___________¹When I say "aerobrake" here, I mean a gentle set of braking maneuvers, with somewhere between 20m/s and 200m/s of delta-v removed per pass, rather than a single-pass aerocapture. All such maneuvers require about 70m/s to raise the perigee at the end of the aerobraking, plus whatever maneuvering and attitude control is required between passes. At 20m/s removed per pass, it takes 75 days to return to 300x300. At 200m/s, it takes 5 days.
As long as aerobraking is possible, it seems like a no-brainer over tanker shuttles with EDL.
Quote from: TheRadicalModerate on 01/13/2025 05:10 pm___________¹When I say "aerobrake" here, I mean a gentle set of braking maneuvers, with somewhere between 20m/s and 200m/s of delta-v removed per pass, rather than a single-pass aerocapture. All such maneuvers require about 70m/s to raise the perigee at the end of the aerobraking, plus whatever maneuvering and attitude control is required between passes. At 20m/s removed per pass, it takes 75 days to return to 300x300. At 200m/s, it takes 5 days.flaps + heatshield => ~20t.70m/sec. One engine firing at one G is about 600kg/sec for an empty Starship, so 70m/sec is 4.2t of propellant.If you don't get your slow aerobrake down in less than 5 passes, you'd be better off doing a single pass aerocapture w/ heatshield. Aka a standard Starship V3 and solve the coupling problems.
Quote from: TheRadicalModerate on 01/13/2025 05:10 pmAs long as aerobraking is possible, it seems like a no-brainer over tanker shuttles with EDL.No concerns about vehicles that can't be serviced? If a depot fails after fueling, it could waste the work of ten tanker trips, so it's not trivial.
Let's use 150 passes @ 5m/s per aerobrake pass, plus about 90m/s for reverse BLT, and 70m/s for circularization: 910m/s.
Quote from: TheRadicalModerate on 01/14/2025 04:29 amLet's use 150 passes @ 5m/s per aerobrake pass, plus about 90m/s for reverse BLT, and 70m/s for circularization: 910m/s. How long would all those aerobraking passes take?
More to the point, what level of depot utilisation might that result in? Would you need 10 times as many depots? 50x? 100x?
If we're not talking about high-volume applications then don't even bother with a new (and disposable) depot variant, just use regular tankers.Then once volumes are high enough for depots to not be disposable, utilisation will probably be something that needs to be considered.
Quote from: steveleach on 01/14/2025 10:16 amIf we're not talking about high-volume applications then don't even bother with a new (and disposable) depot variant, just use regular tankers.Then once volumes are high enough for depots to not be disposable, utilisation will probably be something that needs to be considered.Why do you think regular tankers will work? Even if you solve the QD/GSE gender problem, the boiloff from a tanker will be much higher than from a depot. Disposable or not, I think we're stuck with depots for the nonce.I think a return time of 5 to 10 days is fine. If we wanted, say, monthly missions to Moon Base Alpha, we might need two depots.
We're not talking about a disposable depot here. It just fills up in VLEO from tankers, boosts up to the Final Tanking Orbit (FTO, in the FCC application's lingo), refuels the target Starship, then returns to VLEO for the next mission. The only difference is that it takes longer to return via aerobraking. If you need it sooner, most missions will allow propulsive return. It'll just consume more prop.
Quote from: TheRadicalModerate on 01/14/2025 02:18 pmWe're not talking about a disposable depot here. It just fills up in VLEO from tankers, boosts up to the Final Tanking Orbit (FTO, in the FCC application's lingo), refuels the target Starship, then returns to VLEO for the next mission. The only difference is that it takes longer to return via aerobraking. If you need it sooner, most missions will allow propulsive return. It'll just consume more prop.To be honest, though, I expect the depots will be disposable for quite a while. Their inability to EDL makes it hard for SpaceX to study them to see what parts are suffering wear and tear.Another reason to want someone to build that orbital drydock. :-)
Quote from: FCC Technical AnnexLow Earth Orbit. SpaceX will conduct a range of Starship operations in low-Earth orbit (“LEO”). Each fully reusable Missions beyond LEO will also require a tanker version of Starship for propellant aggregation. During these missions, SpaceX will launch one or more propellant tanker versions of Starship. Some of these tanker variants will remain in LEO as “depots,” and will be filled with propellant by subsequent tanker launches. LEO operations will occur in a circular orbit at 281 km altitude (+/- 100 km) and an inclination ranging from equatorial (0 degrees) to polar.Medium-Earth Orbit/High-Earth Orbit/Final Tanking Orbit. Missions beyond LEO will also require space station operations in medium-Earth orbit (“MEO”) to high-Earth orbit (“HEO”). For example, crewed lunar missions will include a secondary propellant transfer in MEO/HEO, the Final Tanking Orbit (“FTO”). Operations in MEO/HEO will occur in an elliptical orbit of 281km x 34,534 km and an altitude tolerance of +116,000/-24,000 km apogee and +/- 100 km perigee, with inclination between 28 and 33 degrees (+/- 2 degrees).
