Are hydrogen upper stages worth it or not?

[Lars-J]

While avoiding that hydrogen did allow Spacex to get flying sooner it also forced some design decisions such as densified propellants that later came to haunt them when they decided to upgrade their vehicle to EELV class payloads.

This. For all the talk of how LOX/LH2 is so much harder and more expensive to deal with, ULA had had only a few minor issues with Centaur (knock on wood), and SpaceX has had a ton of challenges with densified propellants, failures due to having to bury their pressurant bottles inside the tanks, etc. Sometimes you end up with more complexity in your attempt to avoid other complexity.

~Jon

Let's not pretend that ULA developed Centaur from scratch here... Because that would be very much inaccurate, since the first Centaur variant flew over 50 years ago. If Centaur is available, that is one thing. But if you are working on a clean sheet design, that changes things.

[Pipcard]

For stages that use aerobraking to reenter and land then Methane is superior as stage will be smaller and therefore lighter than Hydrogen. Large light weight stainless tanks are great in vacuum but don't work to well when they have to be integrated into a airframe with heat sheilding.

For lunar missions hydrogen gives better performance plus long term there is option of ISRU fuel. Mars missions long term have option of either ISRU fuel but aerobraking favours Methane.

The question is whether it is better to spend money on developing and having an extra production line for an optimized lunar lander, or use the same methalox Mars vehicle for your lunar missions, even if only the LOX is available for ISRU (but it would be great if the carbon seemingly found by LCROSS was verified). I guess it would depend on the scale of the operations.

I'm pretty confident the answer is an optimized lunar lander. The amount of benefit you get by being able to do full ISRU refueling instead of having to still carry your own fuel is huge. LOX ISRU helps, but full ISRU shines especially with reusable vehicles. And the benefit of full ISRU is added to the top of the benefit from using LH2 vs Methane in the first place.

Both ULA and Blue Origin will have LOX/LH2 upper staged vehicles flying, so developing a kit to enable lunar landings for one of those stages shouldn't break the bank. It might be harder for SpaceX to compete for lunar missions, but that's why it's good to have an industry with multiple providers taking multiple approaches.

~Jon

I like where this thread is going. So this is the real dilemma for the future of space development if it gets to the point of colonization and/or large scale industrialization: is it worth it to have methalox ISRU only, hydrolox ISRU only (if zero boil-off systems are practical), or both?

[Robotbeat]

With hundreds of thousands of people living in space, there's plenty of room for both. Doesn't mean that one might not become dominant, but if they still trade as closely as they do today, there's enough room in such a market for both solutions.

[Pipcard]

With hundreds of thousands of people living in space, there's plenty of room for both. Doesn't mean that one might not become dominant, but if they still trade as closely as they do today, there's enough room in such a market for both solutions.

What about the near-term future? (including the decade following an ITS Mars landing, assuming that the program is successful)

[Robotbeat]

With hundreds of thousands of people living in space, there's plenty of room for both. Doesn't mean that one might not become dominant, but if they still trade as closely as they do today, there's enough room in such a market for both solutions.

What about the near-term future? (including the decade following an ITS Mars landing, assuming that the program is successful)

Assuming ITS is fully successful, almost nothing even on the drawing board could compete with it. Possibly something Bezos is working on (Blue Origin uses both methane and hydrogen) or, on the smaller end, an evolution of Masten Space Systems' XS-1, which uses methane (and, I believe, an ITS-like launch cradle... It was actually Masten Space that pioneered that idea before SpaceX got interested in it).

[Pipcard]

Assuming ITS is fully successful, almost nothing even on the drawing board could compete with it.

Even for lunar missions, in the case that the LCROSS data showing carbon on the Moon were erroneous?

[Robotbeat]

Assuming ITS is fully successful, almost nothing even on the drawing board could compete with it.

Even for lunar missions, in case the LCROSS data showing carbon on the Moon was erroneous?

Easily. A fueled up ITS can go to the Moon and back with a huge amount of payload. No refueling required.

