multi-layer insolation shield like the one JWST uses
Quote from: DanClemmensen on 12/18/2024 03:08 ammulti-layer insolation shield like the one JWST usesSo basically, simplifing, you want the thermal micrometeoroid garment.https://en.wikipedia.org/wiki/Thermal_Micrometeoroid_GarmentThe ISS, Soyuz, and Shuttle have all used similar thermal blanket technology. The outer layer (a strong engineered textile like Beta cloth, Nomex, Ortho-Fabric, etc) acts as a MMOD bumper, the MLI acts as thermal insulation and also 'packs' the Whipple shield for higher efficacy, and optionally you can add an inner textile later (Kevlar or similar) for improved tank wall impact protection where it's most effective because the hypersonic cone has fanned out.
Quote from: OTV Booster on 12/18/2024 02:57 amQuote from: Paul451 on 12/17/2024 11:19 pmQuote from: andrewmcleod on 12/16/2024 09:12 amQuote from: TheRadicalModerate on 12/15/2024 05:33 amEven a pinhole in a tank is a loss of missionIs that true?It's not. But impacts by hypersonic "pins" don't create pinhole leaks. So TRM's broader point is true. MMOD shielding will be vital.This is another argument in favor of a depot. A ship hanging around waiting for tankers would need MMOD shielding. A ship or tanker spending minimum time in LEO would have a lesser need.After discussion on the point we seem to have reached a consensus that where possible the depot should carry the heavy stuff so the tankers and outbound ships don't have to repeatedly haul it up to orbit. MMOD shielding is heavy.Hmmm. A thought. MMOD shielding is a two layer affair with the outer layer taking the hit and turning the intruding particle into a spray of fine low energy particles that get stopped by the second layer. Just noodling here but could metallic heat shielding double as the outer MMOD shield?I think you mean solar insolation shielding, not re-entry shielding. A Depot never needs to re-enter. I think a multi-layer insolation shield like the one JWST uses would make a fine Wipple shield. The problem is that the solar heat flux is not aligned with the MMOD threat.
Quote from: Paul451 on 12/17/2024 11:19 pmQuote from: andrewmcleod on 12/16/2024 09:12 amQuote from: TheRadicalModerate on 12/15/2024 05:33 amEven a pinhole in a tank is a loss of missionIs that true?It's not. But impacts by hypersonic "pins" don't create pinhole leaks. So TRM's broader point is true. MMOD shielding will be vital.This is another argument in favor of a depot. A ship hanging around waiting for tankers would need MMOD shielding. A ship or tanker spending minimum time in LEO would have a lesser need.After discussion on the point we seem to have reached a consensus that where possible the depot should carry the heavy stuff so the tankers and outbound ships don't have to repeatedly haul it up to orbit. MMOD shielding is heavy.Hmmm. A thought. MMOD shielding is a two layer affair with the outer layer taking the hit and turning the intruding particle into a spray of fine low energy particles that get stopped by the second layer. Just noodling here but could metallic heat shielding double as the outer MMOD shield?
Quote from: andrewmcleod on 12/16/2024 09:12 amQuote from: TheRadicalModerate on 12/15/2024 05:33 amEven a pinhole in a tank is a loss of missionIs that true?It's not. But impacts by hypersonic "pins" don't create pinhole leaks. So TRM's broader point is true. MMOD shielding will be vital.
Quote from: TheRadicalModerate on 12/15/2024 05:33 amEven a pinhole in a tank is a loss of missionIs that true?
Even a pinhole in a tank is a loss of mission
I don't know how you attach a blanket to the exterior of the tanks so that:1) It was launchable and withstood dynamic pressure and the vagaries of the boundary layer.2) It could expand/contract with prop loading and unloading.
Quote from: Twark_Main on 12/18/2024 05:11 amQuote from: DanClemmensen on 12/18/2024 03:08 ammulti-layer insolation shield like the one JWST usesSo basically, simplifing, you want the thermal micrometeoroid garment.https://en.wikipedia.org/wiki/Thermal_Micrometeoroid_GarmentThe ISS, Soyuz, and Shuttle have all used similar thermal blanket technology. The outer layer (a strong engineered textile like Beta cloth, Nomex, Ortho-Fabric, etc) acts as a MMOD bumper, the MLI acts as thermal insulation and also 'packs' the Whipple shield for higher efficacy, and optionally you can add an inner textile later (Kevlar or similar) for improved tank wall impact protection where it's most effective because the hypersonic cone has fanned out.I don't know how you attach a blanket to the exterior of the tanks so that:1) It was launchable and withstood dynamic pressure and the vagaries of the boundary layer.2) It could expand/contract with prop loading and unloading.
