Quote from: InterestedEngineer on 12/14/2024 11:51 pmThe primary benefit for VLEO in regards to MMODs is the relative low density. They just don't last very long, so there's no accumulation.In circular orbits, yes. I mentioned that. Density is pretty low up to almost 400 km. But debris after a collision will include particles in elliptical orbits that spend only a little time at 200 km. Yes, those will circularize fairly quickly and then decay, but they're still an issue.
The primary benefit for VLEO in regards to MMODs is the relative low density. They just don't last very long, so there's no accumulation.
Quote from: InterestedEngineer on 12/14/2024 12:48 amQuote from: InterestedEngineer on 12/14/2024 12:18 amMaybe a Depot is a bad requirement. Let's remove it and see what happens.Quote from: Twark_Main on 12/14/2024 12:38 am"Maybe [my favorite opinion this week is right]" isn't a real argument, I'll have you know. If you want to claim that it is actually a bad requirement, you have all your work ahead of you.No, the burden of proof is on the one who wants to keep the part (in this case, a custom depot Starship).Elon isn't being flippant when he says the requirements are probably wrong. 30 years of software engineering tells me the same thing.As for the whole "burden of proof" silliness, attempting to seize the high ground Null Hypothesis isn't a real argument, it's an epistemological sleight-of-hand trick. Nobody is suggesting Elon is being flippant about requirements. I'm just observing that your idea of the requirements can be wrong too. Hence why the quote "cuts both ways."
Quote from: InterestedEngineer on 12/14/2024 12:18 amMaybe a Depot is a bad requirement. Let's remove it and see what happens.Quote from: Twark_Main on 12/14/2024 12:38 am"Maybe [my favorite opinion this week is right]" isn't a real argument, I'll have you know. If you want to claim that it is actually a bad requirement, you have all your work ahead of you.No, the burden of proof is on the one who wants to keep the part (in this case, a custom depot Starship).Elon isn't being flippant when he says the requirements are probably wrong. 30 years of software engineering tells me the same thing.
Maybe a Depot is a bad requirement. Let's remove it and see what happens.
"Maybe [my favorite opinion this week is right]" isn't a real argument, I'll have you know. If you want to claim that it is actually a bad requirement, you have all your work ahead of you.
With the miscellaneous rhetoric issues out of the way, as far as actual engineering we come back to:Quote from: Elon MuskEverything possible must be done to ensure astronaut safety.Elon isn't being flippant when he talks about astronaut safety.
Everything possible must be done to ensure astronaut safety.
1. Some money and development is saved (that's about where the advantages end).2. Either boil-off protection is placed on the final ship (sometimes extra mass) or the prop just boils off.3. Either the final ship or all the tankers have to have a male QD port (have fun with the GSE).4. The final ship has to be launched at the beginning of the refueling campaign instead of the end. 4a. Everything onboard has to be rated for an extended loitering lifetime. 4b. Crew during refueling is absolutely prohibitive now. 4c. Final ship outfitting, testing, and revising cannot continue throughout the refueling campaign (not good for timelines). 4d. Months-long MMOD considerations are now relevant to the expensive final ship rather than to a barrel of prop.MMOD risk to depots between refueling campaigns doesn't even move the needle compared to these disadvantages.
1. Some money and development is saved (that's about where the advantages end).
The requirement to transfer 2000t of fuel in one go is one of those things the depot drags along with it. If you don't have depot, you don't have this requirement.So let's enumerate all the problems with transferring 2000t of fuel in one go, instead of just ignoring it like the quote poster did.
Quote from: InterestedEngineer on 12/15/2024 11:07 pmThe requirement to transfer 2000t of fuel in one go is one of those things the depot drags along with it. If you don't have depot, you don't have this requirement.So let's enumerate all the problems with transferring 2000t of fuel in one go, instead of just ignoring it like the quote poster did.By this I gather you mean that one cost of using a depot is that each liter of propellant gets transferred twice: once to the depot and once to the vehicle. Without the depot, each liter is transferred only once. Since the transfer process has some inefficiencies, this roughly doubles them.I don't see any other issue with this, though. Is there something else you're getting at?
