5 ms·

Faulty assumptions and logic. The “payload” is an experiment that is at most two tanks, a valve and monitoring equipment. However it is pretty clear they will

Faulty assumptions and logic. The “payload” is an experiment that is at most two tanks, a valve and monitoring equipment. However it is pretty clear they will use the existing LOX main and header tanks and valves already present in every Starship. This will meet the requirements of NASA’s 2020 $53 million challenge to “transfer 10 metric tons of cryogenic propellant, specifically liquid oxygen, between tanks on a Starship vehicle”[1] as the smaller header tank holds roughly 20 tons. If you are already attempting to get into orbit it makes sense to include this experiment in case you succeed because you save the time and cost of an entire launch and might get a $53 million award while advancing a key requirement for HLS, a $2.9 billion dollar contract and the current core mission. This experiment has been planned for years, Starship was designed to do it and it makes sense to do it now. SpaceX clearly knows what they are doing. Unfortunately HN is terrible on this subject. [1] https://www.nasa.gov/technology/2020-nasa-tipping-point-selections/ https://www.nasa.gov/technology/2020-nasa-tipping-point-sele...

Tuna-Fish 3y ago
Cryogenic fuel transfer requires a bit more than that. The big issue is that unlike all fuel that has been moved between tanks in microgravity in the past, SpaceX isn't storing their fuel in a flexible tank. Instead, they have the fuel and the pressurant gas in the same tank. Under acceleration (gravity or otherwise), this is not a problem, because the fuel pools naturally "down", with the pressurant above it. However, as soon as the acceleration stops, there is no more down. Instead of having a bunch of fuel on the bottom and pressurant above, the tank will contain a chaotic mixture of floating gas and liquid. The minimum setup for fuel transfer is a maneuvering system capable of a very long, weak impulse (to provide down), and then some kind of pump that moves fuel, because the force provided by the weak impulse of a maneuvering thruster is not enough to do it on it's own.
- pokey00 3y ago
  Your maneuvering system just needs to be able to get tanks spinning.
  - deleted 3y ago
    [deleted]
  - angiosperm 3y ago
    Rotating the pair of (tank and vehicle) around a common center, and pumping "up" through the center, could work. But the pair would be very off-center most of the time, and unstable unless attached end to end.
    - nomel 3y ago
      Are flexible couplings (like a hose) not possible to use, in these cases?
      - angiosperm 3y ago
        Flexible couplings are hard to keep flexible at -200 C. But the real issue is keeping the fluid pooled around the pump intake, and not distributed in blobs throughout the tank, interspersed with gas.
- jauntywundrkind 3y ago
  Could you try to circularly pump a mixture between the two tanks and create some kind of filter in each tank? The idea of circulating a mixture & taking one you need is for example how donating blood plasma platelets works. If we can separate fuel and pressurant without too much fuss, a similar non-propulsive technique for a cryonic fill might be possible?
  - angiosperm 3y ago
    Mixing fuel and oxidizer is a good way to get a dramatic explosion. Good if you wanted the explosion, not so good if you wanted vehicle and tank to still exist afterward.
  - gpm 3y ago
    The "pressurant" in this case is just evaporated fuel (or evaporated oxygen in the liquid oxygen tank). I'm not sure I understand how your pump idea works though.
    - XorNot 3y ago
      I think the point is that the liquid/vapor mixture is still a fluid, just a weird one - you can pump it with the right type of pump it would just be more akin to ventilating a room to get dust out, for example (i.e. moving gas to move floating solids).
- angiosperm 3y ago
