# Is there any kind of research on *sub*-orbital rendezvous-ing with a space tug outside the atmosphere?

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.

• I've removed two irrelevant tags. Apr 5, 2019 at 15:11
• One such "suborbital rendezvous" Apr 5, 2019 at 15:15
• I am adding the tags again as I don't see what else you could meet that was going back into orbit. Apr 5, 2019 at 15:25
• 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. Apr 5, 2019 at 15:44
• 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. Apr 5, 2019 at 16:24

## 3 Answers

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.

• 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. Apr 5, 2019 at 20:19
• @bitchaser Any feature you give to that barge, it's more efficient to give to the ascender. Apr 5, 2019 at 20:24
• I agree that's true, at least in any configuration I can conceive. Apr 5, 2019 at 20:26
• 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. Apr 6, 2019 at 4:45
• @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. Apr 6, 2019 at 5:50

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?

• Are you accounting for the cost of re-circularizing the orbiter? Jun 17, 2020 at 23:37
• Is there an answer here, or is this a long comment? If there is an answer, a tl;dr would be nice. Jun 17, 2020 at 23:45
• 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. Jun 19, 2020 at 1:05
• It occurs to me that a really interesting use case for this would be something like a Mars (or Venus!) sample return, where the added fuel for our "catcher" to match orbit and recircularise may be offset by the saving from lower launch fuel requirements. I wonder if that might be the most likely place to look for analysis of it. Aug 9, 2020 at 13:11
• Good thoughts, and I am pleased somebody else likes the idea! The most serious objection here is about engine reusability: Attitude control or propulsion for deep space missions need to last very long and allow firing many times over years, but those wouldn't cut it because the extended lifetime is bought with poor efficiencies like low specific impulse with resistojets or poor thrust to weight with ion thrusters: Our space tug would require pretty decent specific impulse and good thrust to weight to close the gap from sub- to orbital velocity as in the example above. Nov 30, 2020 at 13:53

To answer your question: not that I'm aware of. However, it would make a lot of sense if you're bringing fuel to a destination, landing, and trying to return. For example, you'd need roughly 30km/s minimum to get from the equator of Jupiter to low Jovian orbit. Instead of dropping one huge rocket into the atmosphere you could drop one rocket with 15km/s and keep an identical rocket in orbit. When it's time to come back you'd dip the one in orbit into the atmosphere to slow it down and skip back off. At the same time, you'd fire the one on the "surface" to rendezvous with the one that just came from orbit. Payload would be passed off like a baton while on a suborbital trajectory. Once in low Jovian orbit you'd switch to yet another similarly sized rocket (hopefully better specific impulse) to achieve escape velocity and transfer towards Earth. The beauty of this flight plan is that it takes a mission which is utterly impossible with chemical propellants (on the order of 75km/s in total) and breaks it down into 3 or 4 independent rockets which are actually feasible.

All the junk you need in low Jovian / moderately negative c3 orbits would start out loosely captured. Apojove would be lowered slowly by atmospheric braking, preferably using something like a magnetoshell to increase drag without excessively heating the spacecraft. Everything that goes suborbital would need a beefy heat shield but nothing as serious as the Galileo probe. Scaling also works out in our favor.

Science part of the mission might be a glider dynamically soaring on the wind. It would be kinda cool to have a sample return from each of the cloud layers. Probably all sorts of interesting chemistry. Sending a human astronaut would also be possible, but likely very risky and would limit g-forces. Quick burns would be preferrable because of the high gravity.

A more realistic near-term application would be sample return from Venus. For ease of calculation send 3 rockets each with 4 km/s delta V and pass the payload between each one. Compare that to one rocket with 8 km/s and one with 4 km/s. Assuming UDMH N2O4 with isp of 330s, each stage would have a mass ratio of roughly 3.5. All in all, suborbital rendezvous cuts fuel required to return a given mass by 43%, leaving a lot more room for science. Mars sample return would only benefit by -19% fuel, which is still nice but maybe not worth the added complexity. But what do I know - missions using ballistic capture have bent over backwards for less reward in the past. Still, the best use cases are planets with horrifically deep gravity wells and substantial atmospheres. Ideally Venus and the gas planets.

It's important to note that this wouldn't really help you get stuff off Earth unless you're bringing in propellent from deep space. And in that case, may I suggest a momentum exchange tether instead? The premise is similar to a space elevator except it's shorter, situated in low earth orbit, and rotating with a tip speed up to 3 km/s. Fly up on a hypersonic plane or single stage rocket so your apogee coincides with the velocity and position of the tip, then grab on to it when you get the chance. As it rotates, it pulls you up, then prograde, then back down. If you let go at the top, you'll be going over escape velocity. To get yourself to LEO you can have the momentum exchange tether laying on its side and just let go earlier. Doing this will change your inclination, but not send you further than you'd bargained for. You could still get to escape velocity if you choose. The tether loses velocity every time it boosts something to orbit, but it's reversible, so it can be recharged when you decide to reenter. You can also use moon rocks to supercharge it and send more stuff up than you currently have in space. There's a whole bunch of interesting material about it online - Curious Droid YouTube channel has a great video on the practical side of things. Cool Worlds YouTube channel has a good video on a lunar space elevator concept that could provide moon rocks for the additional reaction mass. Hope you find this stuff interesting.