I just watched an NSF video and they say it was Ship 30. It's sitting on the engine installation stand, so it wouldn't make sense for it to be Ship 29.
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Buckminster
Well-Known Member
Fascinating. It never registered that the external spindle shape is an entire turbopump unit (CH4). I assume LOX is on the centerline because it's moving more mass (LOX:CH4 combustion mass ratio is 4:1, so a lot more LOX mass is running through that section).
That is a cool animation. It also appears that the CH4 pumps from the top down to about the middle, where the CH4 is then routed to nozzle heat exchangers, and then returns at the bottom where it flows upwards... whereas the LOX flows straight down in to the engine...
The capability of rocket engine pumps is mind boggling. Pumping tons of liquid every second... with statements like "draining an Olympic sized swimming pool every XX seconds", etc... I've also seen some discussions of the pre-burners developing 10's of 1000's of shaft HP to drive them.
Then there's the temps, pressures, and seals...
The engineering is pretty amazing.
Then there's the temps, pressures, and seals...
The engineering is pretty amazing.
The numbers are available, and they're certainly impressive. Each engine has a mass flow of ~510 kg/s of LOX and ~140 kg/s of CH4. Total ~650 kg/s. That's per engine. With 33 engines, that's over 21 tons of propellant leaving the back end of a booster every second.
That one is a bit far-fetched. An Olympic-sized swimming pool is huge. It holds 3,700 tons of water, while a booster holds 3,400 tons of propellant. The propellants are ballpark the density of water, so they're comparable.
What is probably being referenced is a typical in-ground residential swimming pool, which runs around 30,000 gallons. That's 113 tons, and the booster can crank through that in under 5 seconds. For reference, here's a 27,000 gallon swimming pool. So imagine draining that by the count of 5. The booster is moving some serious mass.
The numbers are available, and they're certainly impressive. Each engine has a mass flow of ~510 kg/s of LOX and ~140 kg/s of CH4. Total ~650 kg/s. That's per engine. With 33 engines, that's over 21 tons of propellant leaving the back end of a booster every second.
That one is a bit far-fetched. An Olympic-sized swimming pool is huge. It holds 3,700 tons of water, while a booster holds 3,400 tons of propellant. The propellants are ballpark the density of water, so they're comparable.
What is probably being referenced is a typical in-ground residential swimming pool, which runs around 30,000 gallons. That's 113 tons, and the booster can crank through that in under 5 seconds. For reference, here's a 27,000 gallon swimming pool. So imagine draining that by the count of 5. The booster is moving some serious mass.
You are correct that I'm pretty sure it was the aggregate amount of liquid pumped by all the engines in a booster (such as the Atlas 5, etc...) when I've typically heard those quotes...
This is along the lines of the "Olympic Sized Swimming" pool comments.. they claim that for SLS:
NSF's latest video includes information from NASA on how SpaceX plans to perform the full scale Propellant Transfer Demo.
Two launches within 3-4 weeks, sometime in 2025. Uses the existing quick disconnect ports for transfer. They'll dock, fire settling thrusters, crack the valves and let pressure differential move the propellants. No pumps for now. Docking will rely on a probe-and-drogue system, which is the type used by Soyuz and Apollo.
Two launches within 3-4 weeks, sometime in 2025. Uses the existing quick disconnect ports for transfer. They'll dock, fire settling thrusters, crack the valves and let pressure differential move the propellants. No pumps for now. Docking will rely on a probe-and-drogue system, which is the type used by Soyuz and Apollo.
So one ship will be planned to have significantly more prop mass remaining after reaching orbit, and the other ship will launch with just enough prop to reach orbit?
The first ship will carry whatever payload it wants to transport to Moon or Mars. The 2nd one will have propellant as its payload, and that propellant will refill the first ship whose tanks should be empty when it reached orbit
Ben W
Chess Grandmaster (Supervised)
I'm surprised they wouldn't use centripetal force (spinning the two craft slowly around each other) to do the settling without continuously burning propellant. Or is only a tiny amount of continuous thrust needed for settling? Can the continuous thrust come from only one ship, e.g. via a downward-pointing thruster on the sides of the ship that are nearly in contact (i.e. near combined center of mass)? How do they set up and maintain the pressure differential? (Just venting ullage gas from the depot?) Or is there a reason the freshly-launched tanker might already be at higher pressure than the depot? Maybe rookie questions, just curious!
