Are hydrogen upper stages worth it or not?

[Robotbeat]

For first stage, methane may still be preferable unless you go LOx-rich or use a tripropellant design. For the first stage, it's not so much energy but mass that's needed to throw out the nozzle, and methane would be cheaper per ton than hydrogen even if both made renewably.

[TrevorMonty]

For first stage, methane may still be preferable unless you go LOx-rich or use a tripropellant design. For the first stage, it's not so much energy but mass that's needed to throw out the nozzle, and methane would be cheaper per ton than hydrogen even if both made renewably.

Thread title is about US only, best keep booster fuel types out of the discusses.

[Oli]

For the first stage, it's not so much energy but mass that's needed to throw out the nozzle, and methane would be cheaper per ton than hydrogen even if both made renewably.

If we assume the methanation process is for free, and if we assume mixture ratios of 5 and 2.77 for hydrolox respectively methalox, methalox would be 80% of the cost of hydrolox per kg.

So it might just barely make sense to use hydrolox for a first stage if we look at fuel costs alone.

But this is a purely hypothetical scenario of course. Methane will be available as a fossil fuel for a long time to come and fuel costs are unlikely going to dominate launch costs for an equally long time to come.

[Pipcard]

For first stage, methane may still be preferable unless you go LOx-rich or use a tripropellant design. For the first stage, it's not so much energy but mass that's needed to throw out the nozzle, and methane would be cheaper per ton than hydrogen even if both made renewably.

Thread title is about US only, best keep booster fuel types out of the discusses.

The question is whether or not having commonality is more important than upper stage performance. However, hydrogen is not a good common fuel because of low density and low thrust in the first stage. So which is more economical, having a hydrocarbon first stage + LH2 second stage, or the same hydrocarbon (kerosene or methane) on both stages? And if the latter is more economical, why are Blue Origin and ULA wasting their money on hydrogen upper stages? All that we know so far is that a Falcon 9 is cheaper per kg than an Atlas V, along with anecdotal statements on this forum about hydrogen infrastructure and systems being more expensive, like this one:

LH drives up the cost. All the plumbing and valves have to be LH compatible, the valves and plumbing for Kero are cheaper.

[Robotbeat]

You can't just ignore first stages when deciding whether hydrogen is preferable for the upper stage. If you optimize the two stages completely separately, you will end up with basically two entirely different rockets, which doubles development costs and some operational ones as well. At high enough flight rates, this may not matter as much, but generally it does.

[Katana]

For first stage, methane may still be preferable unless you go LOx-rich or use a tripropellant design. For the first stage, it's not so much energy but mass that's needed to throw out the nozzle, and methane would be cheaper per ton than hydrogen even if both made renewably.

Thread title is about US only, best keep booster fuel types out of the discusses.

The question is whether or not having commonality is more important than upper stage performance. However, hydrogen is not a good common fuel because of low density and low thrust in the first stage. So which is more economical, having a hydrocarbon first stage + LH2 second stage, or the same hydrocarbon (kerosene or methane) on both stages? And if the latter is more economical, why are Blue Origin and ULA wasting their money on hydrogen upper stages? All that we know so far is that a Falcon 9 is cheaper per kg than an Atlas V, along with anecdotal statements on this forum about hydrogen infrastructure and systems being more expensive, like this one:

LH drives up the cost. All the plumbing and valves have to be LH compatible, the valves and plumbing for Kero are cheaper.

Centaur upperstage could go directly to GEO, while Falcon9 could only reach GTO.

[Robotbeat]

For first stage, methane may still be preferable unless you go LOx-rich or use a tripropellant design. For the first stage, it's not so much energy but mass that's needed to throw out the nozzle, and methane would be cheaper per ton than hydrogen even if both made renewably.

Thread title is about US only, best keep booster fuel types out of the discusses.

The question is whether or not having commonality is more important than upper stage performance. However, hydrogen is not a good common fuel because of low density and low thrust in the first stage. So which is more economical, having a hydrocarbon first stage + LH2 second stage, or the same hydrocarbon (kerosene or methane) on both stages? And if the latter is more economical, why are Blue Origin and ULA wasting their money on hydrogen upper stages? All that we know so far is that a Falcon 9 is cheaper per kg than an Atlas V, along with anecdotal statements on this forum about hydrogen infrastructure and systems being more expensive, like this one:

LH drives up the cost. All the plumbing and valves have to be LH compatible, the valves and plumbing for Kero are cheaper.

Centaur upperstage could go directly to GEO, while Falcon9 could only reach GTO.

1) I don't think it's true that Falcon 9 doesn't have the delta-v performance to do direct GEO. However, it doesn't have the lifetime (right now, that we know of). Falcon 9 FT is pretty dang high performance and can hit very high delta-v IF you have a very small payload...

2) Yeah, the advantage of hydrolox is best seen at very high delta-v. For LEO, it almost seems like a waste of time to try hydrogen, but for escape velocity or similar high delta-v (like your example of direct to GSO), hydrogen pulls ahead.

3) But you can get hydrogen-like performance by using more boosters or stages. This is what the Russians do. Just use a ton of stages. But if you're constrained to 2 stages, need to focus on high energy orbits, and have an unlimited budget, hydrogen looks like a very good option. So it fits DOD/three-letter-agency requirements very well.

[Steven Pietrobon]

If we assume the methanation process is for free, and if we assume mixture ratios of 5 and 2.77 for hydrolox respectively methalox, methalox would be 80% of the cost of hydrolox per kg.

Methalox oxidiser to fuel mixture ratio is more like 3.5 to 1.

