Starship On-orbit refueling - Options and Discussion

[oiorionsbelt]

Two starships would the quickest easiest way to transfer 10 tons of LOX from one tank to another.
 Starships by design, plan to have everything needed to accomplish the task and are currently under construction.
Edit: Nope, it says specifically  

between tanks on a Starship vehicle.

[steveleach]

Two starships would the quickest easiest way to transfer 10 tons of LOX from one tank to another.
 Starships by design, plan to have everything needed to accomplish the task and are currently under construction.
Edit: Nope, it says specifically  

between tanks on a Starship vehicle.

Yep.

And as recently noted, 10 tons is about the capacity of a header tank.

I think NASA asked for mission concepts for demonstrating orbital cryogenic refuelling and SpaceX responded with "how about we show you a transfer of LOX between a Starship's main tank and its header tank?" NASA responded with "sounds good, how much will it cost?" so SpaceX produced a bill-of-materials for getting a Starship into orbit.

[cferreir]

It would be awesome if someone could post:

1- How much propellant is transferred by the Progress to the ISS?
2- How do they transfer the two different propellants?
3- How does the hypergolic propellant transfer relate to cryo propellant in terms of temp/pressure?

I tried to search around but have still not found info on it. Anyone?

[steveleach]

It would be awesome if someone could post:

1- How much propellant is transferred by the Progress to the ISS?
2- How do they transfer the two different propellants?
3- How does the hypergolic propellant transfer relate to cryo propellant in terms of temp/pressure?

I tried to search around but have still not found info on it. Anyone?

I found a figure of 650kg of propellant transferred at http://russianspaceweb.com/progress-ms-13.html

There are some additional details under "refuelling module" at https://spaceflight101.com/spacecraft/progress-ms/

I also found this paper on the subject: https://www.researchgate.net/publication/255579266_Technologies_for_Refueling_Spacecraft_On-Orbit

"To avoid the problems of phase
separation a flexible membrane separates the liquid
from the pressurant gas. Then the liquid can be
transferred by pressurizing the tank without worrying
about ingesting vapor. Drawbacks of this system
include life of the membrane, weight and an inability
to deal with vapor evolved from the bulk liquid.
Nevertheless the Progress module includes resupply
tanks holding about 870 kg of propellant (two tanks
of nitrogen tetroxide and two of UDMH hydrazine).
High-pressure nitrogen is used as the pressurant. A
compressor is used to lower pressure in the receiver
tank by transferring nitrogen back into high-pressure
storage bottles. After the lines have been leak checked
the fuel then oxidizer are transferred one at a time to the
station. Separate transfer for each reduces the hazard in
case of a leak. The process can be controlled either by a
ground station or the space station crew."

Also...

"Systems for cryogen propellants such as liquid hydrogen and
liquid oxygen have unique challenges. The large scale of
the systems for which these propellants are attractive
makes any in-tank structure large and complex. No
membrane material that can be used at cryogenic
temperatures has been found. Elastomeric membranes
have poor cycle life in liquid oxygen and hydrogen
diffuses through at an unacceptable rate 43 . At these low
temperatures metal membranes suffer from poor
flexibility and limited life due to cracking. "

[edit: fix quote from paper]

[volker2020]

SpaceX is planing to accelerate the ship, before pumping, which means they don't need a membrane to separate the fluids from the gas.

I think it might be hard, to do the same with the ISS.

I have to admit though, that I don't understand one part of the problem, to me pumping fuel into a engine, is actually the same as pumping it into another tank. We have seen various time, that the process of starting and restarting an engine in space is possible, why should the tanking be any different? I even say it would be easier, because you don't need to pump as much, and bubbles of gas pose no problem.

[ZChris13]

SpaceX is planing to accelerate the ship, before pumping, which means they don't need a membrane to separate the fluids from the gas.

I think it might be hard, to do the same with the ISS.