Ben W
Chess Grandmaster (Supervised)
MagSafe! (Just trying to envision this on Starship scale...)
As long as the approaching tanker has fine-grained 6DOF maneuverability, and the relative positions in space are precisely known (Tesla Vision! At first I was going to say ultrasonic sensors, but those obviously won't work in space, haha), there shouldn't be anything particularly difficult (relatively speaking) about making an arbitrarily precise computer-controlled approach and docking. Much easier in space, where there's no atmospheric turbulence or external forces to deal with, like there would be on landing / catch.
The Space Shuttle was similar size to Starship, and it docked with ISS just fine. That's probably the closest real-life example to work from.
The NASA guy said that they'd use the same sensor suite they use on Dragon, which is LIDAR. Jack said the NASA guy mentioned "radio" as well, but I'm not sure what that's in reference to.
They want something that's brain dead simple for now, so they're following the way Starships transfer propellant today. No changes, except for the settling thruster(s).
As for the thrust, it can come from one steerable thruster. It would point through the changing center of mass of the pair of vehicles. I assume that the thruster would only be strong enough to maintain a tiny velocity for the combined mass of both vehicles, including propellant. If it's roughly 1500 tons, and you want 0.1 m/s2 of acceleration, then you need a 13 ton thruster. At 0.01 m/s2, then you need a 1.3 ton thruster. For reference, SuperDraco is a 7 ton thruster. But we know that SpaceX has been working on methalox thrusters for Starship. It'll be interesting to see how they do it. Maybe they'll brute force it and use ullage gas to translate the two vehicles as well as to prevent spin.
If they have to maintain thrust for an hour of transfer, then 0.1 m/s^2 accelerates the ships by 360 m/s. At 0.01 m/s^2, it's 36 m/s.
This test may not be a full transfer. They may crack the valves, accept that some amount will transfer, and leave it at that. I wonder if their pressure management is so good that they can keep the tanker pressure sufficiently higher than the receiving ship's.
TunaBug
Member
What I imagined from the description is that the thrust is continuous, and thus the "pressure differential" doesn't need to come from pressurizing the tank. It would just be the thrust providing an artificial gravity: connect the tanks and they'll tend to equalize their levels.
Massive ships are easier because f=ma.
Heck, more mass means they theoretically can self dock just due to gravitational attraction.
The one critical part that does get more difficult is final closure speed since the mating interfaces need to absorb all the residual momentum.
I've always wondered why docking vehicles don't slide past each other at a very close distance (under a meter), equalize velocities so they're motionless at that very close distance, then either thrust gently to close the last bit of distance or mechanically reach out and latch. A tangential approach also helps to avoid hitting the other vehicle with your thrusters.
It requires vehicle structure geometries that allow for this sort of thing, but it would seem a worthy tradeoff. Starships could certainly do it so long as they don't put the mating surface in line with the flaps.
Anything more than connected is the same thing though and it's safer to begin a linear approach from a distance to allow more time to react to problems than create a near miss situation to start with.
Also keep in mind both objects are orbiting and can't maintain a fixed gap without continual thruster intervention unless they are pro/retrograde with identical speeds, in which case they aren't closing the gap.
For ISS, the majority of the approach path ensures that if the docking craft goes dark it will not collide.
Ben W
Chess Grandmaster (Supervised)
Starship refueling does have the advantage that both craft are maneuverable, so if either were to lose power on a collision course, the other could take evasive action. (Unlike ISS.)
Starship is also long enough that tidal forces are not entirely negligible. When pointed directly towards/away from Earth in LEO, the endpoints will experience a tidal acceleration of roughly 0.001m/s^2, or around one ten-thousandth of a G. (Someone check my math.) It’s possible that over the course of minutes or hours, this might be enough to settle the propellant all by itself?
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