Are hydrogen upper stages worth it or not?

[jongoff]

Just to add to Jon's point. ULA distributed launch paper gives a 30 day wait in LEO (180km) of 30t drop tank while it waits for LV it is refuelling. The wait assumes a certain amount of boil off (0.9t LH) and doesn't use sunscreens. Move that same tanker to L2 with sunscreen and months for same boil off rate become possible.

A quickly deployable small LV like XS1, used as topup tanker (1-2t LH) could  extend stay of main drop tanks to months for an additional cost. Good insurance to have just in case main payload LV is delayed.

It's an outside chance that Masten will get the XS-1 Phase II nod, but XS-1 and Vulcan/ACES would make a really good team...

~Jon

[jongoff]

But the joker in the pack remains no demonstration of LH2 (or AFAIK any cryogen) transfer on orbit. 

I'm not too worried about LH2 transfer on orbit. It needs to be flight demonstrated, but I don't think it's that high of a risk. If we get our SBIR Phase II, we'll hopefully have hardware that could be flight demo'd by the end of the 2yrs.

~Jon

[john smith 19]

I'm not too worried about LH2 transfer on orbit. It needs to be flight demonstrated, but I don't think it's that high of a risk. If we get our SBIR Phase II, we'll hopefully have hardware that could be flight demo'd by the end of the 2yrs.

Is that SBIR to actually do such a transfer?

This would be excellent news and long overdue.

[Vultur]

I think people ought to look at propane again... from what I've read, the Isp is only a few seconds worse than methane and the density is much better. It might be the ideal SSTO fuel.

IMO if you could make a propane engine with good TWR, that combined with SpaceX's mass-ratio performance, PICA-X TPS, and landing technology would make a VTVL SSTO very practical.

[Katana]

I think people ought to look at propane again... from what I've read, the Isp is only a few seconds worse than methane and the density is much better. It might be the ideal SSTO fuel.

IMO if you could make a propane engine with good TWR, that combined with SpaceX's mass-ratio performance, PICA-X TPS, and landing technology would make a VTVL SSTO very practical.

Propane has isp only a few seconds above kerosene but density much lower, near to methane.

Kerosene SSTO maybe even more close.

[Jim]

IMO if you could make a propane engine with good TWR, that combined with SpaceX's mass-ratio performance, PICA-X TPS, and landing technology would make a VTVL SSTO very practical.

It isn't even possible must less practical.

[jongoff]

I'm not too worried about LH2 transfer on orbit. It needs to be flight demonstrated, but I don't think it's that high of a risk. If we get our SBIR Phase II, we'll hopefully have hardware that could be flight demo'd by the end of the 2yrs.

Is that SBIR to actually do such a transfer?

This would be excellent news and long overdue.

The SBIR Ph II proposal is to develop and qualify a small-scale coupler (initially focused on LOX but traceable to LH2). There's only so much you can realistically jam into a $750k research contract. That said, if we win the Phase II, we'll definitely be talking with as many groups as we can to see if we can push this through LH2 ground testing, and then eventually flight demonstration.

Further conversation should probably move to the Altius thread.

~Jon

[jongoff]

I think people ought to look at propane again... from what I've read, the Isp is only a few seconds worse than methane and the density is much better. It might be the ideal SSTO fuel.

IMO if you could make a propane engine with good TWR, that combined with SpaceX's mass-ratio performance, PICA-X TPS, and landing technology would make a VTVL SSTO very practical.

Propane has isp only a few seconds above kerosene but density much lower, near to methane.

Kerosene SSTO maybe even more close.

Subcooled propane has a nearly identical bulk density to LOX/Kero. But I'm not sure this has much to do with whether LH2 upper stages are worth it or not...

~Jon

[Robotbeat]

IMO if you could make a propane engine with good TWR, that combined with SpaceX's mass-ratio performance, PICA-X TPS, and landing technology would make a VTVL SSTO very practical.

It isn't even possible...

(gets really close to the microphone) Wrong.

[john smith 19]

I think people ought to look at propane again... from what I've read, the Isp is only a few seconds worse than methane and the density is much better. It might be the ideal SSTO fuel.

There's an old post from Steven Pietrobon on this. In truth at either NBP or a bit sub cooled all short chain hydrocarbons come out about the same with the same engine conditions except Proypyne (Methylacetylene to use its non systemic name). The strained triple bond was good for 5-10 extra seconds IIRC.

