Are hydrogen upper stages worth it or not?

[RanulfC]

Cryo-propane still isn't a consideration I suppose?

Randy

[spacenut]

When they went to the moon, it was determined that kerolox was the best first stage and hydrolox was the best for upper stages.  They also had no monitary restraints, just the need to get to the moon first. 

Today it seems that BO and SpaceX are going metholox for both stages.  Cost is the biggest factor today.  Metholox seems to be the best to solve the capability, cost, and re-usability.  Solids are not good for upper stages because they can't be shut down and restarted and weight is a factor.  Hydrogen is expensive and has a supercold storage and boiloff problem especially if loitering.  Liquid methane is about the same temperature as liquid oxygen which is needed for any upper stage being considered. 

[Pipcard]

When they went to the moon, it was determined that kerolox was the best first stage and hydrolox was the best for upper stages.  They also had no monitary restraints, just the need to get to the moon first. 

Today it seems that BO and SpaceX are going metholox for both stages.  Cost is the biggest factor today.  Metholox seems to be the best to solve the capability, cost, and re-usability.  Solids are not good for upper stages because they can't be shut down and restarted and weight is a factor.  Hydrogen is expensive and has a supercold storage and boiloff problem especially if loitering.  Liquid methane is about the same temperature as liquid oxygen which is needed for any upper stage being considered. 

You seem to have forgotten that BO is going for a hydrolox third stage, which is what has been boggling my mind.

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.

But what about guckyfan's claim that a hydrogen upper stage would be prohibitively "complex and expensive" for integration, and that even an expendable methalox upper stage would be more cost efficient for BLEO applications? (e.g. Owlon's proposed "direct-injection/fast trajectories for planetary science missions")

[RanulfC]

When they went to the moon, it was determined that kerolox was the best first stage and hydrolox was the best for upper stages.  They also had no monitary restraints, just the need to get to the moon first. 

Today it seems that BO and SpaceX are going metholox for both stages.  Cost is the biggest factor today.  Metholox seems to be the best to solve the capability, cost, and re-usability.  Solids are not good for upper stages because they can't be shut down and restarted and weight is a factor.  Hydrogen is expensive and has a supercold storage and boiloff problem especially if loitering.  Liquid methane is about the same temperature as liquid oxygen which is needed for any upper stage being considered.

Just to be historically correct, (and a bit nit-picky-ish ) kerolox as a booster was a compromise as it was already 'developed' and in use and hydrolox was in development as the "best" possible chemical propellant as long as you didn't seriously consider 'additive' chemicals or elements. (Which is to say they in fact DID consider just about everything from fluorine to boron additives to aluminum and everything short of the kitchen sink* but only pursued such in specific cases. *=Yes there were studies of suspended ceramic materials a couple of times )

In terms of both cost and performance 'at-the-time' most work agreed that the "optimum" solution was large solid booster stages with all hydrolox upper stages as per the Air Force SLS and later Saturn-II studies. However it can be noted that there were a LOT of 'assumptions' and built-in bias' that are questionable today that were not in fact questioned then.

Big solid boosters were supposed to be quite cheap to build and operate by the metrics of the day. So much so that significant money and effort was put into developing a way to throttle, shut-down and re-start them which was successfully tested but never employed because it, (as you might imagine) significantly increased the cost and complexity of the basic solid motor. In addition formulations were found that greatly decreased the burn instability and "shudder" that most SRBs had but again it costs a lot less to NOT do that if you can get away with it.

Similarly the one thing everyone pretty much agreed on was once we had hydrolox propulsion we could pretty much 'do-anything' from easy SSTO to hydrogen powered airplanes and cars with filling stations on every corner. In truth, eh, not so much. But, again in the 'thinking-at-the-time' anything LESS than hydrolox was not really worth considering seriously as hydrolox was, (obviously) THE best combination despite any engineering or operational concerns. After all fixing those concerns/issues/problems was 'only' engineering and operations work, and of course money, but we had that readily available didn't we?

It's taken decades to break away from most of those "basic" assumptions and 'common-knowledge' to the point where alternatives are becoming more common place. Methalox is currently leading the way due to a combination of users, higher awareness that there ARE alternatives and the need to find more efficient, economic, and effective ways of accessing space. As an 'on-going' process it will continue, er, going on and growing and changing as time goes on but I would not expect it to be a smooth or easy process and fully expect that todays assumptions and 'common-knowledge' will change over time, again.

Randy

[guckyfan]

But what about guckyfan's claim that a hydrogen upper stage would be prohibitively "complex and expensive" for integration, and that even an expendable methalox upper stage would be more cost efficient for BLEO applications?

I was thinking in the context of SpaceX launch operations. Horizontal integration and erection of the stack with the TEL. Can you even do that with a Centaur or ACES?

