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The key difference with a plain old-fashioned orbital rendezvous would seem to be that there'd be limited time to only briefly match velocities and trajectories:

Let's say a vehicle launched from the ground on a suborbital trajectory wants to meet a space tug that had slowed down from LEO for payload handover.

Assuming the tug had slowed to 5 km/s, and the ascender accelerated to exactly that velocity, just under 2/3rds of minimum orbital speed. If either the tug, or the ground-launched vehicle were just 1 second off, the vehicles would be 5 km apart, and you'd have to do the measuring, calculation and navigating to bridge that gap within a few minutes before their trajectories had to diverge again, as the tug would soon have to fire its thrusters to get back into orbit before entering the atmosphere together with the suborbital ascender.

If you were just a millisecond off, it'd be 5 metres, and considering that the relative trajectories were divergent, again there'd be little time to fix it.

I can't find anything on this online. It would seem to be a useful technique as it would lower the delta-v for the vehicle that lifts the payload from the ground to the rendezvous point relative to fully making orbit. (NOTE: Yes, you'd have to get extra fuel to the tug to perform this manoeuver but that's potentially a smaller problem.)

Is it just too hard so no one's ever contemplated trying?

EDIT I: The first answer (now deleted) made before this amendment of the explanation and of the title to make it clearer that the question isn't about planes meeting in the atmosphere was technically correct, aerial re-fueling in the atmosphere is a suborbital rendezvous manoeuver. However, I assume the upper vehicle here is on a trajectory to re-enter orbit either because it's the end of a tether that will spin up by virtue of its momentum or a space tug which would have to fire its engines before entering the atmosphere.

EDIT II: As @bitchaser pointed out below in the comments to the first answer, the slowing down might happen via aerobraking to save fuel, and engines on the tug could be optimized for the purpose of re-injection into orbit.

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  • $\begingroup$ I've removed two irrelevant tags. $\endgroup$ – Magic Octopus Urn Apr 5 '19 at 15:11
  • $\begingroup$ One such "suborbital rendezvous" $\endgroup$ – Chris Apr 5 '19 at 15:15
  • $\begingroup$ I am adding the tags again as I don't see what else you could meet that was going back into orbit. $\endgroup$ – Prototypist Apr 5 '19 at 15:25
  • $\begingroup$ Neither skyhook nor space-tug is applicable to this topic, you can click on each individual tag to see what they actually relate to. Skyhooks are a theoretical "crane from orbit" sky hook and space tugs are orbital transfer vehicles. But I won't edit it again. Also, for reference, the fastest Soyuz rendezvous with the ISS was 3 hours and 46 minutes MET (time from launch). Meaning they likely did 2 or more orbits before docking. $\endgroup$ – Magic Octopus Urn Apr 5 '19 at 15:44
  • $\begingroup$ The definition for skyhook attached to this tag here is "A proposed method for orbital launch and transfers. Utilizes a long wire with fixed rotation to manage momentum." which is exactly what I had in mind, transfer momentum to augment a sub-orbital trajectory so that it becomes orbital. Also, I mean space tug in the sense of sub-orbital to orbital transfer which I think is covered by the meaning. $\endgroup$ – Prototypist Apr 5 '19 at 16:24
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I can't find anything on this online. It would seem to be a useful technique as it would lower the delta-v for the vehicle that lifts the payload from the ground to the rendezvous point relative to fully making orbit. (NOTE: Yes, you'd have to get extra fuel to the tug to perform this manoeuver but that's potentially a smaller problem.)

This is the opposite of a useful technique.

The speeds have to be matched, or you have a collision instead of a rendezvous. An ascending ship on a 5km/s suborbital trajectory meeting an orbiting station at 7.7km/s yields a massive cloud of debris. You can't just toss the payload from one airlock to the other as you go by; it's still got a relative velocity of 2.7km/s.

So in saving X amount of delta-v on the ascending ship, the destination station would have to spend X delta-v to slow down to match, and then spend X delta-v again to get back to orbital speed, plus a little more to recover the altitude lost during the slowdown. The destination station is probably substantially larger than the ascender, so that 2X delta-v probably means much more fuel expenditure as well.

On top of that, if there's any delay in achieving rendezvous, or any problem restarting the circularization thruster on the station, you've lost the whole thing.

