This idea was born from the confluence of three things: (1) the physics of temperature control inside a thermal radiation enclosure, (2) the concept of flying just a shuttle cargo bay and manipulator arm as a permanently-orbiting item, and (3) the promise of the mechanical counter-pressure (MCP) approach as a means to a supple space suit. Update 2-21-14 in red below, just before "conclusions".
Combining these into a real on-orbit repair and assembly facility addresses two needs that we know we have. One is the on-orbit maintenance and repair capability that proved so valuable with the Space Shuttle, as regards the Hubble Telescope repairs, and some others. We already know we need to restore that capability now that the Shuttle is retired from service. (This does mean going to the item needing repair, a significant on-orbit maneuvering capability.)
The other is related to the assembly techniques used for the International Space Station (ISS), but goes far beyond that capability. ISS assembly involved docking of modules, hooking up large quick-disconnects for plumbing and electrical, and the turning of fairly large nuts and bolts (which proved quite difficult). We need to add the capabilities of doing small, detailed work in wiring, plumbing, the turning of very small nuts and bolts, and multiple kinds of riveting.
The ability to do in-space assembly including astronaut’s fine motor skills makes possible the on-orbit assembly of vehicles and objects that are too wide to launch within the payload shrouds of existing (or contemplated) launch rockets. One very pertinent example would be the lander vehicles needed for manned surface missions at Mars. These designs tend to exceed 10 meters diameter, in form factors that are stable for landing on rough ground.
Earlier Related Articles
There are articles already published here on “exrocketman” that relate to two of the list of three things in paragraph 1 above. As regards item (2), see “End of an Era Need Not Be End of a Capability”, dated 8-2-11. This dealt with launching what amounts to a shuttle bay, manipulator arm, and operator’s cabin into orbit as a permanent item manned intermittently at need.
As regards item (3), see “Fundamental Design Criteria for Alternative Space Suit Approaches”, dated 1-21-11. This one dealt with minimum requirements for pure oxygen breathing pressures and the corresponding compression levels balancing it in the body. It has direct application to mechanical counter-pressure suit designs, but applies to any design.
Item 1 Radiation Enclosure for Temperature Control
Objects in space get very hot on the side facing the sun, and very cold on the side facing darkness. This causes all kinds of misfit and alignment problems for any kind of assembly work, due to thermal expansion and contraction. Astronauts handling items in this condition need thick insulated gloves to avoid thermal injuries to their hands. Thick gloves greatly reduce dexterity. Thin gloves with far greater dexterity might be possible for some suit designs, if this insulation requirement were avoidable.
If the assembly and repair space were pressurized and heated as a shirtsleeve environment, the warm atmosphere would equalize and stabilize workpiece temperatures at something tolerable barehanded. But, such a hangar must be pumped down to zero pressure to open the door for access, and that pump-down process is inherently inefficient. Atmosphere gets lost, inevitably.
With the cost of breathing gas launched to space quite high, this loss is intolerable. That points toward an unpressurized work space with astronauts in suits. One benefit of the unpressurized enclosure approach is that it can be very lightweight and easy to build.
If the enclosure were a spaceframe grid of light tubing with six-way corner connectors, then reflective sheeting can be hung between the tubes. With lighting hung on the inside of that frame, light bounces inside such that all physical objects approach whatever equilibrium the power of the lighting supports. The low emittance (reflective outside as well as inside) of the enclosure sheeting controls the power radiated to deep space. The sheeting exterior reflectance also limits what can be absorbed from the sun.
Basically, one adjusts the lighting power inside the enclosure to achieve room-temperature work pieces inside. Having the lighting come from many directions (lights all over the inside of the spaceframe) banishes shadows, making workpiece visibility far better. Inside such an enclosure, the astronaut’s suit need only supply compression for breathing purposes. Thick insulating protection from hot sunlight and cold darkness are not necessary. That makes a supple space suit an integral part of this capability. And, in an unpressurized enclosure, opening the “door” (one wall) loses no breathing gas. This concept is illustrated in Figure 1.
