the more I tinker with the numbers, the more it seems like it isn't the dry mass so much as it is the actual MASS FRACTION. Hidden in that conventional wisdom are built-in assumptions about the relationship between fuel, tankage and weight, but those assumptions aren't etched in stone. If you can increase the fuel density without changing anything (or too much) about the tank design, you can actually increase the mass fraction considerably. And because it's an issue of volume vs. surface area (at least for the tanks) then alot of the time the need for stronger takes can be offset by the fuel itself; even if a 30% increase in fuel density results in a 30% increase in tank mass, the mass of the tanks is so much smaller than the mass of the fuel that the final mass fraction increases considerably (from, say, 80 to 90%).

The use of cryogenics like liquid hydrogen have the effect, IMO, of distorting the importance of tank weight because the mass of the tanks is relatively high compared to the mass of the fuel it carries. But moving away from cryogenics makes both the weight and the sophistication of the tanks less important. Even as toxic as they are, a high-density hypergolic fuel could easily outperform cryogenics, especially if you found a way to get its exhaust velocity up to to 3km/s. The only thing standing in the way of THAT, IMO, is the tendency to make engines that use hypergolic propellants as light and as simple as possible which means the complicated turbopumps and pressure feeds that might boost their performance are almost always excluded from consideration.

SpaceX, I think, has basically proven this with the Falcon-9, with the Kerostine/LOX combination that already gives that booster pretty amazing performance without the need for complicated strap-on SRBs. I'm thinking that some alternative fuel, something denser than RP-1, would actually increase the mass fraction of boosters and orbital craft considerably.

 

[Nebusj]

If you can increase the fuel density without changing anything (or too much) about the tank design, you can actually increase the mass fraction considerably.

Might I inquire whether you have any thoughts about some way to increase the density of hydrazine by thirty percent without changing anything else about the tank design?

I'm also curious whether there are any experienced rocket engineers with good things to say about monopropellants, considering.

 

[Crazy Eddie]

Might I inquire whether you have any thoughts about some way to increase the density of hydrazine by thirty percent without changing anything else about the tank design?

Thoughts? Certainly. NOFBX is supposed to be exactly this but has stability problems. I was thinking about some dissolved salts or metals added to UDMH or Nitrogen tetroxide in such a way that they would only react to the fuel mixture at high temperatures. Adding it to UDMH would make more sense; something like aluminum or carbon nanoparticles might react pretty violently in a high temperature reaction in the presence of a very strong oxidizer. That would increase your fuel density and MIGHT offset the loss of specific impulse due to the additional reaction mass.

I'm also curious whether there are any experienced rocket engineers with good things to say about monopropellants, considering.

Monopropellant gets a well deserved bad rap.

 

[publiusr]

I would also think that hypergolics would also allow for spaceplanes that don't look like beached whales.