Low Earth Orbit. SpaceX will conduct a range of Starship operations in low-Earth orbit (“LEO”). Each fully reusable Missions beyond LEO will also require a tanker version of Starship for propellant aggregation. During these missions, SpaceX will launch one or more propellant tanker versions of Starship. Some of these tanker variants will remain in LEO as “depots,” and will be filled with propellant by subsequent tanker launches. LEO operations will occur in a circular orbit at 281 km altitude (+/- 100 km) and an inclination ranging from equatorial (0 degrees) to polar.Medium-Earth Orbit/High-Earth Orbit/Final Tanking Orbit. Missions beyond LEO will also require space station operations in medium-Earth orbit (“MEO”) to high-Earth orbit (“HEO”). For example, crewed lunar missions will include a secondary propellant transfer in MEO/HEO, the Final Tanking Orbit (“FTO”). Operations in MEO/HEO will occur in an elliptical orbit of 281km x 34,534 km and an altitude tolerance of +116,000/-24,000 km apogee and +/- 100 km perigee, with inclination between 28 and 33 degrees (+/- 2 degrees).
Quote from: TheRadicalModerate on 01/14/2025 02:18 pmWe're not talking about a disposable depot here. It just fills up in VLEO from tankers, boosts up to the Final Tanking Orbit (FTO, in the FCC application's lingo), refuels the target Starship, then returns to VLEO for the next mission. The only difference is that it takes longer to return via aerobraking. If you need it sooner, most missions will allow propulsive return. It'll just consume more prop.To be honest, though, I expect the depots will be disposable for quite a while.
Their inability to EDL makes it hard for SpaceX to study them to see what parts are suffering wear and tear.Another reason to want someone to build that orbital drydock. :-)
Quote from: InterestedEngineer on 01/13/2025 05:33 pmQuote from: TheRadicalModerate on 01/13/2025 05:10 pm___________¹When I say "aerobrake" here, I mean a gentle set of braking maneuvers, with somewhere between 20m/s and 200m/s of delta-v removed per pass, rather than a single-pass aerocapture. All such maneuvers require about 70m/s to raise the perigee at the end of the aerobraking, plus whatever maneuvering and attitude control is required between passes. At 20m/s removed per pass, it takes 75 days to return to 300x300. At 200m/s, it takes 5 days.flaps + heatshield => ~20t.70m/sec. One engine firing at one G is about 600kg/sec for an empty Starship, so 70m/sec is 4.2t of propellant.If you don't get your slow aerobrake down in less than 5 passes, you'd be better off doing a single pass aerocapture w/ heatshield. Aka a standard Starship V3 and solve the coupling problems.The only propulsive delta-v per pass is attitude control and a few m/s for course correction. Then add 70m/s for circularization and 90m/s for NRHO-to-BLT insertion.Returning propulsively from NRHO, even via a reverse BLT, is about 3340m/s. But a tanker sitting in VLEO is useless unless you bring it back to EDL, which is another... 150m/s for landing delta-v? 3490m/s.v3 tanker dry mass = ~160t @ Isp=369s, total prop consumption = 233t.Let's use 150 passes @ 5m/s per aerobrake pass, plus about 90m/s for reverse BLT, and 70m/s for circularization: 910m/s. Depot dry mass should be about 140t, same Isp, so prop consumption is 40t.I could be wrong about course correction and attitude delta-v per pass, and long-term boiloff is an issue, but this is a depot; it should be pretty good at managing boiloff. Seems like a pretty definitive win.And that's before you're factoring a second pair of RPODs for each campaign.
