Dale Christopher

Looking for opinions about how optimal NASA’s Lunar Gateway plan...

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5 minutes ago, Dragon01 said:

They're bending metal for Vulcan, too (there was a post in the ULA thread about that a good while ago). And their engine is not any more flight-proven than BE-4, unless you count the flying test stand that is Starhopper. I'd say that Vulcan, with its more conservative design (Delta-IV derived tanks, a fat Centaur for the upper stage), is more real than Starship, if anything.

Be-4 has not had a full power test on the stand yet, but yeah, they are bending metal on the tanks, which is great.

The Raptor is far more real than Be-4 right now, as it actually exists as an engine that has been attached to a rocket. That makes it far more flight proven, even if just a static fire (that's all the initial hop tests were, if they had launch clamps it would not have left the ground at all).

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Posted (edited)

There's relatively little difference between being attached to a rocket like Starhopper and being attached to a test stand. Be-4 also exists, though it's a little bit earlier in development than Raptor (it is a much bigger engine, though). The real difference is that Vulkan is basically fuel tanks and engines, like just about any other rocket (I don't think that the recoverable skirt will be used from the start). It's a conservative proposal, but one in which the only uncertainty is Be-4, which seems to be doing well enough. Starship has many more unknowns involved in its design.

Edited by Dragon01

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1 hour ago, Dragon01 said:

There's relatively little difference between being attached to a rocket like Starhopper and being attached to a test stand.

None the less, there are differences between a production engine, and one easily worked on on the test stand.

1 hour ago, Dragon01 said:

Be-4 also exists, though it's a little bit earlier in development than Raptor (it is a much bigger engine, though).

I have no doubt Be-4 will work, but it was supposed to have been done a couple years ago. The last we heard was that they hoped to fire it full power by the end of the year. Raptor has already fired full power, and has been tested as a flight article engine.

1 hour ago, Dragon01 said:

The real difference is that Vulkan is basically fuel tanks and engines, like just about any other rocket (I don't think that the recoverable skirt will be used from the start). It's a conservative proposal, but one in which the only uncertainty is Be-4, which seems to be doing well enough. Starship has many more unknowns involved in its design.

Starship has many unknowns, true. Vulcan has fewer (other than the never flow, never full power tested engines), but it's also a throw away rocket. Super Heavy, OTOH, is just a large F9 booster, it's conservative for SpaceX---which means they have a giant booster they can build whenever that is 100% reusable with very little risk. Minus stage 2 reuse, it still outclasses any other vehicle available in every way (they could test stage 2 recovery by launching large payloads to LEO, then trying to land, after all). If it works, or when it works it changes everything, otherwise it's still a powerful SHLV.

Regardless, I think that minus full S2 reuse, I don't see prop depots as making much sense if the deliveries are coming from small rockets.

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Take what is probably the coolest semi-traditional upper stage that's being considered right now, ACES.

ACES is a full-blown spacecraft. RCS, power, autonomous docking, prop transfer. Woot!

Fly ACES to LEO. Refill ACES in LEO, and then you have a powerful tug to move things... except that to refill ACES, it will take some number of ACES flights, and every one of those flights throws away an ACES (4 to refill?). Once you have one, you can then move it to the Moon---which will result in it getting there less than full. If the props are transferred to another vehicle, you now have a empty/dead ACES at Gateway. This only becomes marginally useful if you have excess props on every other flight to Gateway that can be transferred to it, and that those props can last until needed.

The only solution to this issue is full stage 2 reuse, or ISRU (and a reusable craft that can actually go from the lunar surface to Gateway with enough props for a round trip up and back, AND excess for the depot. That seems unlikely.

 

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Posted (edited)

Lol, Starship would provide a good solution for logistical support to moon missions... it’s almost overkill really. I haven’t seen any mock-up of its internal habitat volume but I’m assuming it would dwarf the gateway... it could serve as a mobile base, a massive fuel depo (even though we are talking methane) a storage and logistic hub, transport, emergency return vehicle... I mean if we are flying Starships in Lunar space it kinda changes the game.

*I suppose there is still the same question as with the Gateway though. Would there be a real benefit from having the capability in orbit.

Edited by Dale Christopher

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40 minutes ago, Dale Christopher said:

Lol, Starship would provide a good solution for logistical support to moon missions... it’s almost overkill really. I haven’t seen any mock-up of its internal habitat volume but I’m assuming it would dwarf the gateway. If landed on the surface it could serve as a large mobile base, a massive fuel depo (even though we are talking methane) a storage and logistic hub, emergency return vehicle... I mean if we are flying Starships in Lunar space it kinda changes the game.