People underestimate the cost of getting lunar ISRU up and running. The environment is much harsher than Mars and the water much rarer. And it won't be exactly easy on Mars, either. (Also, mining water for export from the Moon will be easily out competed by $9/kg in LEO by at least one variant of ITS.) Even a space elevator couldn't compete.

This is all /assuming/ ITS is fully successful. I /don't/ think that is currently most likely to happen.

[jongoff]

I like where this thread is going. So this is the real dilemma for the future of space development if it gets to the point of colonization and/or large scale industrialization: is it worth it to have methalox ISRU only, hydrolox ISRU only (if zero boil-off systems are practical), or both?

Let's not pretend that ULA developed Centaur from scratch here... Because that would be very much inaccurate, since the first Centaur variant flew over 50 years ago. If Centaur is available, that is one thing. But if you are working on a clean sheet design, that changes things.[/quote]

I'm torn to be honest. For the Moon, my guess is it'll mostly make sense to go with LOX/LH2. I'm just skeptical that there really are enough hydrocarbons relative to water to enable LOX/Methane on the moon. And a full-ISRU system will always beat an oxidizer only ISRU system. Any relative higher cost of LH2 launch infrastructure and propulsion hardware is swamped IMO by this consideration. At least for cislunar space once ISRU is operational.

On Mars... I can see good arguments for either LOX/Methane or for "both". For landing and ascent, LOX/Methane probably wins. But for Earth Return, LOX/LH2 might not be out of the question, if your earth return vehicle uses LOX/LH2. I guess I just don't think that SpaceX will hold a monopoly on in-space transportation, and if you can make LOX/CH4, you can also make smaller quantities of LOX/LH2. By definition.

~Jon

[jongoff]

Easily. A fueled up ITS can go to the Moon and back with a huge amount of payload. No refueling required.

I strongly disagree. Even if ITS really has the magical price/kg people believe, a similarly reusable system that gases up on the moon is always going to be cheaper than hauling propellant up and down the gravity well. The delta-V from LEO to the lunar surface and back (assuming aerobraking at earth) is about 9km/s. With LOX/Methane that's a lot of propellant per unit payload compared to something where you can refuel along the way.

People underestimate the cost of getting lunar ISRU up and running. The environment is much harsher than Mars and the water much rarer. And it won't be exactly easy on Mars, either. (Also, mining water for export from the Moon will be easily out competed by $9/kg in LEO by at least one variant of ITS.) Even a space elevator couldn't compete.

First off, most of the cost of setting up ISRU is driven by the high cost of transportation to and from the Moon. In a world where that's dropping because of RLVs, the cost of setting up ISRU will plummet too. And I don't buy for a second that Elon has an approach that'll get anywhere near $9/kg in LEO. Yes, if you assume magic on your side and no magic on the other side, the magic side is always going to look better.

This is all /assuming/ ITS is fully successful. I /don't/ think that is currently most likely to happen.

And even if it is "successful" does that mean it'll be anywhere near $9/kg in LEO successful?

~Jon

[notsorandom]

Let's not pretend that ULA developed Centaur from scratch here... Because that would be very much inaccurate, since the first Centaur variant flew over 50 years ago. If Centaur is available, that is one thing. But if you are working on a clean sheet design, that changes things.

Historically hydrogen has been a bit harder to tame. The Centaur and other LH2 stages around the world had plenty of early difficulties and it is harder to design an LH2 engine. However Blue Origin went ahead and developed the BE-3 and New Shepard under a commercial, non-government sponsored program. The system so far has a good if not short flight record.

The Centaur may be a legacy from a time when these things were more difficult. Blue has shown that a company can develop and operate a LH2 rocket economically with a stable budget for a fraction of the cost of the Centaur. Its still harder to develop and it might not be a good idea for a company with a more constrained budget but it doesn't seem to be an impossible thing anymore.

[TrevorMonty]

Blue didn't quite start from scratch they hired a lot of people with LH2 knowledge. In a way it works out better for them as clean sheet design can be more efficient than modifying legacy systems.