  I haven't seen any plausible indication of how they will manage what is called "ullage". Putting propellers on electric motors inside the tanks, to make fluid circulate axially around the center line, would propel it to intakes on the circumference, with a growing bubble running down the center. Then they would not need to accelerate the vehicle throughout the process, which alternative seems wholly impractical. It would require clever design of the anti-slosh baffles (never shown in cutaway diagrams) to be compatible with such circulation. It might be tricky to keep the whole tank from switching to tumbling end-over-end.
  - chris_va 3y ago
    They brought me in here to do a job, they asked me to stir the damned tanks, and I stirred the tanks!
    - bandyaboot 3y ago
      Oh yeah smart guy? What were the gauges reading before you flipped the switch?
  - Robotbeat 3y ago
    It's as simple as using thrusters to keep the propellant settled. milli-gee thrust (or even lower) has been shown to be sufficient for settling cryo propellant in upper stages during long coasts. SpaceX has been experimenting/analyzing settling of cryo propellant (LOx) during hours-long coasts for years, now. It's a requirement for doing direct-GSO insertion for an upper stage (or even doing any relights or deorbit burns, for that matter), which they've already done. "But wait, it's supposed to be more complicated than that!" No, really, they've been doing this stuff for years, and settling propellant for upper stage coasts really is virtually the same thing as settling for propellant transfer (transferring to the engine(s) for relight vs transferring to another stage... in fact you have to be MORE careful to avoid bubbles when feeding propellant to an engine than you do when transferring propellant to another tank). https://www.youtube.com/watch?v=mVAGoWJuDKk https://www.youtube.com/watch?v=mVAGoWJuDKk They still need to do the whole launch-two-starships-then-dock-them bit, plus quick connect fittings, but they already have years of experience with autonomous rendezvous and docking from operating Dragon at ISS (Dragon 2 has that capability, and it has been used for the uncrewed Dragon 2 demo mission plus the recent cargo Dragon 2 missions, plus I think it's the nominal choice even for crewed missions).
- actionfromafar 3y ago
  So that's why they had to start spinning the space station in Armageddon (1998) before refueling the shuttle! To pool the propellant. Great attention to detail, very scientific movie.
  - avmich 3y ago
    Russian components, American components, all made in Taiwan! Couldn't be said more precisely!
- AYBABTME 3y ago
  Or spin.
- idlewords 3y ago
  This is very well put. There's also a second issue, which is chilling down the receiving tank quickly and efficiently enough that the fuel being transferred doesn't just flash bil and vent off into space. Like rocket combustion, this is very hard to model and sensitive to nozzle geometry, how fast you spray, the phase of the moon etc.
- schiffern 3y ago
  > as soon as the acceleration stops... the tank will contain a chaotic mixture of floating gas and liquid The traditional solution is to replace acceleration/gravity with surface tension, using various types of Propellant Management Devices (PMDs).[0][1] When last we saw our heroes, the stated plan was to solve the problem with milli-g acceleration via small thrusters.[2] That was 2017, but so far there's been no indication this part of the plan has changed. [0] https://en.wikipedia.org/wiki/Propellant_management_device https://en.wikipedia.org/wiki/Propellant_management_device [1] https://www.google.com/search?q=Propellant+management+device&tbm=isch https://www.google.com/search?q=Propellant+management+device... [2] https://youtu.be/S5V7R_se1Xc?t=2872 https://youtu.be/S5V7R_se1Xc?t=2872
- nine_k 3y ago
  A smaller impulse to rotate the whole thing, so that tanks are on different sides relative to the center of rotation, may be easier. At least the system will have as much time as needed to do the pumping after the spin-up. Then another impulse would stop the rotation.
ramesh31 3y ago
>If you are already attempting to get into orbit it makes sense to include this experiment in case you succeed because you save the time and cost of an entire launch and might get a $53 million award while advancing a key requirement for HLS, a $2.9 billion dollar contract and the current core mission. Literally KSP career mode IRL