Agreed, I was only mentioning it as a vehicle that can be refilled, and not thrown away (as a cargo carrier, not a crew vehicle).

Tugs make sense, but only if they can move the propellants where they need to go without massively draining the propellant they carry in the process.

Regardless of who does it, or how, any propellant depot should first have a quickly reusable stage 2 to supply it with props.

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On 6/12/2019 at 12:45 AM, tater said:

Current ideas for a Gateway-->Moon lander have:

Ascent Module, AM (returns to Gateway for refilling with props, presumably).

Descent Module, DM (brings Ascent M to lunar surface from LLO)

Tug (brings AM/DM stack from Gateway to LLO, and possibly AM back to Gateway).

The problem of course is that any "tanker" to bring propellants from Earth to Gateway might as well be a throw away stage. Even refilling the small AM takes an autonomous vehicle that might as well be a new one, particularly given the at best 1/year flight rate. To make any sense at all, a lunar prop depot pretty much requires Starship, or something functionally identical: Relatively inexpensive to fly to LEO, that can be topped off in LEO, that can then deliver propellants to Gateway, and return to Earth for reuse. I can't see a prop depot case closing without a 100% reusable (and not reused via expensive refurb, either) vehicle.

If FH/NG/etc can get the 3 lander elements to Gateway, then any use of them to get just props to the same location actually wastes capacity. The only savings is the cost of the vehicles. The tug should be no more expensive than any tanker, since any tanker has to be able to operate and dock at Gateway, anyway. That only leaves the cost of the ascent stage. Making such a stage so that it can refill, AND such that it can live in space and be reused over years is likely more expensive than making a 1-off version that gets thrown away.

This. 
Now in KSP having an fuel depot in Mun orbit makes lots of sense as you can pretty easy do landing and back to orbit single stage and you can either use an oversize transfer stage or simply do an second launch with an fuel tank rater than an lander and the science instruments are an significant part of the cost for an small lander. 

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22 hours ago, tater said:

This is not an ethical experiment. Expose people to known, high radiation, just to see how they are harmed? The radiation environment is already well understood, as are health effects. Putting people at Gateway doesn't help at all.

It allows us to engage in biochemical experiments- you act like humans have to be the test subject when we've been using animal and cellular test subjects for decades. It forces us to engineer new technologies to solve the radiation problem for cheaper since we can actively test our designs in small scale close to home, instead of waiting for months to send it up on it's own vehicle through SpaceX or ULA.

22 hours ago, tater said:

Starship/Super Heavy is more real than Vulcan or NG right now. It has a tested, flight-ready engine, and they are actually bending metal.

To be fair, Vulcan also has a tested engine and is bending metal. Before you comment, no I don't consider Starhopper more real than a test stand engine burn seeing as Starhopper doesn't fly, and is basically a giant water tank in a steel shell. Quite a large leap from a rocket.

22 hours ago, tater said:

All uncrewed Gateway plans right now are predicated on 1 extant SpaceX vehicle (FH), and one ULA vehicle, DIVH. All other launches require (not counting Starship) vehicles that don't actually exist, Vulcan/NG.

Which is fine since all launches are slated for 2020 at the soonest. So if those vehicles end up delayed or worse cancelled, NASA will pick up the slack with SLS. Undoubtedly missing the 2024 landing date but likely to see SLS pick up after what NG/Vulcan was intended to launch.

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1 hour ago, ZooNamedGames said:

To be fair, Vulcan also has a tested engine and is bending metal. Before you comment, no I don't consider Starhopper more real than a test stand engine burn seeing as Starhopper doesn't fly, and is basically a giant water tank in a steel shell. Quite a large leap from a rocket.

It not a 'large leap from a rocket' at all. Its days away from being ready for its first untethered flight. It already has been released from its umbilicals and the Raptor engine should arrive soon. The road is closed from June 17th to 19th, which means they will likely start flying starhopper like an actual rocket within those windows, although it could be delayed.

Starship development is going quite fast.

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There is zero reason to do radiation tests on humans, and Gateway won’t be occupied long enough at a stretch to be terribly useful. And non-human experiments could be done with free fliers.

Gateway is only a thing because Orion can’t do anything else.

 

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On 6/12/2019 at 4:25 AM, Dale Christopher said:

Another thing I’m wondering about is that the propellant most likely to be derived from the moon for all this is Hydrogen+Oxygen and so now aren’t we also having to deal with boil-off @_@! (Not conducive to long term storage!)

Yes, you do. You could try the other option, Al+O,  but the big names seem to have turned away from it.

