Earlier this summer, I stumbled on a fascinating paper while trying to find some quotes for my Space 2009 Propellant Depot paper. The paper I found, Boom Rendezvous Alternative Docking Approach, written by Joseph Bonnometti of MSFC, discussed an interesting alternative to the standard method of bringing spacecraft together. It also provided an interesting insight into the early development of rendezvous and docking systems during the Apollo Era.
Technology Lock-in
One of the most interesting points made in the paper was how often technology can get locked-in by early decisions in a field, often made in situations of very limited data. As documented in the paper, the engineers developing the Apollo docking system admitted that “The selection of a docking system for the Apollo Program was based on limited knowledge because experience with actual hardware in space or from ground-based docking simulations was almost nonexistent.” These early conceptual downselects, in this case with only a few half-scale air-bearing experiments to provide any data at all, often are not revisited in future developments. This puts us in a situation where hasty decisions made early-on, based more on “engineering judgment” end up getting stuck with for many decades after the fact.  While this paper was focused on rendezvous and docking techniques, there are plenty of other examples of similar technology lock-ins in aerospace and elsewhere in industry.
Illustration of Boom Rendezvous
I found the following illustration, pulled from a NASA presentation given by Kirk Sorensen of MSFC (one of Joseph’s coconspirators on many topics including air launch, MXER tethers, and Thorium Liquid Flouride Reactors), to be the best illustration of the concept:
Much like modern mid-air refueling of helicopters and jets, a low-inertia connection is made at the end of booms extended from both vehicles, instead of trying to actively fly the two vehicles into each other.  In this case, the boom is on the order of 10-100m long, giving plenty of space to avoid collisions while hooking up the booms.
Boom Design Concept
The preferred approach for the extendable boom was to use a system like the Bi-STEM, which has been manufactured by Northrop-Grumman for in-space applications for decades:
The Bi-STEM system is sort of like a pair of tape-measures. The coils of spring-steel form into arcs as they are spooled out, and their ends are interlocked, creating a tube that can be actively lengthened and shortened using electric motors or the spools. A polymer tether can be easily run down the center of the assembly, adding greatly to the system’s tensile strength:
The whole assembly can be mounted on a gimbal or a Camfield Joint allowing full 6DOF pointing, using electric actuators.
Advantages
The main advantages of Boom Rendezvous, as detailed in the paper linked to, include:
- Greatly decreased probability of collisions during rendezvous. Of all the possible rendezvous failures, collisions are the most likely to badly damage one or both vehicles. Being able to greatly reduce the probability of damage due to failed docking is critical for operations like propellant depots that may require hundreds or thousands of successful docking operations over their lifetime. With Boom Rendezvous, a missed connection goes from being a serious hazard to being the kind of thing you can easily try again, with the only risk being the ego risk of getting razzed by your fellow astronauts after the fact.
- Greatly reduced propellant requirements for the docking maneuvers. Instead of the hunting problem often faced in current real-world docking operations, the closing is performed almost entirely by electric motors.
- Elimination of plume impingement problems. When maneuvering two rocket-powered vehicles close together, impingement of the jets from the maneuvering vehicle on the other vehicle structures can be a severe problem.  Impinging plumes can spall off structural material or contaminate surfaces and optics. Since all the maneuvering close-in is done using the booms, this is eliminated.
- Much lighter and simpler connection interfaces, since the booms can eliminate any remaining rotational or angular misalignments, and since the booms up close have enough compressive strength that you can very precisely control the final connection loads. Without those extra loads, you can eliminate the heavy backing plates, shock absorbers, and guide petals common in modern docking adapters. And without having to have those in the middle, the latching mechanisms, seals, and fluid/electrical connections can be made a lot more straightforwardly.
- Reduced sensor requirements for the rendezvous/docking. You no longer need to know anywhere near as much about the target vehicle’s velocity and orbit, which allows you to use less sensitive, more robust sensors to make the hookup.