Quote from: TheRadicalModerate on 12/18/2024 10:02 pmI don't know how you attach a blanket to the exterior of the tanks so that:1) It was launchable and withstood dynamic pressure and the vagaries of the boundary layer.2) It could expand/contract with prop loading and unloading.I don't either. I would launch a multi-layer shield like the one for JWST as a separate launch, but with a conical shape as recommended on some thread here in the last two years. Unfold this piece of origami and then carefully ease Depot into it. Depot would emerge for transfer operations and then sneak back into its cocoon and go back to sleep.
Good catch (although I haven't verified your results). Two things jump out:1) It's non-intuitive why mass loss from ullage gas would be less than from liquid prop. Gas has much lower viscosity. Of course, there's only so much gas available, but if the pressure drops to near-zero, the prop will boil like crazy. Did you take this into account? It's possible that the pressure is still very low at equilibrium, but that's a much more complicated calculation.2) If your hole opens up on a gas pocket, the thrust from the escaping gas will accelerate the vehicle in the opposite direction, which will cause liquid prop to settle around the hole. Given the outcome of the boiling calculation, that may be a good thing. Or not...
I'm concerned with how they'll balance the ullage thrust from the depot and the tanker to avoid imparting unwanted angular momentum. If you're venting gas to make the transfer happen, then it seems one side has a lot of ullage gas to work with while the other side has relatively little. If most of the mass is in the receiving vehicle, maybe this isn't a problem; a nearly full depot loading prop from a tanker could probably just angle the thrust a little. But in the case of transfer from a fully loaded depot to an empty tanker, I don't think that's possible.So it seems like you'll need some amount of ullage thrust from both vehicles for the duration of the transfer, but I still don't see how you can do this and keep them balanced.
1) Intuitively it makes sense to me I think? Something to do with the relationship between pressure, density and velocity. The pressure difference presumably drives the velocity through the hole in some way?
You're converting power from the pressure drop (volumetric flow × pressure drop) into power from kinetic energy (1/2 mass flow × velocity2), minus any friction losses.
It's not really an issue if you're using combusting gas: both vehicles will have some amount of highly pressurized LCH4 and LOX in COPVs, and that'll handle the settling burn.¹ The vehicles will have to communicate and decide which one imparts what thrust at what time to keep the system controlled, but that's a pretty standard guidance and control problem.
Quote from: Twark_Main on 12/19/2024 08:05 pmYou're converting power from the pressure drop (volumetric flow × pressure drop) into power from kinetic energy (1/2 mass flow × velocity2), minus any friction losses.Doesn't help them since it's velocity that they're asking for, without which volumetric flow, hence mass flow, is also unknown. The only variable they have is pressure drop (~6 bar.)
Quote from: TheRadicalModerate on 12/19/2024 09:21 pmIt's not really an issue if you're using combusting gas: both vehicles will have some amount of highly pressurized LCH4 and LOX in COPVs, and that'll handle the settling burn.¹ The vehicles will have to communicate and decide which one imparts what thrust at what time to keep the system controlled, but that's a pretty standard guidance and control problem.In this case, though, it seems as though they'd have to develop a whole new rocket engine. That seems to me to be a really big deal. And they can't just buy one off the shelf because, until recently, no one was developing methalox engines. Yeah, there's a lot written about throttling liquid-propellant engines, but, boy-oh-boy, talk about adding a complex new part!It's possible that this is actually the biggest technical risk as far as making depots work goes.
In the most recent artwork of the LSS, it looks like there are 3 clusters of 3 thrusters around the waist, spanning about 120º, if I'm eyeballing correctly. That would imply a total of 27 thrusters. On landing, assuming that the LSS has a dry mass of 95t (no TPS, which may now be wrong) and let's arm-wave the crew module and its payload at 15t, for a total inert mass of 110t.To get back to NRHO, we need about 170t of prop (just trust me on this, but note that I'm using v2 as a baseline, not v3), but let's goose it up to 200t just to be sure, for a gross landing mass of 310t. If the max thrust required allows the LSS to hover, then in 1.62m/s² lunar gravity, we need a total of about 500kN of thrust. Let's allow for a couple of thruster failures, and assume we have 25 working. So each one needs to generate a max thrust of 20kN.