On 6/12/2019 at 8:27 PM, tater said:

The radiation environment is already well understood, as are health effects.

Not really, not that kind of radiation.

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Posted (edited)
3 hours ago, RCgothic said:

Ionising radiation only comes in three types.

Alpha, beta, gamma, proton and neutron, are five out of those three right? :wink:

Edited by Nefrums

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4 hours ago, DDE said:

Not really, not that kind of radiation.

GCRs? It's ~2X what it is at ISS.

The other issue would be the Earth's magenetotail. The Apollo missions avoided this, but did operate outside the Earth's magnetic field (other side of magnetopause).

222898main_orbit2_20080416_HI.jpg

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Personally I don't understand the need for a lunar gateway. If you want to study the effects of deep space on the human body it's much easier to do that in simulated tests here on Earth, although as @tater points out this would still raise ethical issues, namely that blasting people with radiation to see what happens isn't going to be good for them. So excluding space biology, there's no new science that can be done at the gateway that you couldn't do on the ISS or in a capsule, all of the new science is on the lunar surface. So although the engineering challenges and the cost is greater it makes more sense to me to build a propellant depot and lunar base on the surface first. And from there you could support long duration and ranged missions with massive scientific pay-off, demonstrate the feasibility of ISRU and manufacturing, demonstrate the feasibility of long term stay on another body, and set the grounds for a permanent lunar colony later in the century. The fact that a lunar base is currently being pegged as a secondary goal to the gateway worries me, because it might be that the gateway is all we get.

I could very well be mistaken, but only time will tell how this will play out.

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Posted (edited)

The Gateway is now basically being baselined as a propellant depot though. Or rather, a space truck-stop of sorts. There's accommodations for visiting travelers, but the reason you're really there is to refuel your "truck." Except in this case your "truck" is actually a human lunar lander.

The idea is that after the initial couple of lunar sorties, NASA could start working on developing an ISRU descent stage to replace the expendable descent stages they ship with each landing, upon which you ship that over there on a really big rocket, and you don't need to ship them there anymore.

Such a descent stage would have to be much larger than the expendable descent stage, because it would take over both ascent and descent duties. The old "ascent" stage would then function as a sort of abort system, allowing the astronauts to abort to lunar orbit if the descent stage fails and removing the black zones you had on the Apollo-era landings.

Granted, all this is pretty far in the future, and we all know that NASA plans don't always work out. But I think the concept's solid, and NASA is being smart in laying the groundwork for this. For example, in their RFP for human lunar landers, they specifically stated a design preference for hydrolox over hypergolics, the opposite of what you'd want if you weren't taking ISRU into account in your design.

By the way, that ISRU lander stage would almost certainly be in the ~30t range, compared to the ~10t expected for the expendable descent stages. It'd need a SLS Block IB cargo launch all to itself just to get to TLI.

Edited by jadebenn

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Posted (edited)

I disagree with your ~30t estimate for an ISRU lander. Assuming:

-An Isp of 460s (similar to high performance hydrolox engines like the RL-10).

-A delta-V expenditure of 5km/s for a single stage landing and return.

 

Plugging those numbers into the equation edV/Ve=m0/mf gives us an approximate propellant mass fraction of 0.7, assuming a dry mass of about 5t (2t for the crew module like the LM, 1t for the ISRU system, 1t for the tankage/landing legs/etc and 1t for any extra science equipment or other payloads) this would only mean a total mass of 17 tonnes.

This assumes that the lander lands empty, refuels, takes off and lands again when the next lot of astronauts arrive (this does require the propellant to not boil off too much, which is the only reason I can think off for having a gateway, so you can keep all of the heavy insulation in orbit). These are all made up numbers, but I can't imagine that they would differ by much (then again, I could've screwed up my math somewhere - please correct me if I did). The only lander design that comes close to 30t was the Altair lander (46t), but that was designed for a crew of 4 not a crew of 2 as currently envisioned in the Artemis programme.

Edited by Ol’ Musky Boi

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Altair was not just large because of the crew, it was large because they never got the stack to be able to do Apollo style mission architectures, and the lander descent stage had to do the LOI burn.

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Posted (edited)
1 hour ago, Ol’ Musky Boi said:

I disagree with your ~30t estimate for an ISRU lander. Assuming:

-An Isp of 460s (similar to high performance hydrolox engines like the RL-10).

-A delta-V expenditure of 5km/s for a single stage landing and return.

 

Plugging those numbers into the equation edV/Ve=m0/mf gives us an approximate propellant mass fraction of 0.7, assuming a dry mass of about 5t (2t for the crew module like the LM, 1t for the ISRU system, 1t for the tankage/landing legs/etc and 1t for any extra science equipment or other payloads) this would only mean a total mass of 17 tonnes.