- Less precision required in the initial rendezvous orbit. This may allow for the upper stage of the launch vehicle to do the rendezvous burn, allowing the payload to be much simpler and “dumber” than your typical modern prox-ops stage (like Dragon or Cygnus). This will be important for depot operations as well, because the less smarts the tanker has to have, the lower it’s cost can be, and the more space can be left for the actual useful cargo or propellant.
Applications of Boom Rendezvous
While Boom Rendezvous has many benefits compared to the existing probe-and-drogue based docking systems in-use today, it has a few areas where it really shines:
- Rapid Rendezvous situations. These include MXER tethers, apogee tugs, exo-atmospheric suborbital refueling, and other situations where the vehicle needs to hook-up very quickly. Trying to do that with the high-inertial close-in maneuvering typical of today’s rendezvous and docking systems is begging for a crash.
- Depots and other space facilities. The ability to have the actual docking with a depot occur several meters away from sunshields, tanks, and other hardware increases the odds of the depot being able to last long enough to be economically useful. In fact, it may be possible using Boom Rendezvous for the Tanker/Tankee to offload or onload propellants without ever actually touching the depot itself.
- RLVs. Most early-generation RLVs are likely going to be rather weight constrained. By providing a potentially lighter docking system that doesn’t require as many demanding subsystems, more weight can be reserved for payload and recovery systems.
- Space Tugs. Boom Rendezvous makes it a lot easier to divide the docking up into as many tugs as are necessary for safe operations. Much like how more than one tug boat can be used when bringing a large oceanliner in to dock.
Concluding Thoughts
Boom Rendezvous and Docking is a rather promising approach that I hope sees more investigation. The cool thing is that with the advent of suborbital vehicles, this is the kind of system that could be rapidly matured and demonstrated “in-space” for a tiny fraction of what it would cost to do with purely orbital systems. Hopefully, the changing technological maturation situation provided by reusable suborbital launch vehicles can allow us to finally revisit hasty decisions made during Apollo.
Jonathan Goff
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The existing remote arms could also do this. Would just need practice.
For something of very large mass, a stronger arm could do it.
Something like this ought to be done–it’s risky and a little silly to have the shuttle move right up to the ISS.
Charles,
While it would be possible to use existing remote arms to get some of the benefits of boom rendezvous, I don’t think they’re long enough/low-enough inertia to really give you all the benefits. IIRC the station arm is just under 18m fully extended, whereas the booms we’re talking about here are probably more in the 50-100m range….
~Jon
Let it not be said nothing useful ever comes out of MSFC…
How do you dock the booms without precision measurements? Okay, it needs less precision, still…
Kind of reminds me to the catchers for space tethers.
What about the rotational momentum? Booms can compensate for some velocity mismatch, but if docking velocity mismatch vector is random, that will almost always result in a significant angular momentum. Speed of rotation increases when the boom reels in. Maybe that can be used to make booms less stiff?
Axel,
You handle rotational momentum by using more than one boom. With multiple booms it’s just a question of getting the reeling rates right.
~Jon
It probably reminds you of the catchers for space tethers because that’s exactly what I was working on when I thought the idea up in the first place.
I suspect you’d need at least three booms for complete control, otherwise the arriving vehicle could still pivot about a pitch/yaw axis. I noticed that multiple booms are mentioned in the paper.
Tim,
Honestly I’m not entirely sure. Three sounds right, but there may be some trickiness that would give it to you with two and the right kind of gimbal or camfield joint system on both ends. Not sure.
~Jon
A single probe plus an RCS to rotate the spacecraft may be sufficient. You only need one fishing line and one hook when catching a fish.
The probe firing mechanism may need to move left-to-right and up-and-down. If the probe attaches to the side of the hatch a method of performing the final aligning the two spacecraft will be needed. It only has to work over say the last 6 inches, it can use a conventional electric motor but may need to rotate the spacecraft through +/- 180 degrees.