Also integrating LH into the TEL. They might have to vertically integrate that stage together with the payload, if that is possible. A huge headache and something SpaceX would not do and would cost a lot in their structure. If you design a rocket and a pad all dedicated to that LH upper stage the situation may be different. But then again only for use in cislunar space including earth departure burns to outside cislunar. A different propulsion system would be needed on arrival at the destination.

[Pipcard]

But what about guckyfan's claim that a hydrogen upper stage would be prohibitively "complex and expensive" for integration, and that even an expendable methalox upper stage would be more cost efficient for BLEO applications?

I was thinking in the context of SpaceX launch operations. Horizontal integration and erection of the stack with the TEL. Can you even do that with a Centaur or ACES?

Also integrating LH into the TEL. They might have to vertically integrate that stage together with the payload, if that is possible. A huge headache and something SpaceX would not do and would cost a lot in their structure. If you design a rocket and a pad all dedicated to that LH upper stage the situation may be different. But then again only for use in cislunar space including earth departure burns to outside cislunar. A different propulsion system would be needed on arrival at the destination.

So what you're saying is that it would be okay for the ITS booster (BFR) to have a hydrolox upper stage (super-ACES in the place of the ITS spaceship) that was supplied by a third party that wanted to develop industry on the Moon*, now that the plan has been revealed to involve vertical integration (via giant crane) and Pad 39A (which has already handled launch vehicles with hydrolox)?

*I know LCROSS apparently found carbon on the Moon, but then one poster (jongoff) claims that it's disputed based on statements by Paul Spudis. (who is an ITS skeptic)

[jongoff]

But what about guckyfan's claim that a hydrogen upper stage would be prohibitively "complex and expensive" for integration, and that even an expendable methalox upper stage would be more cost efficient for BLEO applications?

I was thinking in the context of SpaceX launch operations. Horizontal integration and erection of the stack with the TEL. Can you even do that with a Centaur or ACES?

I don't see why not. Didn't at least some of the old Atlas missile silos have the rockets stored horizontally and then tipped up for firing? I really don't see anything about Centaur or ACES that would preclude horizontal processing if you really felt it was necessary.

Also integrating LH into the TEL. They might have to vertically integrate that stage together with the payload, if that is possible. A huge headache and something SpaceX would not do and would cost a lot in their structure. If you design a rocket and a pad all dedicated to that LH upper stage the situation may be different. But then again only for use in cislunar space including earth departure burns to outside cislunar. A different propulsion system would be needed on arrival at the destination.

Why would integrating LH2 for an upper stage into a TEL be that hard? Sure, it's less dense and colder than LOX, but these are solvable problems. The Centaur LH2 umbilical hardware I got to examine at SLC-3 back in Vandenburg didn't really look that hard to deal with compared to their LOX umbilicals.

I also don't get why an LH2 upper stage is only useful in cislunar space for earth departure burns.

~Jon

[Nathan2go]

I think the hydrogen fueled rocket stage question depends on your goals; these issues are probably more important than the launch pad issues:

- If you want to refuel on Mars, then methane is the easiest fuel to make.  This works well with the SpaceX vision for Mars.  Methane is also easier to store in LEO than LH2 (LEO is much warmer than in higher orbits due to proximity to warm mother Earth).

- If you want to refuel on the Lunar poles or at Lagrangia with Lunar-made fuel, then hydrogen is the easiest fuel to make.  This works well with the Blue Origin vision for lots of people living and working in near-Earth space.   One could also argue that for trips to Lagrangia or the Moon, that stopping to refuel in LEO would add too much mission complexity and travel time, thus adding a third stage is preferred (LEO refueling of Mars ships is great as a LEO fuel depot allow utilization of the launch facility and ground crew outside of the Mars departure window).

- If you want the US government to pay you to develop an engine using the highest performing fuel, or if you want to tell your customers that your rocket builds on decades of rocket heritage, then an RL-10 derived hydrogen engine is the right answer (thus the ULA choice).  Also, hydrogen is more suitable to expendable upper stages which are paired with expendable first stages, since in that case, the upper stage is a smaller part of the whole, thus cost increases for the small upper stage have less impact to the total cost (paring an expendable upper stage with a reusable first stage makes the upper stage a bigger percentage of the total flight cost).

I don't think there is much reason to try propane as a launch fuel.  As I understand it, the performance would be about the same as methane, except that the heavier-than-air fumes could collect on the ground around the rocket, creating a fire/explosion hazard; plus it's a more expensive fuel.

On the other hand, propane could be interesting as a separate fuel for long storage periods and landing, since it's storage temp (Tboil =231 K, Tmelt=86K) is more compatible with people, without freezing in the presence of LOX (Tboil=90 K).  I suspect that a Martian plant for making methane could easily co-produce propane as 5-10% of the produced fuel, by simply tweaking the catalyst.