But it's horse for course. For absolute maximum Isp LH2 is  the way to go. Once other factors come into play  (IE you're not on a cost plus government contract) the choice becomes more complicated.

[Robotbeat]

For absolute maximum Isp on a chemical rocket: fluorine, lithium, hydrogen tripropellant.

[jongoff]

For absolute maximum Isp on a chemical rocket: fluorine, lithium, hydrogen tripropellant.

*Shudder*

For the highest non-terrifying Isp, LOX/LH2 with TAN and either a dual-expander or a very low-end staged combustion, so you can get away with a big expansion ratio while still getting good liftoff T/W, and if you go LOX-rich on the TAN, you might even be able to claw back a not-too-crappy bulk density.

If I were still running a propulsion R&D shop, I'd be trying to pitch NASA or AFRL on funding something like this.

~Jon

[Vultur]

I think people ought to look at propane again... from what I've read, the Isp is only a few seconds worse than methane and the density is much better. It might be the ideal SSTO fuel.

IMO if you could make a propane engine with good TWR, that combined with SpaceX's mass-ratio performance, PICA-X TPS, and landing technology would make a VTVL SSTO very practical.

Propane has isp only a few seconds above kerosene but density much lower, near to methane.

Kerosene SSTO maybe even more close.

Yeah, that might be better given that there's far more experience with it... OTOH for an SSTO those few seconds matter.

IMO if you could make a propane engine with good TWR, that combined with SpaceX's mass-ratio performance, PICA-X TPS, and landing technology would make a VTVL SSTO very practical.

It isn't even possible must less practical.

Impractical I can see, but impossible seems really unlikely.

 The Titan II first stage could have made it to orbit by itself, IIRC, and the Mercury-Atlas only dropped engines - the Merlin 1D's TWR and Isp are more than good enough to send a rocket with Mercury-Atlas mass fraction (or significantly worse - Merlin 1D's current TWR is really high).

Building an expendable SSTO would be completely straightforward with today's technology. For reusability, SpaceX has already demonstrated vertical landing, so the big remaining question is TPS.

(Well, and speed/cost of reuse, but that's a "practicality" question not a "possibility" one.)

[Pipcard]

Okay, it seems that this thread has strayed too far from what it was supposed to be about - upper stages, not SSTOs - so can we please return to that? Thank you.

[Vultur]

Yeah, I'll start a new thread in Advanced Concepts.

[Vultur]

For absolute maximum Isp on a chemical rocket: fluorine, lithium, hydrogen tripropellant.

Aw, can't we get ozone in there somehow?

Li/F/H2 is definitely the highest demonstrated chemical Isp (542 seconds, IIRC). Is it the highest theoretical? I've seen claims for Be/O2/H2 holding that title - mostly just online rumors, but this actual NASA paper does seem to support it:

 "Current Evaluation of the Tripropellant Concept" by Robert L. Zurawski https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19860018652.pdf
"The Be/H2/O2 tripropellant offers the highest specific impulse of any chemical propellant combination".
It also says the Isp is 69.2 sec better than H2/O2.  They're modeling 1000 psi chamber pressure though -- I wonder if the advantage would be as significant at, say, an SSME's much higher pressure?

[Robotbeat]

Definitely not the highest theoretical. There is a list of increasingly insane chemical compounds that one could use. The problem is two fold: one, is that if you try to put more energy into the propellant, you'll melt the engine. That's why extra hydrogen is used, as it keeps the speed of the exhaust high while keeping temperatures lower. Theoretically, at very high expansion ratios (such that all the chemical energy gets converted to kinetic energy eventually), the highest Isp is actually with a stoichiometric mix as it has the highest specific energy.

The second is that these higher energy propellants will kill you if you even THINK about them too long.

[WBailey]

Free radical propellants could deliver thousands of seconds specific impulse, if they could be kept from recombining instantly upon formation.
Edit: Also metallic hydrogen, which may or may not have the same problem (we'll see soon)

https://www.history.nasa.gov/conghand/propelnt.htm
https://ntrs.nasa.gov/?N=0&Ntk=All&Ntt="Free radical" propellant
edit: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19910012841.pdf

[Robotbeat]

Atomic hydrogen. Will kill you if you think about it too long, even theoretically.

[Chris Bergin]

Reacting to report to mod. Don't have time to read the thread, but trust the reporter, so....back on topic from this point onwards.  This thread isn't about SSTOs or the craziest propellant combos. It's about whether LOX/LH2 makes any sense relative to LOX/CH4.

Thanks.