AM,
Part of the point of boom rendezvous is to avoid needing to use RCS near the other spacecraft, so you don’t have to worry about plume impingement. Adding a third boom isn’t that big of a deal. It doesn’t have to be anywhere near as long as the first boom. You could probably get away with the first boom set being in the 50-100m, and the 2nd and 3rd being only in the 5-20m range.
~Jon
If the system is symmetrical the first pair of probes could be connectors attached to cables on winches. They would bring the
two spaceships close together. Solid booms can then be used for accurate alignment and to stop unwanted movements, particularly roll.
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This reminds me generally of a “tri-stirrup” arrangement I came up with for a specialized neutral bouyancy docking mechanism. The idea was to be able to stick a single “toe” in a frustrum of a triangular pyramid stirrup — with three of the stirrups arrayed 120 degrees apart around the central docking port.
As far as the distal probe ends of the booms are concerned — you end up thinking about male-vs.-female geometry questions and whether you want “keyed” geometries so things can mate in only one orientation or attitude.
It occurs to me that you may want a ferro-magnetic fluid clutch/brake on a volumes of revolution geometry male/female pair of parts for the distal end of the first boom pair — to dampen torques in a gradual process controlled by the receiving “dockee’s” docking operations attitude control computer. Once the relative rates of rotation have been dampened to a minimum through the single made rotary joint — the second boom can be used to complete the “summation” of the combined inertias — with only a small residual inertial delta for the dockee attitude control to deal with.
This definitely is a promising alternative to what is effectively 50-year-old SOP for rendezvous and docking — if for no other reason than the kinetic physics of it.
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As long as we’re being inventive, I don’t see why we need all of the links from the two spacecraft to be booms of similar type.
The boom is at least semi-rigid in 6 DOF (perhaps least so in torsion around its axis, at least for STEM types). But even with a lightweight connector on the end, you won’t be able to move it laterally very quickly once you have it out to 100 meters, and stopping its whip oscillations would be an interesting control problem.
So it looks to me like you want one long stiff boom (from one side only) which can be shot toward the approximate location of the target spacecraft. Then, you want a much lighter-weight and shorter boom from the target spacecraft. Once contact is made between the lightweight boom and the stiff boom, the lightweight boom can be retracted all the way to its spacecraft, thus making a stiff connection between the stiff boom and the target spacecraft.
(In an extreme version of this, the lightweight booms wouldn’t be booms at all, but ropes with free-flying remote-controlled microthruster pods. But I digress.)
If the stiff boom is on-orbit permanently, then its length does not need to be variable. It can simply be left extended, pushed back and forth through a gimbaled ring for 3DOF positioning. At that point, you can think about using something like NASA’s SAILMAST for the boom: http://nmp.nasa.gov/st8/tech/sailmast_tech1.html
Sailmast is impressive: very stiff in 6 DOF, and 35 grams per meter. If you had a direct spacecraft-to-spacecraft connection with Sailmast, you probably would not need a second boom. Which is good, because if you have trouble stopping relative rotation with one boom, you’ll probably have even trouble with two similar booms. Once you have two booms connected, if you don’t stop rotation within the first half-spin, then the booms will twist up in a very weak and probably destructive configuration. (Don’t cross the booms!)
A permanently-extended boom would, of course, extend beyond the docking adapter, but it wouldn’t need a whole lot of angular room to swing in. Or, it looks like a partially-deployed Sailmast is stiff for all but the last meter or so, so you might be able to extend and retract it usefully, with the force-transfer points a meter from the coil, as long as you don’t extend it fully and let it snap into its locked-extended configuration.
A separate question: I don’t understand why it would be useful for the main tether to be able to pull with more force than it can push. If you start the spacecraft moving with a forceful pull, you’d better be able to stop them with a forceful push. In theory, you could pull hard on a long boom, then wait till it got close, and when the boom was short enough to be stiff, give it a hard push to stop it. But that seems overly risky. (I’m remembering the line from Heinlein’s _The Rolling Stones_: “He started it with a pull; he thought he could stop it with a shove. They had to amputate both his legs, but they saved his life.”)
Chris