Selenian Boondocks

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.