Tuna-Fish 3y ago

Cryogenic fuel transfer requires a bit more than that. The big issue is that unlike all fuel that has been moved between tanks in microgravity in the past, SpaceX isn't storing their fuel in a flexible tank. Instead, they have the fuel and the pressurant gas in the same tank. Under acceleration (gravity or otherwise), this is not a problem, because the fuel pools naturally "down", with the pressurant above it. However, as soon as the acceleration stops, there is no more down. Instead of having a bunch of fuel on the bottom and pressurant above, the tank will contain a chaotic mixture of floating gas and liquid. The minimum setup for fuel transfer is a maneuvering system capable of a very long, weak impulse (to provide down), and then some kind of pump that moves fuel, because the force provided by the weak impulse of a maneuvering thruster is not enough to do it on it's own.

pokey00 3y ago
Your maneuvering system just needs to be able to get tanks spinning.
- deleted 3y ago
  [deleted]
- angiosperm 3y ago
  Rotating the pair of (tank and vehicle) around a common center, and pumping "up" through the center, could work. But the pair would be very off-center most of the time, and unstable unless attached end to end.
  - nomel 3y ago
    Are flexible couplings (like a hose) not possible to use, in these cases?
    - angiosperm 3y ago
      Flexible couplings are hard to keep flexible at -200 C. But the real issue is keeping the fluid pooled around the pump intake, and not distributed in blobs throughout the tank, interspersed with gas.
jauntywundrkind 3y ago
Could you try to circularly pump a mixture between the two tanks and create some kind of filter in each tank? The idea of circulating a mixture & taking one you need is for example how donating blood plasma platelets works. If we can separate fuel and pressurant without too much fuss, a similar non-propulsive technique for a cryonic fill might be possible?
- angiosperm 3y ago
  Mixing fuel and oxidizer is a good way to get a dramatic explosion. Good if you wanted the explosion, not so good if you wanted vehicle and tank to still exist afterward.
- gpm 3y ago
  The "pressurant" in this case is just evaporated fuel (or evaporated oxygen in the liquid oxygen tank). I'm not sure I understand how your pump idea works though.
  - XorNot 3y ago
    I think the point is that the liquid/vapor mixture is still a fluid, just a weird one - you can pump it with the right type of pump it would just be more akin to ventilating a room to get dust out, for example (i.e. moving gas to move floating solids).
angiosperm 3y ago
I haven't seen any plausible indication of how they will manage what is called "ullage". Putting propellers on electric motors inside the tanks, to make fluid circulate axially around the center line, would propel it to intakes on the circumference, with a growing bubble running down the center. Then they would not need to accelerate the vehicle throughout the process, which alternative seems wholly impractical. It would require clever design of the anti-slosh baffles (never shown in cutaway diagrams) to be compatible with such circulation. It might be tricky to keep the whole tank from switching to tumbling end-over-end.
- chris_va 3y ago
  They brought me in here to do a job, they asked me to stir the damned tanks, and I stirred the tanks!
  - bandyaboot 3y ago
    Oh yeah smart guy? What were the gauges reading before you flipped the switch?
- Robotbeat 3y ago
  It's as simple as using thrusters to keep the propellant settled. milli-gee thrust (or even lower) has been shown to be sufficient for settling cryo propellant in upper stages during long coasts. SpaceX has been experimenting/analyzing settling of cryo propellant (LOx) during hours-long coasts for years, now. It's a requirement for doing direct-GSO insertion for an upper stage (or even doing any relights or deorbit burns, for that matter), which they've already done. "But wait, it's supposed to be more complicated than that!" No, really, they've been doing this stuff for years, and settling propellant for upper stage coasts really is virtually the same thing as settling for propellant transfer (transferring to the engine(s) for relight vs transferring to another stage... in fact you have to be MORE careful to avoid bubbles when feeding propellant to an engine than you do when transferring propellant to another tank). https://www.youtube.com/watch?v=mVAGoWJuDKk https://www.youtube.com/watch?v=mVAGoWJuDKk They still need to do the whole launch-two-starships-then-dock-them bit, plus quick connect fittings, but they already have years of experience with autonomous rendezvous and docking from operating Dragon at ISS (Dragon 2 has that capability, and it has been used for the uncrewed Dragon 2 demo mission plus the recent cargo Dragon 2 missions, plus I think it's the nominal choice even for crewed missions).
actionfromafar 3y ago
So that's why they had to start spinning the space station in Armageddon (1998) before refueling the shuttle! To pool the propellant. Great attention to detail, very scientific movie.
- avmich 3y ago
  Russian components, American components, all made in Taiwan! Couldn't be said more precisely!
AYBABTME 3y ago
Or spin.
idlewords 3y ago
This is very well put. There's also a second issue, which is chilling down the receiving tank quickly and efficiently enough that the fuel being transferred doesn't just flash bil and vent off into space. Like rocket combustion, this is very hard to model and sensitive to nozzle geometry, how fast you spray, the phase of the moon etc.
schiffern 3y ago
> as soon as the acceleration stops... the tank will contain a chaotic mixture of floating gas and liquid The traditional solution is to replace acceleration/gravity with surface tension, using various types of Propellant Management Devices (PMDs).[0][1] When last we saw our heroes, the stated plan was to solve the problem with milli-g acceleration via small thrusters.[2] That was 2017, but so far there's been no indication this part of the plan has changed. [0] https://en.wikipedia.org/wiki/Propellant_management_device https://en.wikipedia.org/wiki/Propellant_management_device [1] https://www.google.com/search?q=Propellant+management+device&tbm=isch https://www.google.com/search?q=Propellant+management+device... [2] https://youtu.be/S5V7R_se1Xc?t=2872 https://youtu.be/S5V7R_se1Xc?t=2872
nine_k 3y ago
A smaller impulse to rotate the whole thing, so that tanks are on different sides relative to the center of rotation, may be easier. At least the system will have as much time as needed to do the pumping after the spin-up. Then another impulse would stop the rotation.

ramesh31 3y ago

>If you are already attempting to get into orbit it makes sense to include this experiment in case you succeed because you save the time and cost of an entire launch and might get a $53 million award while advancing a key requirement for HLS, a $2.9 billion dollar contract and the current core mission. Literally KSP career mode IRL