[Robotbeat]

Let's not pretend that ULA developed Centaur from scratch here... Because that would be very much inaccurate, since the first Centaur variant flew over 50 years ago. If Centaur is available, that is one thing. But if you are working on a clean sheet design, that changes things.

Historically hydrogen has been a bit harder to tame. The Centaur and other LH2 stages around the world had plenty of early difficulties and it is harder to design an LH2 engine. However Blue Origin went ahead and developed the BE-3 and New Shepard under a commercial, non-government sponsored program....

...That is actually an urban legend. BE-3 was actually a paid milestone under CCDeV. They directly got NASA funds for it. Additionally, they got a lot of free access to NASA test facilities and/or personnel as part of an unfunded SAA like the one SpaceX is using for Red Dragon.

Here's one news article on it (it'd be nice if someone found the actual contract showing it's a paid milestone: https://www.flightglobal.com/news/articles/blue-origin-completes-full-power-tests-on-thruster-for-orbital-vehicle-377721/ )

So it most certainly was done with government funds helping. Don't be surprised that Blue Origin/Bezos doesn't advertise this much. It's better PR if it seems like it's totally their own doing.

(This is secondary to your main point that hydrogen doesn't have to necessarily be extremely expensive.)

Just like SpaceX's COTS and CC, this is a good example of efficient use of government funds. The contract was competitively awarded, and the company contributed to development as well. This is EXACTLY what we want: government funds helping a transformative nascent industry get off the ground that otherwise would've taken longer or wouldn't have been able to make progress. And being competitively awarded is key to this.

[Pipcard]

Are zero-boil off systems prohibitively complex for a hydrolox-based architecture?

Also, in the SpaceX ITS presentation, hydrogen was deemed a bad choice for in-space refueling. So why is ULA planning distributed launch with hydrolox refueling for ACES?

[Steven Pietrobon]

Also, in the SpaceX ITS presentation, hydrogen was deemed a bad choice for in-space refueling. So why is ULA planning distributed launch with hydrolox refueling for ACES?

Its relatively easy for ULA to do hydrolox propellant transfer as they have a good idea on how to do it, based on the experiments they've done on the ground. For SpaceX its probably in the "too hard" basket.

[jongoff]

Are zero-boil off systems prohibitively complex for a hydrolox-based architecture?

I wouldn't say ZBO LOX/LH2 systems are prohibitively complex, but they are complex enough that they're often not worth the hassle compared to making sure your system has a very good passive thermal design (like what ULA is trying to do with ACES). In most cases the passive thermal system is "good enough", and in the cases where it isn't, having a good passive thermal design makes the active thermal systems easier to design, since you've minimized the heat leak into the LH2 tanks that you're trying to actively remove.

FWIW, by passive thermal design I mean things like good multi-layer MLI or sun-shield insulation, minimizing penetrations into the tanks, being creative about how you mount stuff that needs to be warm relative to the tanks, using tanks with low CTE materials, vapor cooled skirts, etc.

From the ULA papers I've seen, some key considerations are:
1- Location of the stage: if you're in low orbit around a planet, that's a big warm body taking up half the sky, so that's a more challenging thermal environment than say L2 or deep space. IIRC they were saying boiloff rates at EML2 would be 1/10th what they are in LEO.
2- Duration: if you're talking missions measured in days or weeks, passive is clearly the way to go, but as you start getting into the months/years range, active cooling starts looking better.

Also, in the SpaceX ITS presentation, hydrogen was deemed a bad choice for in-space refueling. So why is ULA planning distributed launch with hydrolox refueling for ACES?

The ITS presentation was SpaceX's perspective on the problem, not some independent statement of objective truth. As others have pointed out ULA and others have a lot more experience with LH2. So some of it may just be due to them being more comfortable with other fluids. Some of it stems from being Mars focused, where the easy availability of CO2 from the atmosphere changes the equation. Some of it can be just smart people coming to different conclusions--note that throughout the space age, really brilliant people don't always come to the same conclusion, even for the same problem. I almost get more worried when I see everyone agreeing... :-)

I'm pretty sure that with ULA's decades of experience (including pre-ULA experience) in LH2, and their economic situation, that LH2 really is the best upper stage propulsion choice for them as a company.