I think a single shuttling depot is going to always beat out a shuttling tanker between two depots, as long as the depot can aerobrake¹ back into VLEO. The aerobraking maneuver takes anywhere from a few days to a few months, depending on how aggressively you reduce delta-v per pass, but lunar missions are only going to happen every few months anyway. Note the following:1) Shuttling tankers (probably) have higher dry masses than shuttling depots.2) Shuttling tankers lack whatever boiloff-reduction tech is on the depots. This is especially important when the tanker needs to use a BLT to get to a lunar orbit, since transit will then take 2-3 months.3) A tanker's direct EDL likely requires more delta-v for the landing burn than a depot's aerobraking campaign requires to manage attitude, course corrections to line up the next pass, and perigee raise at the end of the aerobraking.4) Shuttling depots are completely standalone once they go to high orbit. Their QD system will work directly with the target ship. Shuttling tankers always need a depot on each end to get the QD gendering to work out, or they need the development of a completely new and androgynous QD for all Starships.As long as aerobraking is possible, it seems like a no-brainer over tanker shuttles with EDL.___________¹When I say "aerobrake" here, I mean a gentle set of braking maneuvers, with somewhere between 20m/s and 200m/s of delta-v removed per pass, rather than a single-pass aerocapture. All such maneuvers require about 70m/s to raise the perigee at the end of the aerobraking, plus whatever maneuvering and attitude control is required between passes. At 20m/s removed per pass, it takes 75 days to return to 300x300. At 200m/s, it takes 5 days.
I don't see the part about a one-pass aerobrake with a heatshield and circularize. That's what I was proposing. far less complicated conops, you are now in LEO and ready to act as a depot again for getting prop to higher energies.
I also don't get how you adjust the altitude of the 5m/sec aerobrake. Each pass drops the altitude, accelerating the aerobrake, and eventually you run into thick enough atmosphere that without a heat shield you burn up (or rip off the solar panels, or scar them with plasma etc). That requires some sort of deltaV at apogee to raise the the perigee which has to be done very every single pass. 668 passes, in the case of 5 m/sec. per pass.668 passes through the van allen belts, into the very busy LEO to VLEO space is probably not the best idea from a logistics, scheduling, and traffic management standpoint.
Quote from: InterestedEngineer on 01/14/2025 05:20 pmI don't see the part about a one-pass aerobrake with a heatshield and circularize. That's what I was proposing. far less complicated conops, you are now in LEO and ready to act as a depot again for getting prop to higher energies.But one-pass requires TPS that's able to scrub off more than 3km/s of speed in a single pass--with the entry speed being 11km/s, instead of ~7.8km/s. That's effectively an EDL-capable Starship.If that kind of TPS has the properties needed to store prop in a low enough boiloff state, then a plain vanilla Starship probably does as well, and depots are superfluous. But that doesn't sound like what's happening.
Quote from: Greg Hullender on 01/14/2025 02:43 pmQuote from: TheRadicalModerate on 01/14/2025 02:18 pmWe're not talking about a disposable depot here. It just fills up in VLEO from tankers, boosts up to the Final Tanking Orbit (FTO, in the FCC application's lingo), refuels the target Starship, then returns to VLEO for the next mission. The only difference is that it takes longer to return via aerobraking. If you need it sooner, most missions will allow propulsive return. It'll just consume more prop.To be honest, though, I expect the depots will be disposable for quite a while. Their inability to EDL makes it hard for SpaceX to study them to see what parts are suffering wear and tear.Another reason to want someone to build that orbital drydock. :-)Yep. Same problem with HLS or indeed any non-EDL Starship that may have multiple missions. At least it's easy to replenish Depot, since it's whole function is to transfer fuel. HLS and other Ships will be harder to reprovision. NASA, et. al. have kept ISS up and more or less operational for 25 years, so it's at least conceivable that these Ships can be refurbished by a semi-permanent dry dock, but I suspect an EDL-capable repair ship makes more sense, since it can do the refurbishment in any LEO orbit.