I have to admit though, that I don't understand one part of the problem, to me pumping fuel into a engine, is actually the same as pumping it into another tank. We have seen various time, that the process of starting and restarting an engine in space is possible, why should the tanking be any different? I even say it would be easier, because you don't need to pump as much, and bubbles of gas pose no problem.

The main difference is that once your engine is started it produces thrust, which will continually settle your tanks. No such easy mechanism exists when you're capturing the propellant in another tank once it passes through your pump.

[r8ix]

SpaceX is planing to accelerate the ship, before pumping, which means they don't need a membrane to separate the fluids from the gas.

I think it might be hard, to do the same with the ISS.

I have to admit though, that I don't understand one part of the problem, to me pumping fuel into a engine, is actually the same as pumping it into another tank. We have seen various time, that the process of starting and restarting an engine in space is possible, why should the tanking be any different? I even say it would be easier, because you don't need to pump as much, and bubbles of gas pose no problem.

The main difference is that once your engine is started it produces thrust, which will continually settle your tanks. No such easy mechanism exists when you're capturing the propellant in another tank once it passes through your pump.

AIUI, the plan has always been to use slight continuous acceleration (~0.1g?) to keep the propellant settled for the duration of the transfer...

[aero]

Would it be possible to just blow the LOX into the other tank? That is, fan gaseous O2 from the full tank to the empty with globs of LOX entrained, then return the gaseous O2 via a filter that mostly keeps the globs of LOX in the tank being filled.

[steveleach]

SpaceX is planing to accelerate the ship, before pumping, which means they don't need a membrane to separate the fluids from the gas.

I think it might be hard, to do the same with the ISS.

I have to admit though, that I don't understand one part of the problem, to me pumping fuel into a engine, is actually the same as pumping it into another tank. We have seen various time, that the process of starting and restarting an engine in space is possible, why should the tanking be any different? I even say it would be easier, because you don't need to pump as much, and bubbles of gas pose no problem.

The main difference is that once your engine is started it produces thrust, which will continually settle your tanks. No such easy mechanism exists when you're capturing the propellant in another tank once it passes through your pump.

AIUI, the plan has always been to use slight continuous acceleration (~0.1g?) to keep the propellant settled for the duration of the transfer...

That's my understanding, yes, though they can potentially stop the acceleration one the propellant is settled and just use pressure & surface tension to keep it that way.

This is one of the benefits of using the primary propellants in your RCS system though; you don't have to worry about running out of RCS fuel from running them for a long time.

I suspect (based on nothing at all) that they'll use electric pumps to speed up the transfer as well.


So no fundamental issues to overcome, but you are working in micro-gravity with cryogenic propellants going through a full day/night cycle every 90 minutes, while trying to minimise losses. No-one has done that before, so you can't just dismiss it as trivial.

[tbellman]

AIUI, the plan has always been to use slight continuous acceleration (~0.1g?) to keep the propellant settled for the duration of the transfer...

More like 0.001 g ("milli-g acceleration" is the words they have used), but otherwise correct.  At 0.1 g, they would very quickly leave LEO...  (And of course, quickly consume the propellant.)

[rsdavis9]

SpaceX is planing to accelerate the ship, before pumping, which means they don't need a membrane to separate the fluids from the gas.

I think it might be hard, to do the same with the ISS.

I have to admit though, that I don't understand one part of the problem, to me pumping fuel into a engine, is actually the same as pumping it into another tank. We have seen various time, that the process of starting and restarting an engine in space is possible, why should the tanking be any different? I even say it would be easier, because you don't need to pump as much, and bubbles of gas pose no problem.

The main difference is that once your engine is started it produces thrust, which will continually settle your tanks. No such easy mechanism exists when you're capturing the propellant in another tank once it passes through your pump.

AIUI, the plan has always been to use slight continuous acceleration (~0.1g?) to keep the propellant settled for the duration of the transfer...

That's my understanding, yes, though they can potentially stop the acceleration one the propellant is settled and just use pressure & surface tension to keep it that way.

This is one of the benefits of using the primary propellants in your RCS system though; you don't have to worry about running out of RCS fuel from running them for a long time.