Quote from: Greg Hullender on 12/16/2024 12:38 amQuote from: InterestedEngineer on 12/15/2024 11:07 pmThe requirement to transfer 2000t of fuel in one go is one of those things the depot drags along with it. If you don't have depot, you don't have this requirement.So let's enumerate all the problems with transferring 2000t of fuel in one go, instead of just ignoring it like the quote poster did.By this I gather you mean that one cost of using a depot is that each liter of propellant gets transferred twice: once to the depot and once to the vehicle. Without the depot, each liter is transferred only once. Since the transfer process has some inefficiencies, this roughly doubles them.I don't see any other issue with this, though. Is there something else you're getting at?The length of time it takes to transfer a whole tank in one shot vs. the a bunch of small transfers (which you already have to do)The added risks of all fuel eggs in one basket while doing a risky maneuver.I think depot drags along the requirement for hot gas thrusters.
Sorry, but I seem to have lost track. Has anyone seen a stated requirement to transfer propellant from a Tanker to any ship other than a Depot? Has anyone seen a stated requirement to transfer propellant to any non-Depot from a ship other than a Depot?
Quote from: DanClemmensen on 12/16/2024 02:28 amSorry, but I seem to have lost track. Has anyone seen a stated requirement to transfer propellant from a Tanker to any ship other than a Depot? Has anyone seen a stated requirement to transfer propellant to any non-Depot from a ship other than a Depot?Nope. The thought experiment is to ask "what happens if we remove the depot requirement?" After all, it wasn't that long ago that Elon declared "depots suck" (or something to that effect).
I think depot drags along the requirement for hot gas thrusters.
[...]Reason #1 is by far the biggest problem for depots, and may indeed require putting shielding around the tankage. Even a pinhole in a tank is a loss of mission--and potentially the loss of a bunch of previous tanker missions.
In October 2020 up-thread, the best guess was that the necessary settling acceleration is on the order of 1e-4 m/s2. Also, keeping the depot pointing toward the sun to minimize illuminated surface area and therefore boil-off is probably important. Either way, using drag for the acceleration doesn't really make sense, because the ship would have to be brought up to altitude again using propulsion. Besides, boil-off will create ullage gas that will have to be vented anyway; they might as well use it for settling.
Quote from: TheRadicalModerate on 12/15/2024 05:33 am[...]Reason #1 is by far the biggest problem for depots, and may indeed require putting shielding around the tankage. Even a pinhole in a tank is a loss of mission--and potentially the loss of a bunch of previous tanker missions.Is that true? That Progress had a reasonable-sized hole in it for ages without the ISS suddenly depressurizing...Let's throw some poorly-considered numbers at a random formula on Wikipedia...First, to estimate the density of methane at its (sea-level) boiling point of ~110K and a pressure of 6 bar, using the ideal gas equation you get a density of 10.52 kg/m^3.https://en.wikipedia.org/wiki/Orifice_plateThen, if I plug in some inappropriate values to this equation for a 1mm^2 hole leaking methane *gas* (discharge coefficient of 0.85, methane density of 10.52 kg/m^3, expansibility factor 1 even though that's inappropriate for a compressible gas, pressure 6 bar and pressure differential of 1) then I get a mass loss rate of 260kg/day. If I used a more accurate expansibility factor (which I think is less than 1 for compressible gases as the flow becomes choked), it would be even lower. Not ideal, but not a total disaster if you are leaking gas rather than liquid.If I use a density for liquid methane (420kg/m^3) then I get 1.6 tons per day which is rather more problematic. Anything much more than 1mm^2 is obviously more problematic.Major limitations:1) I've used an expansibility factor of 1 which is inappropriate for gas (correct for liquid), but (I think) conservative2) I've used an arbitrary but conservative discharge coefficient of 0.853) I've used a pressure of 6 bar; reducing pressure would reduce leakage rates (but only as the root of the pressure)4) I've used a sea-level boiling point of 110K instead of the actual boiling point at 6 bar (and assumed this is the temperature the methane will be at, which is likely)So a pinhole is not necessarily completely disastrous (a 0.1mm x 0.1mm pinhole would only be 16 kg/day, or ~6 tons per year, and you'd probably never notice it) but larger holes (>1mm^2) do become problematic quickly.And obviously a hole might cause structural failure of the tank which would be Bad.
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
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: 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?