Figure 1 – Temperature Control in a Radiation Enclosure
Item 2 Having Shuttle Bay Servicing Capability Without a Shuttle
The tremendously valuable repair capability we had with the Space Shuttle derived from its cargo bay (with its work-piece restraints), and its manipulator arm. The arm operator had a pressurized cabin with windows, which made many jobs far easier to do. In addition, the arm itself was a stable place for spacewalking astronauts to “stand” while working outside. The problem was the huge expense of launching all of this gear each and every time it was needed, as part of a very large craft.
A less expensive but equivalent capability would be the essential cargo-bay restraint frame with manipulator arm, and operator’s cabin, launched once, and simply left on-orbit. This facility would be intermittently manned when needed, using a much smaller craft as the crew taxi. This basic concept is shown in Figure 2. To make it practical, this facility would also need a very significant on-orbit maneuver capability, since many of the items that might need servicing would not have maneuvering capability for themselves.
Figure 2 – “Shuttle Bay Only” On-Orbit Concept
Item 3 Supple Space Suit
Modern space suits are gas balloons around the human body. These must be well-insulated against hot and cold extremes, and must include some sort of cooling system to deal with the waste heat and moisture produced by human exertion. No such balloon suit so equipped has allowed dexterous activity. Small plumbing and wiring, riveting, and turning small nuts and bolts, are all simply impossible, when dressed like this for space.
An alternative vacuum protection approach is the so-called mechanical counter pressure (MCP) space suit. In terms of its internal tissue pressures, the body does not know gas pressure from mechanical pressure exerted upon the skin. What is required is tissue pressures that balance the breathing gas pressure in the lungs, which in turn must supply an adequate partial pressure of oxygen. This is complicated by the “displacement” effect of water vapor at body temperature inside the very wet spaces of the lungs. (From a suit stiffness standpoint, lower compression is better dexterity, no matter the approach.)
Minimum compression is achieved with pure dry oxygen breathing gas feeds. A rebreather with carbon dioxide absorber and a makeup oxygen supply is the most practical approach to that kind of breathing gas supply. The minimum partial pressure of oxygen is a fuzzy limit; there is really little reason to make this any higher than in sea level air (1 atm total pressure). Many aircraft applications do not require the use of supplemental oxygen until one reaches 10,000 foot (3.0 km) altitudes (0.6878 atm total pressure). The truth probably lies between those figures.
Air composition (20.94 volume percent oxygen) and altitude pressure determine the partial pressure of oxygen out in the dry air. That would be 0.2094 atm at sea level, and 0.1440 atm at 10,000 feet. In the wet lungs, one must “displace” some of the dry air with water vapor at body temperature (0.0620 atm at 98.6 F or 37.0 C). Subtracting this reduces the partial pressure of the dry air (and its components) at the same total pressure. Those in-lung partial pressures of oxygen would be 0.1474 atm at sea level, and 0.0820 atm at 10,000 feet. The displacement pressure is absolute: it does not scale with total.
Adding that water vapor pressure back in determines the total dry pure-oxygen breathing gas feed in that kind of a space suit. That is the compression the suit must achieve on the body, regardless of how it is achieved, in order to balance the breathing gas total pressure. All of this is shown in Figure 3.
Figure 3 – Determining Oxygen Pressure and Suit Compression from Equivalent Altitude Conditions
In a two-gas system, one adds the water vapor and in-lung oxygen pressures together to determine the dry oxygen partial pressure in the dry two gas mixture being fed. Its composition percentage then determines the total breathing gas pressure, and thus the required suit compression level.
The first successful MCP suits were a series of elastic leotard garments developed by Dr. Paul Webb in the late 1960’s and early 1970’s. These garments squeezed the body for tissue compression, and were combined with a pressure breathing helmet and breathing tidal volume bladder, as shown in Figure 4. Note the thin compression gloves. Complete with backpack and helmet, this rig weighed 85 pounds. Dr. Webb typically used breathing gas pressures between 0.22 and 0.29 atm, more than necessary.