I suspect (based on nothing at all) that they'll use electric pumps to speed up the transfer as well.


So no fundamental issues to overcome, but you are working in micro-gravity with cryogenic propellants going through a full day/night cycle every 90 minutes, while trying to minimise losses. No-one has done that before, so you can't just dismiss it as trivial.

Except just about every time a second stage has been relight.

[Stan-1967]

AIUI, the plan has always been to use slight continuous acceleration (~0.1g?) to keep the propellant settled for the duration of the transfer...

More like 0.001 g ("milli-g acceleration" is the words they have used), but otherwise correct.  At 0.1 g, they would very quickly leave LEO...  (And of course, quickly consume the propellant.)

Some quick calculations suggest a 250ton SS ( 100 tons propellant) would need around a 2.5kN ullage thruster for .001g acceleration.  If the thruster had a  ISP of around 280 (i.e hypergolic level ISP) it would only consume around 1kg/s.  If the thruster could maintain that .001g for 3600 seconds ( 1 hour), the mass penalty for the burn is then 3.6tons.  Seems reasonable.

As to leaving LEO, you don't have to burn in a prograde direction.  Do the burn 90 degrees from prograde and so as to not change perigee or apogee so that you only create slight inclination change.

[steveleach]

That's my understanding, yes, though they can potentially stop the acceleration one the propellant is settled and just use pressure & surface tension to keep it that way.

This is one of the benefits of using the primary propellants in your RCS system though; you don't have to worry about running out of RCS fuel from running them for a long time.

I suspect (based on nothing at all) that they'll use electric pumps to speed up the transfer as well.


So no fundamental issues to overcome, but you are working in micro-gravity with cryogenic propellants going through a full day/night cycle every 90 minutes, while trying to minimise losses. No-one has done that before, so you can't just dismiss it as trivial.

Except just about every time a second stage has been relight.

So what's your take on this then? NASA are simply so totally incompetent that they have to pay SpaceX tens of millions to do something trivial?

[tbellman]

Some quick calculations suggest a 250ton SS ( 100 tons propellant) would need around a 2.5kN ullage thruster for .001g acceleration.  If the thruster had a  ISP of around 280 (i.e hypergolic level ISP) it would only consume around 1kg/s.  If the thruster could maintain that .001g for 3600 seconds ( 1 hour), the mass penalty for the burn is then 3.6tons.  Seems reasonable.

At a minimum, you will be pushing 340 tonnes: two Starships (120t each) docked together, and 100 tonnes of propellant.  3.3 kN needed, using 1.2 kg/s (assuming your I_sp).

But you will also need to be able to push ~1600 tonnes: two Starships, 150 tonnes of cargo, 1200 tonnes of propellant (from a full accumulation tanker to an empty payload ship, or the last transfer from a tanker to the accumulation tanker), plus ~10 tonnes of propellant for the tanker to land.  In this case, 15.7 kN thrust is needed to achieve 9.8 mm/s², using 5.7 kg/s, or roughly 20 tonnes/hour.

Then it depends on how long a propellant transfer takes (I'm guessing they will aim for around 30-90 minutes for a full 1200 tonne transfer), and the specific impulse of the RCS thrusters.  And "milli-g" is propably just order-of-magnitude correct, so our calculations could be off by a factor 3, or even a factor 5, in either direction...

As to leaving LEO, you don't have to burn in a prograde direction.  Do the burn 90 degrees from prograde and so as to not change perigee or apogee so that you only create slight inclination change.

My point with that remark was just that 0.1 g of settling acceleration would not be reasonable to use.  Just 10 minutes of thrust at ~100 cm/s² gives 600 m/s Δv, which would cause a 4.4° inclination change (if I'm calculating that right), or raise the orbit by several hundred kilometers.  It would also use somewhere between 72 and 340 tonnes of propellant (depending on if tanks are almost empty or almost full).