 This assumes that the lander lands empty, refuels, takes off and lands again when the next lot of astronauts arrive (this does require the propellant to not boil off too much, which is the only reason I can think off for having a gateway, so you can keep all of the heavy insulation in orbit). These are all made up numbers, but I can't imagine that they would differ by much (then again, I could've screwed up my math somewhere - please correct me if I did). The only lander design that comes close to 30t was the Altair lander (46t), but that was designed for a crew of 4 not a crew of 2 as currently envisioned in the Artemis programme.

I was cribbing the mass numbers for a two-stage lander from here (slide 19). I suppose they might not be accurate for this purpose.

Hydrolox boiloff should be able to be mitigated with the right equipment. Depending on how well it can be protected against, the margin of fuel needed to be left-over after ascent changes. I suspect NASA would err on the side of caution in that regard, and include quite a bit more than is strictly needed (just in-case), which would bump up the size of the ISRU stage.

The crew of two is supposed to be a temporary thing. I'm not sure how practical it would actually be to bump that up later, though. That really depends on the lander design. NASA has stated it wants the capability to "evolve" to four-person landings in its RFP, but nothing's concrete right now. The proposals might come back differently.

I think that whatever design NASA picks will most likely have:

  • A hypergolic ascent module - it won't benefit from ISRU anyway, and hypergols have safety benefits for abort scenarios
  • A hydrolox descent module - NASA said they won't discount proposals using alternative fuels, but it will affect their ultimate choice
  • May or may not have a transfer module - all the proposals we know of right now are two-stage landers. If it does have a transfer module, probably hypergolics - needs to be a fuel that Orion could bring along with it each time it arrives at the Gateway.

SLS Block 1B's co-manifested payload capacity should be roughly equivalent to an expendable FH's throw capacity to NRHO, at around 10t-15t. Anything above that will need something beefier, like a cargo-only SLS launch.

Edited by jadebenn

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Cargo only SLS is not a thing for a long time, and it doesn't do much good unless it could launch close to a crew SLS launch.The problem with any prop depot architecture is that the stage that brings the props gets there partially empty, It's like sending multiple tugs and descent stages to have one complete one. Whatever they did to send the descent, and tug stages basically needs to be done for every sortie. Unless they can comanifest with crew for the ascent stage props, might as well just send a new one.

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4 minutes ago, jadebenn said:

The crew of two is supposed to be a temporary thing. I'm not sure how practical it would actually be to bump that up later, though. That really depends on the lander design. NASA has stated it wants the capability to "evolve" to four-person landings in its RFP, but nothing's concrete right now. The proposals might come back differently.

Ahh I must have missed that. All of those lander proposals seem massive to me, especially when they've not got a surface base planned that would necessitate that kind of capacity. It almost seems as if they are designed so that they can only be launched on the SLS, if you'll forgive my tinfoil hat, maybe this is an attempt to justify it's further development? If the lander was made smaller it could easily launch on commercial heavy lift LVs.

One possible use of the Hydrolox boil-off is to generate power, much like in ULA's "ACES" concept and Blue Origin's "Blue Moon" lander. This could remove then need for the Gateway's power and propulsion module, although it would use up fairly significant amounts of fuel so you would need to have quite good margins (it would still be more efficient to pack a little extra fuel than launching the several tonne gateway). Speaking of which, how massive will the Gateway be? Wikipedia says 75t but I can't find anything on the updated design.

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Posted (edited)
33 minutes ago, tater said:

Cargo only SLS is not a thing for a long time, and it doesn't do much good unless it could launch close to a crew SLS launch.The problem with any prop depot architecture is that the stage that brings the props gets there partially empty, It's like sending multiple tugs and descent stages to have one complete one. Whatever they did to send the descent, and tug stages basically needs to be done for every sortie. Unless they can comanifest with crew for the ascent stage props, might as well just send a new one.

The tug and ascent stage are supposed to be reusable. Aside from Artemis 3, the descent stage is also supposed to be co-manifested on the SLS (on Artemis 3, since Block 1B isn't predicted to be available by then, a timed FH launch would be necessary).

Hypergolics are pretty stable. They don't boiloff very quickly at all. It's the cryogenics you have to worry about.

Cargo-only SLS would only be necessary for an ISRU descent stage, which (I assume) would be too massive to be co-manifested in a SLS Block 1B launch. If my assumption is incorrect and it could be done within the 10t-15t payload envelope instead, a Cargo-only SLS launch would not be necessary, and it could fly on either commercial or as an SLS co-manifested payload.