~Jon

[jongoff]

Also, in the SpaceX ITS presentation, hydrogen was deemed a bad choice for in-space refueling. So why is ULA planning distributed launch with hydrolox refueling for ACES?

Its relatively easy for ULA to do hydrolox propellant transfer as they have a good idea on how to do it, based on the experiments they've done on the ground. For SpaceX its probably in the "too hard" basket.

They've also done a lot of post-mission in-space experiments, like that DMSP launch from a few years back. SpaceX wasn't the first company to realize you could do useful experiments after the main customer is away, at least on mission with spare performance (that DMSP one left over 11000lb of prop in the tanks after spacecraft separation, so it was an unusually good one).

As for the in-space transfer part (the one part they haven't done), LH2 isn't really that much harder in space than transfering any other propellant. It's bulky and cold, but LOX and Methane are already cold enough that you have to use cryogenic seals. It's just a matter of picking the right seal, selecting the right coupling materials, and the right surface treatments and finishes. Altius has some recent SBIR work related to this.

~Jon

[TrevorMonty]

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.

[Jim Davis]

A quickly deplorable small LV like XS1...

Deplorable? Ah, autocomplete!

[notsorandom]

Let's not pretend that ULA developed Centaur from scratch here... Because that would be very much inaccurate, since the first Centaur variant flew over 50 years ago. If Centaur is available, that is one thing. But if you are working on a clean sheet design, that changes things.

Historically hydrogen has been a bit harder to tame. The Centaur and other LH2 stages around the world had plenty of early difficulties and it is harder to design an LH2 engine. However Blue Origin went ahead and developed the BE-3 and New Shepard under a commercial, non-government sponsored program....

...That is actually an urban legend. BE-3 was actually a paid milestone under CCDeV. They directly got NASA funds for it. Additionally, they got a lot of free access to NASA test facilities and/or personnel as part of an unfunded SAA like the one SpaceX is using for Red Dragon.

Here's one news article on it (it'd be nice if someone found the actual contract showing it's a paid milestone: https://www.flightglobal.com/news/articles/blue-origin-completes-full-power-tests-on-thruster-for-orbital-vehicle-377721/ )

So it most certainly was done with government funds helping. Don't be surprised that Blue Origin/Bezos doesn't advertise this much. It's better PR if it seems like it's totally their own doing.

(This is secondary to your main point that hydrogen doesn't have to necessarily be extremely expensive.)

Just like SpaceX's COTS and CC, this is a good example of efficient use of government funds. The contract was competitively awarded, and the company contributed to development as well. This is EXACTLY what we want: government funds helping a transformative nascent industry get off the ground that otherwise would've taken longer or wouldn't have been able to make progress. And being competitively awarded is key to this.

Should have been more specific. Pretty much any engine developed by a commercial company has been directly funded by government money or can trace a substantial heritage to a previous government program. The difference I was trying to emphasize was between the BE-3 and engines like the RL-10 and J-2X. Those being the result of a government run program.

[john smith 19]

As for the in-space transfer part (the one part they haven't done), LH2 isn't really that much harder in space than transfering any other propellant. It's bulky and cold, but LOX and Methane are already cold enough that you have to use cryogenic seals. It's just a matter of picking the right seal, selecting the right coupling materials, and the right surface treatments and finishes. Altius has some recent SBIR work related to this.

IIRC ULA also mentioned that a settling thrust of about 10 x 10^-6g is enough to cut boil off by 50%. One of the reasons for their interest in IVF.

There is also the NAIC funded work on a surface coating  blocking solar absorption and allowing a tank to continue emitting heat down to an equilibrium temperature of about 47K, significantly cutting the heat load you'd need to expel long term.

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