I’ve talked a lot about the concept of on-orbit propellant transfer, and how important it is to reducing the costs of interplanetary and cislunar transportation. However, I realize that there aren’t a lot of easy to find resources discussing the problem, and many readers may not know all of what’s involved in the process. While I’m not an expert in the area by any stretch of the imagination, I think I know enough about the problem and the various ideas suggested that I should be able to give a brief introduction for those unfamiliar with the topic.
Rendezvous and Docking
The first step in on-orbit propellant transfer is actually getting the tanker to the vehicle it is refueling (or the orbital propellant depot). I won’t spend too much time on this, because although this is a definitely complicated task, it is one that has been discussed thoroughly elsewhere. The basic idea of orbital rendezvous if for the vehicle being launched to try and match positions and velocity vectors with the object it is wants to dock with.
There has been a lot said about the difficulty of orbital rendezvous, but one needs to keep in perspective the fact that the US has been doing this for decades now. NASA did have some problems with their DART spacecraft, but that stems more from the fact that they were trying to do a fully autonomous rendezvous, as opposed to a rendezvous that is either performed by a pilot on board, or remotely by telepresence. For on-orbit refueling in cislunar space, there really isn’t a need to do the rendezvous autonomously, because communication lag is so short that even if you don’t have a pilot on-board, telepresence is adequate.
A quick distinction also ought to be made between the two standard ways of mating two vehicles in orbit,docking and berthing. Docking is basically flying the two vehicles together using a heavy mechanism that latches the two together after they’ve contacted. Berthing uses some sort of robotic arm to connect the two vehicles together in a much gentler manner, requiring a much lighter connection system. The Russians have prefered docking systems in the past, while the US has been more a fan of berthing systems. A orbital propellant depot might very well have a robotic arm to allow for berthing of tankers and visiting vehicles, while a refueling system that connects directly to the vehicle might just use a docking system. It might also be possible to have a small robotic arm or set of small arms on the tanker if that turns out to be a useful idea.
Once the two vehicles have been mated, some method for connecting the propellant tank on the tanker to the receiving vehicle must be implemented. A manned tanker might have manually attached plumbing umbilicals, while an unmanned tanker might have automatically connecting ones. Depending on how good the alignment is during docking/berthing, this could be a relatively complex or relatively simple. The number of connections that need to be made depends on how many fluids are carried on one tanker. A tanker could carry just one propellant, or it could carry several liquids and gasses. The umbilicals may need to be able to transfer gas from the ullage of the tank being filled back to the tank being emptied. Lastly, umbilicals will also likely need to provide at least some data to the tanker if possible to let it know when to start and stop.
Probably the most difficult part of on-orbit propellant transfer is the propellant management. One of the difficulties in designing propellant tanks that function in zero-G is controlling where the liquid is within the tank. The reasons why propellant location within the tank is important are:
- Many rocket engines can be damaged if their feedlines are sucking gas in, particularly if the engine is turbopump fed.
- If the vent line isn’t uncovered, you could end up venting liquids as well as gasses–this is wasteful, and could be dangerous depending on the liquid.
- If the propellant is floating around unconstrained, the vehicle’s CG can move substantially, particularly for high mass ratio vehicles like lunar transfer tugs. Sloshing propellants can make it very tough to dock with, among other problems.
With these reasons in mind, you generally want to keep the liquid near the outlet (usually at the engine end of the tank), and the gas near the vent port, especially during docking and propellant transfer, and immediately prior to engine firing.
This is particularly important for propellant transfer. As you transfer propellant from one tank to the other, it is a lot easier if you can also either vent the excess gas from the one tank, or suck it off and use it to pressurize the transfer tank. This requires making sure that the outlet of the transfer tank is always covered with liquid (so you aren’t just passing gas between the tanks–that would be impolite to say the least), and that the vent port on the receiving tank is always uncovered.
For storable (ie room temperature or non-cryogenic) propellants, there are some rather easy ways of dealing with this problem. The best being using a flexible diaphragm. The diaphragm is basically a flexible sheet of plastic or elastomer that separates the gas from the liquid. If the diaphragm is impermeable, you can always assure that the gas and liquid are where they’re supposed to be. This also gives you a lot more flexibility with how you pressurize the tank.
The problem is that it’s hard to find diaphragms that are both compatible cryogenic propellants (particularly LOX), while still being sufficiently flexible at those temperatures to avoid cracking and eventually leaking. While the flourocarbon that XCOR is using as the matrix for their LOX compatible composites might just do the trick, there’s no way of knowing how many cycles it will last for. Not to mention that cryogenic liquids tend to have boiloff issues as heat enters the propellant tank from the outside environment. This can quickly create gas bubbles on the liquid side that now need to be dealt with.