Figure 4 – Webb’s “Elastic Leotard” Version of an MCP Space Suit
These experimental suits provided a great deal of dexterity and mobility. They were a bit difficult to don and doff, being 6 or 7 layers of essentially very tight pantyhose material, but were much lighter, and eliminated completely the need for a cooling system (just sweat right through the porous garment). Small tears do not lead to loss of compression. See Figures 5 and 6 for examples of high mobility, done in the atmosphere at elevated breathing pressures counterbalanced by suit compression, using a “hookah rig” air supply. See Figure 7 for a photo of the test subject pedaling an ergonometer in deadly vacuum, wearing nothing but the elastic leotard rig.
Figure 5 – Backbend Mobility in MCP “Elastic Leotard”
Figure 6 – Ladder-Climbing Mobility in MCP “Elastic Leotard”
Figure 7 – Webb’s MCP “Elastic Leotard” Being Tested in Lethal Vacuum
Dr. Webb’s design was never developed for use (although in my opinion, it should have been). NASA in recent years has funded Dr. Dava Newman at MIT to experiment with another MCP variant that she terms the “Bio-Suit”. This is a compression garment that is simpler and easier to don than Dr. Webb’s design, being based on fewer layers of substantially-more sophisticated, directionally-tailored material combinations.
Dr. Newman claims to have achieved compression levels near 0.25 atm, and thinks 0.30 atm to be feasible. Actually, those levels she has achieved are pretty close to what Dr. Webb achieved, so as vacuum protection, the garments are really quite comparable. Her design is clearly very supple and mobile, as depicted in Figure 8. And it is already fundamentally adequate as a compression garment.
Figure 8 – Newman’s "Bio-Suit" MCP Garment at MIT
The fundamental equivalent-altitude compression requirements range from 14% atm at 10,000 feet equivalent to 21% atm at sea level equivalent. Both the older Webb “Elastic Leotard” and the newer Newman “Bio-Suit” designs already exceed these compression requirements. What has held this technology back the most seems to be a 33% atm compression requirement from NASA, to match the pressures used in their current suits.
This higher compression level seems to be driven more by safe but immediate decompression from near 1 atm air to much lower-pressure pure oxygen, without risking the bends from the nitrogen dissolved in the blood. However, it might be worth incurring the necessary decompression time in order to obtain the other advantages of the MCP suit at currently-feasible compression values, while still maintaining the safety of “real air” in the habitat (a fire safety issue).
The On-Orbit Repair and Assembly Facility
Putting all of this together looks like the concept depicted in Figure 9. There, a restraint bed with a manipulator arm is located inside a spaceframe covered in reflective sheeting, with lights on the inside. One wall is the “door” through objects may be admitted to, and withdrawn from, the work enclosure. There is a pressurized cabin with a window located at one side, in which the operator of the manipulator arm may work in shirtsleeves.
Adjacent to the arm operator cabin is an airlock chamber for astronauts, that leads either inside the work bay, or to outside the facility, as needed. There is also a habitat module adjacent to the arm operator’s compartment, in which off-duty astronauts may sleep, eat, and recreate (this could be one of the Bigelow inflatables). At its extreme end is a multiple docking adapter for any of a variety of crew transfer vehicles (such as the Russian Soyuz, the Spacex Dragon, the Boeing CST-100, or the Sierra Nevada Dreamchaser).
An adjacent service module contains an orbital maneuvering engine and its propellant supply, makeup breathing oxygen, and other life support supplies. All are brought up with the crew, and transferred from the taxi to this service module.
Individually, all of these modules could be launchable by existing rockets, and assembled by docking and hookup, exactly as the ISS was. The spaceframe is an easily-erectable structure put together much like “Tinkertoys”. The reflective sheeting could be attached by nothing more sophisticated than Velcro. The lighting merely clamps to the frame tubes on the inside, powered by extension cords that also tie to the frame tubes with Velcro.
