Today I gave a lecture on the Future In-Space Operations Telecon Series on the idea of using LEO propellant depots for interplanetary smallsat missions.
Here’s a link to the archive page, which has both the presentation itself and an .mp3 recording of the talk and the associated Q&A/discussions: http://fiso.spiritastro.net/telecon16-18/Goff_11-28-18/
We went over a lot of the same material that I discussed in the previous two posts, but with more illustrations, and some description of what we were doing that hopefully helps make the idea more clear. The main new addition was a “RAAN sweep analysis” we did to quantify the costs of using this 3-burn departure. tl;dr is that it’s not very painful–less than 3% dV hit compared to using a single-burn departure, and if you’re doing a human mission, and launch the crew to rendezvous on the last phasing loop, you can keep the flight-time penalty to <10days. All told, I was really excited to give this talk. It’s a neat topic, and I’m becoming more and more convinced that there may be a commercial path forward for propellant depots for providing dedicated smallsat launches to MEO, GEO, and beyond. Way beyond.
Latest posts by Jonathan Goff (see all)
- NASA’s Selection of the Blue Moon Lander for Artemis V - May 25, 2023
- Fill ‘er Up: New AIAA Aerospace America Article on Propellant Depots - September 2, 2022
- Independent Perspectives on Cislunar Depotization - August 26, 2022
Jon, I don’t understand how refueling with the sticky booms would work. If you’re having the refueler and the refuelee stand off from each other via the booms, how do you synchronize the ullage control burns? Seems like doing those burns open-loop on both the depot and the target vehicle is going to cause them to drift, which is going to put some torques on the system through the booms–or pull off the booms completely.
Can you do closed-loop control on the thrusters of the two vehicles? How big a mod would that be to the GNC software of the target vehicle? Are the impulse increments of the thrusters on the targets small enough to be controllable? For that matter, are the impulse increments on the Centaur 5 adequate?
You use the Sticky Booms to capture at a distance, but you then retract them almost all of the way to pull the two vehicles together. The thrust levels you need for propellant settling are in the milligee to tens of microgees level, so with the booms retracted most or all of the way it’s doable. There are also ways to do the propellant transfer in microgravity directly, but I’m a fan of the settling burns. There is definitely a coupled control problem, but the thing most people don’t realize is that as you retract the sticky booms they become much stiffer in bending (inversely proportional to the deployed length), so that at short distances they’re way stiffer than a traditional robot arm. Even more so if you have 3-4 of them providing a parallel load path structure intsead of a serial one.
Does it need work to figure out the details–possibly. But I don’t think it’s a showstopper. With how low of thrust we’re talking about, if the accelerations on both sides aren’t perfectly balanced, the torque should be low enough to detect and deal with I would think.
I’d missed the retractile nature of the sticky booms. That definitely helps.
I guess the other option would be to hard-berth the target, on-axis, to the front of the depot kit. But then you have flow the various fluids through/around the payload.
…Although it’d be nice to have a flow-through/around connector, for heavier missions. If you had such a thing standardized, it’d be easy just to launch big honkin’ tanks of storables and attach as many as you needed to the front of the stack on-orbit. That would give you ample prop for outer planets missions, and you could jettison the empty tanks as you go to chop the dry mass down.
I’d suggest you run some system mass numbers on settling vs. microgravity handling/transfer. Even though you may have small thrust requirements – usually it means adding a set of thrusters and the mass of propellant used is not non-consequential.
I agree on the complexity of the coupled loads problem, especially if your orbital period is less than the duration of the transfer. This may drive you towards a hard docking requirement, especially on the larger applications.
I’d probably use settling for transfer, and spin settling for handling. I agree that even if you only need 10s of migro-gees of acceleration to settle things, that the impulse adds up over time, so only using it during transfer seems reasonable. I’m not sure if you can realistically avoid having some level of RCS on the depot just for stationkeeping and collision avoidance maneuvers.
On the hard docking, I really don’t think that will be necessary. When fully retracted, if you have three booms, you have a parallel structure that should actually be plenty stiff. Maybe I’m biased but hard docking systems seem potentially mass inefficient to get the same stiffness as a three point parallel structure like what I’ve been envisioning. But we’ll see more when I get to a point where I have the resources to run some numbers.
Agree you’ll need some level or RCS on the depot. Perhaps using the stored commodity as the propellant. I don’t think you’ll find many scenarios where even settling during transfers is mass efficient. This is mainly because I don’t think there will be as many drivers during transfer to shorten the duration as there is on the Earth.
If you have three hard points connected, aren’t you essentially docked? If one RCS system can control both craft, aren’t you, for all practical purposes, docked?