[Robotbeat]

For the record, the hydrogen third stage for New Glenn strikes me as a good idea. Most launches won't need it, but it should /double/the performance to high energy orbit. At least double.

[Pipcard]

Most launches won't need it

Doesn't that mean that it doesn't get amortized enough (low flight rates)?

[Oli]

Methane is also easier to store in LEO than LH2 (LEO is much warmer than in higher orbits due to proximity to warm mother Earth).

If you use subcooled methalox you almost certainly need cryocoolers on both depot and departure stages though.

[TrevorMonty]

Most launches won't need it

Doesn't that mean that it doesn't get amortized enough (low flight rates)?

I'd surprised if 3rd stage doesn't sharing a lot of NS components, maybe even tanks.

[Pipcard]

Most launches won't need it

Doesn't that mean that it doesn't get amortized enough (low flight rates)?

I'd surprised if 3rd stage doesn't sharing a lot of NS components, maybe even tanks.

The New Glenn third stage is apparently depicted as having the same diameter as the rest of the rocket (7 m). But yes, the BE-3 is going to be used for both.

[hkultala]

Most launches won't need it

Doesn't that mean that it doesn't get amortized enough (low flight rates)?

I'd surprised if 3rd stage doesn't sharing a lot of NS components, maybe even tanks.

The New Glenn third stage is apparently depicted as having the same diameter as the rest of the rocket (7 m). But yes, the BE-3 is going to be used for both.

both what?


Same diameter for methalox 2nd stage and hydrolox third stage makes a lot of sense as methalox is much more dense than hydrolox, but upper stages should always be much lighter, so the tank sizes do not differ very much.

Just like Saturn V had same diameter in it's first(kerosine) and second(hydralox) stages.

[Pipcard]

I meant both the New Glenn upper stage and New Shepard, as mentioned by TrevorMonty; sorry for the confusing language.

[Robotbeat]

Most launches won't need it

Doesn't that mean that it doesn't get amortized enough (low flight rates)?

Not necessarily. Think of the hydrogen upper stage as their "heavy." And the hydrogen tech will be very well proven from suborbital flights.

Most Atlas V flights don't use solids, but the tech is still well amortized. (Though pad requirements are easier with solids, of course.)

[Jim]

Just like Saturn V had same diameter in it's first(kerosine) and second(hydralox) stages.

Not a relevant comparison.  The relative size of the LOX tank to the fuel tank was the opposite in both stages.

[notsorandom]

For the record, the hydrogen third stage for New Glenn strikes me as a good idea. Most launches won't need it, but it should /double/the performance to high energy orbit. At least double.

I know you are aware that most launches these days are to high energy orbits. So I'm curious to hear thoughts on why you think most launches will not need the 3rd stage. Do you think that Blue will be doing a different mix of mission types or do you think that the 2 stage NG will still have the performance to do those missions economically?

[Nathan2go]

If you use subcooled methalox you almost certainly need cryocoolers on both depot and departure stages though.

For a departure stage which lingers in LEO for only a few hours, certainly no coolers would be needed for reasonably low boil-off.  And boil-off can be made to happen at any desired temperature between Tfreeze and Tcritical, by adjusting the tank pressure: lower pressure means lower temperature boiling (on Earth, tank pressure can't go below 1 bar to avoid tank collapse, which is why subcooling creates temperature gradients in the rocket, but this is not an issue in vacuum conditions).  Just remember to bring the tanks to flight pressure a few seconds before starting the engines.

For storage of many weeks at a LEO depot, yes maybe cro-coolers are needed.  But holding 95k for methane and 60k for LOX is 3x easier than 20k for LH2.  Also, the Earth departure burn requires only 60% as much delta-V as the trans-Earth injection from Mars surface, and 78% of the delta-V for the second stage to reach orbit; so deep subcooling is not as critical.  So maybe the depot holds the propellants at more like 95/80K (4x easier than LH2).

[Nathan2go]

For the record, the hydrogen third stage for New Glenn strikes me as a good idea. Most launches won't need it, but it should /double/the performance to high energy orbit. At least double.

What?  Were you thinking of hydrolox vs solids (like the cancelled Shuttle-Centaur vs IUS)?  Methalox and hydrolox are much closer in performance.

The rocket equation for mass ratio is exponential in Delta-V/Isp, so it grows faster as the Delta-V gets bigger.  The Isp of hydolox should be 460 s, versus 380 s for methalox, or 21% better.  For low mass ratio/low delta-V burns (eg. LEO to GTO, or LEO to Vescape), all you get is about a 21% benefit. 

For two stages, the delta-V (and mass ratio) of stage 2 must be higher, thus, you'd get a bigger benefit for hydrolox: maybe 40% to Vescape.  But with three stages, each stage contributes less performance, thus there's less benefit to upgrading.