Figure 9 – The On-Orbit Repair and Assembly Facility
As it says in the figure, astronauts inside the enclosed bay need no insulating garments, once the lighting has brought the bay and its contents up to room temperatures. The MCP suit used inside need be nothing more than just the “vacuum-protective underwear” that Dr. Webb tested long ago: a basic compression garment and a breathing rig. Inside the facility, the self-contained oxygen backpack isn’t really needed, since a “hookah rig” tapping facility supplies could be substituted, thereby enhancing mobility and dexterity further.
For astronauts venturing outside the facility, more would be required, but these need only be conventional insulating clothing, almost exactly what we use here on Earth. For example, simply add a white insulating coverall (or pants and coat), plus white insulating gloves, and white insulated overshoes, all worn over the very same compression garment.
Add the same helmet, and the standalone backpack, plus a safety tether to the facility. It might even be convenient to fit the helmet with a broad-brimmed hat to shade the head from the sun. This is depicted in Figure 10. Use hard hiking boots, and the astronaut is ready to explore on Mars or the moon.
Figure 10 – MCP Suit as “Vacuum-Protective Underwear”, with Outer Garments Added as Appropriate
The upshot of accidents and experiments since about 1960 is that exposure of a body part to vacuum results in tissue edema (swelling), as the liquid in the blood diffuses through the blood vessel walls into the intercellular spaces. But, this requires time: it is not immediate. In fact, on Capt. Kittinger’s first balloon flight for the high-altitude bailout tests, one of his suit gloves failed to pressurize. That hand was exposed to lethal vacuum for hours, and swelled up, was painful, and was useless. A couple of hours after landing, the recompressed hand was normal and functional.
Combine that with the piecemeal nature of an MCP suit and the room temperature of items inside the unpressurized work bay described here. MCP suits are not one piece garments. You can take pieces off, such as gloves and booties, without disturbing the compression achieved in the rest of the garment. The experiments and Capt. Kittinger’s experience indicates it is 20 to 30 minutes before swelling begins, after exposure of a hand to vacuum.
What that means is that for very fine work, fine enough that even the thin compression gloves of an MCP suit interfere, you can remove the gloves and work barehanded for a short period of time, as long as nothing is cold enough or hot enough to cause thermal injury. The work bay described ensures “room temperature workpieces”, so thermal injury is not an issue inside this space, once thermal equilibrium is achieved. The astronaut might spend 10 to 20 minutes working safely barehanded in vacuum inside the facility, before needing to recompress his hands with the gloves (or going inside the pressurized space).
That startling possibility is the direct consequence of what we know about physiological responses to vacuum exposure, the radiation physics of enclosed lighted spaces, and the modular nature of MCP space suit construction. This combination of technologies, plus that possibility of barehanded work in vacuum for really fine work, is a revolution waiting-in-the-wings for astronaut dexterity.
Putting together these technologies into an on-orbit repair and assembly facility is actually a very necessary prerequisite to many of the things we might wish to do in space.
A facility like this makes major repair and maintenance of on-orbit satellites and vehicles very possible once again, as it once was with the Space Shuttle: things like Hubble and its descendants. Yet, launching the facility once, and manning it intermittently at need with smaller vehicles, is by far less expensive than Shuttle ever was.
Being able to assemble large objects on-orbit from components makes possible vehicles and equipment too large to launch within current payload shroud dimensions. That makes manned landers “large enough to do a proper job” feasible, for future missions to Mars, the moon, or even Mercury. Those design concepts are entirely over-constrained by shroud dimensions today, among other things.
There is nothing “far-future” or “blue-sky” about the technologies proposed here. Only the combination is new. Everything is already demonstrated to be fundamentally feasible today. Only the “final debugging” of the spacesuit and spaceframe items still needs to be done, and it is quite clear that those could happen very quickly, and for not a heart-breaking expense.
The MCP suit in particular, “done right” as a mix-and-match suite of conventional outer clothing worn over a basic set of vacuum-protective underwear, is exactly what our astronauts need in order to do much more effectively their exploration, or just about any activity imaginable.
For maximum crew self-repair and self-rescue capability on a long mission (like going to Mars), it might be wise to include a version of this facility as a part of their ship. It could come in very handy making critical repairs.