At ~1 cm/s² (0.001 g) on the other hand, a full hour of thrusting is just 36 m/s Δv.  You definitely need to take heed of it in your orbital calculations, but it's not going to give a huge change in either altitude or inclination.

[eriblo]

Then it depends on how long a propellant transfer takes (I'm guessing they will aim for around 30-90 minutes for a full 1200 tonne transfer), and the specific impulse of the RCS thrusters.  And "milli-g" is propably just order-of-magnitude correct, so our calculations could be off by a factor 3, or even a factor 5, in either direction...

A quick search shows that 1e-4 g and possibly as low as 1e-5 g is considered enough for propellant settling. Centaur apparently does 8e-5 g for longer coasts to keep the propellant settled to simplify handling and lower boil-off. I am guessing that one of the major points of interest will be what transfer rates require what settling accelerations. As has been pointed out settling of propellants is standard but large volume transfers without an running engine is not.

[Morgun]

Is there any reason they couldn't just spin the starships and use centripetal acceleration instead of continuous thrust?

[jabe]

Is there any reason they couldn't just spin the starships and use centripetal acceleration instead of continuous thrust?

direction of fluid transfer would  go away from middle ..want from "left to right"

[Thunderscreech]

Is there any reason they couldn't just spin the starships and use centripetal acceleration instead of continuous thrust?

It's been my painful experience that almost any time I use the word 'just' to describe a solution to a problem that experts see as complicated, there ends up being a reason why they didn't do the thing that seemed obvious and simple to me.

It's possible there will be propellant transfer solutions at some point that involve spinning a pair of docked vehicles along one axis or another, but the latest planned method they've shared with us for refueling Starship involves a very modest ullage burn so I have to assume the mechanical/procedural challenges for that are less than they'd be for something involving spinning, at least right now.

[rsdavis9]

That's my understanding, yes, though they can potentially stop the acceleration one the propellant is settled and just use pressure & surface tension to keep it that way.

This is one of the benefits of using the primary propellants in your RCS system though; you don't have to worry about running out of RCS fuel from running them for a long time.

I suspect (based on nothing at all) that they'll use electric pumps to speed up the transfer as well.


So no fundamental issues to overcome, but you are working in micro-gravity with cryogenic propellants going through a full day/night cycle every 90 minutes, while trying to minimise losses. No-one has done that before, so you can't just dismiss it as trivial.

Except just about every time a second stage has been relight.

So what's your take on this then? NASA are simply so totally incompetent that they have to pay SpaceX tens of millions to do something trivial?

Well there is still docking 2 SS's together solidly and hooking up the LCH4 and LOX leaklessly and robotically. Doesn't sound too hard.

[steveleach]

That's my understanding, yes, though they can potentially stop the acceleration one the propellant is settled and just use pressure & surface tension to keep it that way.

This is one of the benefits of using the primary propellants in your RCS system though; you don't have to worry about running out of RCS fuel from running them for a long time.

I suspect (based on nothing at all) that they'll use electric pumps to speed up the transfer as well.


So no fundamental issues to overcome, but you are working in micro-gravity with cryogenic propellants going through a full day/night cycle every 90 minutes, while trying to minimise losses. No-one has done that before, so you can't just dismiss it as trivial.

Except just about every time a second stage has been relight.

So what's your take on this then? NASA are simply so totally incompetent that they have to pay SpaceX tens of millions to do something trivial?

Well there is still docking 2 SS's together solidly and hooking up the LCH4 and LOX leaklessly and robotically. Doesn't sound too hard.

But that wouldn't match the wording of the NASA award, which was to "transfer 10 metric tons of cryogenic propellant, specifically liquid oxygen, between tanks on a Starship vehicle".

I fully agree that autonomously docking a pair of Starships and transferring hundreds of tons of propellant is a lot harder, but I don't think that's what this award is for.

Like I said up-thread, I think this is NASA giving some money to SpaceX to get access to the data from some of the tests they were planning anyway. And I think they are doing that because they don't have this kind of data yet, and there are a bunch of things they consider still risky about it.