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    $\begingroup$ I picture a space barge / space tugboat leaving ISS, slowing, picking up load, regaining speed. I don't think that would change the answer much, but conceivably the barge could use aerobraking to slow, and/or be extra efficient using fuel. $\endgroup$ – Bit Chaser Apr 5 '19 at 20:19
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    $\begingroup$ @bitchaser Any feature you give to that barge, it's more efficient to give to the ascender. $\endgroup$ – Russell Borogove Apr 5 '19 at 20:24
  • $\begingroup$ I agree that's true, at least in any configuration I can conceive. $\endgroup$ – Bit Chaser Apr 5 '19 at 20:26
  • $\begingroup$ I wrote space tug as I meant barge, as in a specialized vehicle for this purpose. And yes you'd have to match velocities or you get debris, same with any kind of rendezvous. You'd have less time to match velocities as I wrote, that's why I theorized that it would be more difficult than doing an orbital rendezvous. The question actually was if there is existing research/experiments. Possibly not then, perhaps as the usefulness itself needs to be established first. $\endgroup$ – Prototypist Apr 6 '19 at 4:45
  • $\begingroup$ @RussellBorogove replying to your reply to bitchaser's comment about the ascender being more efficient at what the barge would do: Assuming the same fuel and engines as with an upper stage that does the orbital injection, you might be right in that the barge would require extra air braking and docking gear, making it heavier. However the barge wouldn't need any kind of air drag optimizing fuselage, making it potentially lighter again, and could be re-used as many times as the engines last for. $\endgroup$ – Prototypist Apr 6 '19 at 5:50
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Okay, so I've been super jazzed about this same idea for a while now, so I can't help but chime in. For starters, the problems with rendezvous are quite important, as already stated-- a second off can mean kilometers, etc, etc. Having said that, we've seen computer controlled rockets do perfect suicide burns; not every time, there's still room for improvement, but clearly computers performing absurdly precise maneuvers is not outside the realm of technological possibility. In fact, I would argue that unless we make some crazy unpredictable advances to the efficiencies of our rocket motors (which, being unpredictable, I can't foresee us doing), the only ways to really improve the practicality of our space programs is by improvements in techniques, and the creation and use of infrastructure. Reusable rockets via propulsive landing is a good example of the former; or ability to make computers do very difficult things is a lot stronger than our ability to make thermodynamic systems do impossible-seeming things. I would argue that suborbital rendezvous is similar, in that it is an extremely difficult technique, and different in that it also represents the construction of infrastructure. I think that a pair of onboard computers actively coordinating with each other through every instant of the launch can get the job done. First try? Probably not. The system will need some fine-tuning. But with our tech where it is, I'd rather face down a problem of skill than a problem of physics any day.

And then come the problems with physics anyways. Putting aside the question of whether or not this actually significantly improves delta-V calcs, we should ask how reusable an in-orbit tug like this would actually be. Space is an extreme environment in terms of temperature, radiation, static electricity (in some circumstances), and any sort of maintenance done on any orbital hardware becomes a space mission all its own, even if the tugs have a station to dock to. Sooner or later, someone's gonna have to fly up some spare parts. A critical problem? Not necessarily, but important to consider. Personally, my biggest worry is about the reusability of engines. Within recent years we've seen rocket engines reflown several times ala spaceX-- and indeed, the space shuttle before that. I believe the most times a specific engine has been used on missions is 6 (space shuttle), but I could be remembering wrong. It's probably a reasonable question how many times even the best-designed rocket-engine can be used without a reasonable expectation of failure. Rockets are extreme physical systems.

But lets put all that aside, because this kind of maneuver is super cool-looking, and that also has to count for something, probably. What about delta-V? Here's where it gets kind of fun, I think. Let's take a moment and think about some engineering quantities.

Launch fuel: this is how much fuel our initial booster needs to get the payload module into the desired sub-orbital trajectory. This is a function of the mass of the payload module (payload and fuel included), the efficiency of the booster engines, and the dry mass of the booster itself, return fuel included.

Return fuel: This is the extra fuel included in the dry mass of the booster, so we can land it, because why not?

Payload mass: How much does our satellite weigh, basically?

Payload fuel: how much fuel do we have to add to our payload module to replenish the fuel on the tug spent slowing down to dock and then speeding back into orbit afterwards?

I wrote some MATLAB script to play with all these values, and I found that with the right configurations of engines and payload you can actually get pretty significant improvements to mass fraction, which pairs nicely with the two-stage reusability. I don't think my scripts actually prove anything from a practical sense, just to be clear, but rather demonstrate that as OP suggests, this might be a question worth asking more seriously-- and perhaps more generally; how can we reapply our existing technology to improve space activity without having to wait, fingers crossed, for giant leaps forwards in physics and/or material science?

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    $\begingroup$ Are you accounting for the cost of re-circularizing the orbiter? $\endgroup$ – Russell Borogove Jun 17 at 23:37
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    $\begingroup$ Is there an answer here, or is this a long comment? If there is an answer, a tl;dr would be nice. $\endgroup$ – Organic Marble Jun 17 at 23:45
  • $\begingroup$ Just a long comment. As for the cost of recircularizing the orbiter, it's one of those scale-y things where if you make all the parts big enough, it starts to pay for itself. There's a degree to which the size requirements are a little absurd, but absurd is a relative term. $\endgroup$ – Beebokenobi Jun 19 at 1:05

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