[Oli]

If we assume the methanation process is for free, and if we assume mixture ratios of 5 and 2.77 for hydrolox respectively methalox, methalox would be 80% of the cost of hydrolox per kg.

Methalox oxidiser to fuel mixture ratio is more like 3.5 to 1.

I got my numbers from http://www.braeunig.us/space/propel.htm.

I know it depends on the pressure, but I'm merely interested in the mixture ratios relative to each other.

[Steven Pietrobon]

This page is more reliable.

http://web.archive.org/web/20090203154304/http://dunnspace.com/alternate_ssto_propellants.htm

[Oli]

This page is more reliable.

http://web.archive.org/web/20090203154304/http://dunnspace.com/alternate_ssto_propellants.htm

It does not say at what combustion chamber pressure, which I find a bit suspect.

Anyway, with those numbers methalox would be ~75% of the cost of hydrolox per kg, under my simplifying assumptions of free methanation and no cost difference in storage.

[Proponent]

Dunn states that he assumes "optimal" mixture ratios, except in the case of lox-hydrogen, where he assumes 6, to increase bulk density.  By "optimal," I presume he means vacuum maximum specific impulse.  But maximum specific impulse does not in general correspond to maximum performance, especially in the case of a ground-launched stage.

Attached is a plot of the specific impulse of lox-methane as a function of mixture ratio for chamber and exit pressures respectively of 1000 psi and 1 atm (the conditions assumed above in the source suggesting 2.77 as a mixture ratio).  This was generated with RPA Lite 1.2.8 with shifting equilibrium and default efficiencies.

Specific impulse peaks at ratio of about 3.2 on the ground and about 3.4 in vacuo.  You might well want to operate at higher ratios, to improve bulk density at the expense of a bit of specific impulse, but I can't see why you'd want to operate at a lower ratio.  And for an upper stage, where mass is particularly important, you're likely to want to stay pretty close to maximum specific impulse, i.e., about 3.5.

[edkyle99]

1) I don't think it's true that Falcon 9 doesn't have the delta-v performance to do direct GEO. However, it doesn't have the lifetime (right now, that we know of). Falcon 9 FT is pretty dang high performance and can hit very high delta-v IF you have a very small payload...

Correct that Falcon 9 appears on paper able to lift significant mass directly to GEO, but first the Falcon 9 second stage will have to demonstrate, or be given, a reliable second restart (third burn) capability after a 5.5 or so hour coast.  I think that this capability will be required for some EELV missions that SpaceX likely wants to win.

 - Ed Kyle

[Pipcard]

https://forum.nasaspaceflight.com/index.php?topic=35754.msg1294057#msg1294057

ULA has a lot of experience with handling liquid hydrogen and the infrastructure is already in place so for them it's difficulty may not be as big an issue as it would be for a company trying it for the first time.

Legacy infrastructure means legacy costs.

Why don't they just abandon it, then?

(I know, I've asked this before on the Vulcan thread. But "legacy costs" were not emphasized enough in that discussion.)

[Robotbeat]

Because:
1) It still works for their bread and butter high energy orbits.
2) Their primary customer (govt) doesn't like change.
3) ULA doesn't have such freedom.
4) And hydrogen really isn't a bad solution for an upper stage at all. Once you have everything else, it IS pretty high performance.

[Pipcard]

Because:
1) It still works for their bread and butter high energy orbits.
2) Their primary customer (govt) doesn't like change.
3) ULA doesn't have such freedom.
4) And hydrogen really isn't a bad solution for an upper stage at all. Once you have everything else, it IS pretty high performance.

I can understand reasons 2 and 3, but aren't they still falling for the same old "optimize for performance" trap?

[Robotbeat]

Optimizing for performance isn't always a bad idea. I mean, think about it this way:

You've built a launch pad, a nice first stage and maybe some boosters. Optimizing the upper stage will help get a big payback by making all those bits more productive.

Rocketry is exponential, so compensating for a non-optimum design by brute force can end up costing a lot more than careful attention to performance.

The Russians take a different approach and just use a bunch of stages. But that's not necessarily cheaper and can have negative reliability consequences.

I think that picking a single propellant combo makes more sense if you're vertically integrated than if you're not.

[Pipcard]

Optimizing for performance isn't always a bad idea. I mean, think about it this way:

You've built a launch pad, a nice first stage and maybe some boosters. Optimizing the upper stage will help get a big payback by making all those bits more productive.

Rocketry is exponential, so compensating for a non-optimum design by brute force can end up costing a lot more than careful attention to performance.

I thought brute force was what amortized the fixed costs of launch infrastructure and made them more productive, and what might make the MCT (now ITS) architecture more economical - by "brute forcing" the launching of propellant and disregarding IMLEO optimization.

You're not going to save money by developing 5 different rocket stages and a couple different EDL schemes at large scale over one BFS which can refuel multiple times.

Things look very different when IMLEO is no longer the primary constraint!

The Russians take a different approach and just use a bunch of stages. But that's not necessarily cheaper and can have negative reliability consequences.

And yet Blue Origin is wanting to do both, a hydrolox third stage. And despite what you've said, Russia is also developing a hydrolox third stage (KVTK), for Angara A5.

I think that picking a single propellant combo makes more sense if you're vertically integrated than if you're not.

Okay, then.

[Robotbeat]

ITS is optimized to all heck.

[TrevorMonty]

Blue covered both bases, common fuel and engine for LEO 2nd stage and light high performance hydrogen 3rd stage for BLEO missions.

The 3rd stage benefits directly from NS development. For NS a LNG engine would have been better choice as fuel costs are lot lower along with associated components. Blue made a strategic decision to develop a LH engine for BLEO applications, with NS being its first use.