There are fortunately several avenues that could be explored for solving this problem, depending on what sort of propellant transfer scheme is used. Since this is just an introduction, I’ll just list a few toss a few out (some conventional, some rather wacky) without elaborating too much for now:
- Gravity Gradients or Tethers–When a vehicle is said to be in a specific orbit, it is actually the CG of the vehicle that is in that orbit. Any portion of the vehicle closer to the gravity well is actually going slower than the orbital velocity of a chunk of matter at it’s precise distance from the center of the planet, while any portion of the vehicle further out than the CG is actually going a little bit faster. This acts like a very slight outward acceleration on anything past the CG, and a very slight downward acceleration on anything planetside of the CG. In small vehicles, these forces are almost negligible. Just enough that some ultraprecise microgravity experiments can get thrown off, but not enough to do too much for short-term propellant settling. These settling forces need to be strong enough to offset any unsettling forces caused by things like equipment vibrations. Over the space of days or weeks, even a constant 1 millionth of a G (in abscence of any other disturbing forces) is sufficient to settle propellant tanks, but in the presence of disturbing forces, and with the time constraints inherent in economically viable propellant transfer, the accelerations need to be bigger.
For very long propellant tanks, large propellant depots oriented with their axis pointing down toward the center of the earth, or vehicles hooked to a sufficietly long tether, the accelerations can be sufficient to settle the propellants in a reasonable amount of time. This requires at least a dozen or more meters long between the two ends of the vehicle, or between the two ends of the station (or the end of the tether and the opposite end of the vehicle). For permanent propellant depots, it is even better if the tether is of the electrodynamic sort, being used for reboost. The constant, but small, acceleration caused by an electrodynamic tether is more than sufficien to insure adequate propellant settling for quick propellant transfers.
A quick google search on tethers and propellant settling can get you more details.
- Tank Pistons–If the propellant tanks are cylindrical, lightweight pistons can be used instead of a diaphragm. Propellant tanks on space vehicles tend to be very light compared to the liquid they hold. It isn’t unheard of for LOX tanks to be only about 1% of the mass of the LOX they hold for low pressure tanks. Adding an extra tank segment long enough to put a tank piston/float that won’t cock will only add another 20-30% to the tank mass, plus the mass of the piston. This might make the tank 2-3% of the propellant mass instead of just 1%. Liquid Hydrogen is low enough density, and requires enough insulation that tanks for it tend to be a lot heavier per lb of propellant, and getting good seals that work at LH2 pressures is more challenging, but this still may be a valid solution to the problem.
- Surface Tension Screens–These are the preferred method on most satellites, but usually end up being a lot more complicated than they sound. Basically, screens, baffles, and other structures are placed througout the tank so that the propellants will stick to the screens. Unfortunately, these can get heavy fast, tend to result in large amounts of unused propellants stuck in the tank at the end of firing, and can cause boiling issues with low temperature cryogens like LH2. But they are still an option
- Fans–one could use a fan with a magnetic coil outside the tank (think flowmeter in reverse) to create sufficient force to send the liquid to one end of the tank. Besides the problem of having moving parts, this might also result in lots of gas entrainment in the liquid if the velocity is too high. It might also augment heat transfer between the liquid and the gas (which may or may not be bad). This would probably need to be used in conjunction with a surface tension screen, but could be used to reduce the complexity of that system.
- Metallic diaphragms–there have been ultra thin-walled metallic diaphragms that could work down to cryogenic temperatures. However, due to wrinkling and other issues, these tended to be single-use items. With modern materials, though, this may now be feasible.
- Spin Gravity–if you are docking the tanker directly to a vehicle instead of depot, it may be possible to spin the two end over end, using the vehicles’ RCS systems, producing some centrifugal forces. Only a very slow rotation is needed, probably on the order of .001-.01G might be sufficient, which implies very low RPMs.
- Propulsive Venting–if one of the propellants is LH2, the boiloff from that can possibly be vented through a low-pressure cold-gas thruster to produce just enough thrust to settle propellants a bit. If you’re going to toss it anyway….
- Ion drives–ion drives have such low T/W that you could possibly fire an ion drive to settle tanks, and so long as you transfer the propellant fast enough, the actual orbital velocity change should be relatively minute. Not to mention that you don’t waste as much propellant. Could cause some interesting issues with ion jet impingement….
There’s probably more ways to skin the cat, but this article is just supposed to be an intro, and it’s already too long as it is.