Quote from: Greg Hullender on 12/19/2024 10:31 pmQuote from: TheRadicalModerate on 12/19/2024 09:21 pmIt's not really an issue if you're using combusting gas: both vehicles will have some amount of highly pressurized LCH4 and LOX in COPVs, and that'll handle the settling burn.¹ The vehicles will have to communicate and decide which one imparts what thrust at what time to keep the system controlled, but that's a pretty standard guidance and control problem.In this case, though, it seems as though they'd have to develop a whole new rocket engine. That seems to me to be a really big deal. And they can't just buy one off the shelf because, until recently, no one was developing methalox engines. Yeah, there's a lot written about throttling liquid-propellant engines, but, boy-oh-boy, talk about adding a complex new part!It's possible that this is actually the biggest technical risk as far as making depots work goes.Well, they're going to have to develop at least one new rocket engine, because there's no way to land a Starship on the Moon without falling into the crater dug by exhaust impingement from Raptors. That's what the waist thrusters are for on the LSS, and I assure you that they're combusting gas engines.The real question is whether they need to develop a second new rocket engine. That would be necessary if the waist thruster can't throttle down enough to support settling acceleration efficiently. Let's model that.In the most recent artwork of the LSS, it looks like there are 3 clusters of 3 thrusters around the waist, spanning about 120º, if I'm eyeballing correctly. That would imply a total of 27 thrusters. On landing, assuming that the LSS has a dry mass of 95t (no TPS, which may now be wrong) and let's arm-wave the crew module and its payload at 15t, for a total inert mass of 110t.To get back to NRHO, we need about 170t of prop (just trust me on this, but note that I'm using v2 as a baseline, not v3), but let's goose it up to 200t just to be sure, for a gross landing mass of 310t. If the max thrust required allows the LSS to hover, then in 1.62m/s² lunar gravity, we need a total of about 500kN of thrust. Let's allow for a couple of thruster failures, and assume we have 25 working. So each one needs to generate a max thrust of 20kN.I'd guess that it's quite simple to throttle a pressure-fed methalox thruster down to 50%, so we can go as low as 10kN of settling thrust, using a single thruster. (If one of them fails, just fire up another and wait for things to settle again.)The combined mass of a depot should be 110t dry for the LSS, maybe the same for the depot with insulation, and let's say we need 1800t of prop in the combined system. So combined wet mass is 2020t. If we can throttle down one of the landing thrusters to 10kN, that should result in a system acceleration of 5mm/s². That's more than we need, but not by a huge amount. If the thruster has an Isp of 320s, it'll have 3.2kg/s of mass flow.Up-thread, I estimated that if on-orbit prop transfer happens at a tenth the rate of pad prop transfer, moving 1500t of prop would take 9.3hrs. That would imply that settling consumed 107t of prop--which ain't great. It's way too much to put into COPVs at the start of the transfer operation. It's even too much to put into header tanks in the depot. (Note that there's no need for depot header tanks except maybe for this purpose, and the LSS can't have header tanks, because they'd block the docking tunnel.)So: either we need a second kind of combusting-gas thruster, or we need a thruster that can support much, much lower throttle settings--like maybe 10%, which would be 2kN thrust, 1mm/s², a mass flow of 0.6kg/s, and settling prop consumption of 21t, which is still pretty terrible.So let's work the problem backwards. Let's say we want to limit settling prop consumption to 5t, with 5kN thrust (throttle = 25%, which is aggressive but not unreasonable). At Isp=320s, that'd be a mass flow of 1.6kg/s. So we'd have to limit the prop transfer to about 55mins. For 1500t, that's a prop transfer mass flow of 480kg/s.That's very close to the fill rate on the pad. We know the plumbing exists to handle that rate, but we have no hydrostatic pressure to play with, and we can't geyser or slosh a lot.¹I'm not sure what conclusions to draw here. Looks like we'd either need a second, smaller thruster (not particularly difficult, but guaranteed to stick in Elon's craw), or a really high prop transfer rate._______¹This is where I'll put in a plug for Team Pump. All things are possible with a self-priming pump and enough electrical power.