27 minutes ago, Ol’ Musky Boi said:

Ahh I must have missed that. All of those lander proposals seem massive to me, especially when they've not got a surface base planned that would necessitate that kind of capacity. It almost seems as if they are designed so that they can only be launched on the SLS, if you'll forgive my tinfoil hat, maybe this is an attempt to justify it's further development? If the lander was made smaller it could easily launch on commercial heavy lift LVs.

One possible use of the Hydrolox boil-off is to generate power, much like in ULA's "ACES" concept and Blue Origin's "Blue Moon" lander. This could remove then need for the Gateway's power and propulsion module, although it would use up fairly significant amounts of fuel so you would need to have quite good margins (it would still be more efficient to pack a little extra fuel than launching the several tonne gateway). Speaking of which, how massive will the Gateway be? Wikipedia says 75t but I can't find anything on the updated design.

The three-stage lander proposal is meant to be small-enough that each part is pretty much launcher-agnostic. Could be co-manifested as part of an SLS launch, or put on a commercial launch (of which the FH would be the rocket of choice for mass to TLI performance). I think the main reason we haven't seen it in proposals so far is due to the added complexity it creates. A two-stage lander is easier to work with. However, all the proposals I've seen predate NASA's request, so it's possible what designs NASA gets will look quite a bit different.

A two-stage lander can work with the SLS, but excludes commercial launches and co-manifested payloads for one of the stages (at least, according to NASA), and a single-stage lander is just too big for anything.

I'm beginning to suspect that those numbers may be so much bigger than the sole historical example partly because they account for the ISRU scenario. As-in, they don't expect a two-stage expendable lander to weigh north of 40t, but they do expect a two-stage ISRU lander to do so. I don't know for sure, because no matter what, any modern proposal is going to be beefier than the Apollo LEM. That design would be absolutely unacceptable from a safety standpoint today.

If you'll permit me to wear my tinfoil hat for a bit: just because they haven't officially said anything about a lunar surface base doesn't mean they aren't thinking about one. :wink: There are some reasons NASA might not want to publicly state such plans at this point in time: sticker-shock being chief among them. With them trying to get funding for the Gateway and the human lunar lander, they've got more than enough on their plate for the time being. Any base plans can wait until after those two get underway.

I expect the PPE to stick around. It's a very useful module and it makes the station a lot more flexible in what architectures it can be used for. Plus, design-wise, it's really just an oversized satellite bus. Also, NASA already contracted with MAXAR for it.

Edited by jadebenn

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Posted (edited)

@jadebenn

hmmm, I thought they had mentioned a permanent base on the moon was the goal, and that the high value spots were the South Pole in the areas that have both permanently shaded craters and access to sunlight.

(I think, I’m thinking of one of Bridenstine’s speaches)

Edited by Dale Christopher

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All good points. I'm not sure how heavy an ISRU system would really be, all you need is a way to extract the water ice from the regolith (I saw an interesting proposal that involved beaming microwaves at it) a system to electrolyse the water and a system to condense the hydrogen and oxygen into fuel. I can't imagine that weighing much more than a few tonnes, then again I haven't done the math so I could be wrong...

After doing some back of the envelope math using the data obtained from the microwave study, I calculated that to fill up a 17 tonne lander with 12 tonnes of hydrolox propellant in a month you would need to extract water at a rate of about 278 g/min. This would correspond to a power requirement of about 139kW (possibly overkill given how the experiment was done with literally just a commercial microwave), which could be obtained from either 3.2 tonnes worth of NASA's 10kW "kilopower" reactors, or 1.8 tonnes of solar panels (which brings it's own host of problems, namely the month long lunar night). Adding on the mass of compressors and electrolysis systems and the entire thing could mass in at several tonnes. Add, say 3 tonnes, to the dry mass of my previous estimate for a lander and it now masses in at 24.2t, so I'm starting to see where that 30t figure comes from (my bad for making silly assumptions). And this assumes that you will have the life support to sit on the surface for a month, which you probably won't if you don't have a surface base.

In this regard, putting ISRU directly on the lander doesn't make so much sense, carrying it with you rapidly inflates the mass of your lander. So it might make more sense to have a permanent ISRU system on the surface, fuelling the lander for a return to Earth once every month before solar power runs out, then coming back a month later to continue setting up the lunar base. This way you only have to haul the ISRU around once.

 

Sorry if my waffling is becoming a little incoherent or inaccurate. It's all too easy to go into "armchair engineer mode" if you can call it that.

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