External Tanks in Orbit

Circularization and Maintenance of Orbit

The first technical challenge to any use of an external tank (ET) in orbit is how to circularize its orbit and then keep it there. This generally requires that some form of attitude jets be attached, as well as considering the problem of how to remotely control them.

Tom Abbott: In all on-orbit External Tank space station conversion proposals, a propulsion system is installed just as soon as is practical and the External Tank remains attached to the Space Shuttle until this is accomplished. Positive control of the ET at all times is the only acceptable way to operate.

Lawrence Trowbridge: The ET has exactly the same velocity as the Orbiter at MECO. If there is a discrepancy in the velocity needed to reach planned apogee, it must be made up by an OMS-1 burn. This never happened on my watch. The OMS-2 burn is where the velocity is added to circularize the orbit. That one is not much of a kick, if I remember correctly. A similar delta-v to the ET should put it in orbit, then. Since the Orbiter has a typical on-orbit mass of the order of 220,000 lb, and the ET is 66,000, it would only take 1/3 as much fuel to do this as it does to orbit the Orbiter.

Darrell Holloway: [the ET will] have to be re-engineered to provide for a non-propulsive method of venting off the residual cryogenics onboard. I have seen still photos taken on a mission in the not too distant past where the 2 inch gaseous hydrogen disconnect valve on the ET failed to close properly (spring loaded to the closed position once the tank separates from the Orbiter) and the venting GH2/LH2 produced an coupled pitch/roll motion. Not a motion you'd like to see your future space station module get cozy with before you have a chance to attach stabilizing avionics and attitude control devices. Also, the ET tumble valve would of course have to be disabled prior to flight.

prb@clark.net (Pat): However, you can save a fortune on construction that may make money available for either station-keeping logistics fuel or for a high impulse Ion engine. Maybe something really cool, like a solar-drive. Large mirrors to collect light to power a high temperature bed.

melanied@erols.com (Dave M.): The large mirrors just make the ballistic coefficient worse, not better.

fidevos@eduserv1.rug.ac.be (Filip De Vos): It is not dumb: the solar drive has a much higher efficiency. Since your reaction mass comes from Earth, like the conventional thrusters' fuel and oxydant, you can still come ahead, and have additional savings in the supply train (the ELVs or whatever that have to bring your reaction fluids up).

Tom Abbott: Rather than using mirrors why not wrap the ET in solar cells to provide the power to the ion engines (and for station power, too).

Cleanup - Evacuation of Residual LOX and LH2

When the Space Shuttle jetisons the external tank (ET), there are from 5 to 20 tons of residual fuel remaining in the tank. In order for the ET to be occupied, something has to be done with this material.

Derek Ho: When I spoke to people at NASA about the concept (ETs in Orbit), I kept hearing problems with the idea that I hadn't thought of or heard before, e.g., the tank insulation would outgas and is poisonous to humans in vapor form, cryogenic fuels in the tank are volatile and have a high probability of causing a catastrophic explosion if left in an unshielded ET for any great length of time, etc.

Thomas Simpson: Question: if you put an ET in orbit with it vented (the valves open to space) and perhaps with a little tank of helium to help purge it, wouldn't the H2 an O2 tanks pretty well vent themselves clean, leaving nothing to possibly detonate or contaminate the interiors for future habitation?...

Darrell Holloway: There is some difficulty in venting the ET once in orbit. While there is an oxygen vent valve which is still operable, and could be rigged to open on command from onboard sensors, you'd have to provide long term power to operate it and keep it open (spring loaded to shut position). The valves which connected the tank to the Orbiter (LO2, GO2, LH2, and GH2) are inoperable after separation of the Orbiter since the driving mechanism is on the Orbiter. The LH2 tank doesn't have a vent which can be operated like the LO2 does, once the hydrogen vent arm drops away at T-0 that valve closes for good (held open by the vent arm connection on the ground). So the LH2 side poses more of a problem.

The helium tank for a purge would probable have to be tied in to some sort of logic circuit which would allow the vents to be closed while the tanks pressurized slightly with helium and then opened to vent the helium (and GO2/GH2 with it), and this would have to be repeated several (many) times to get the residual hydrogen out, aka pulse purging.

nhendrix@ro.com (Doug Hendrix): I'm certainly no expert on this but seems like I remember seeing film shot from inside a propellant tank that showed the propellant reaction to MECO. The propellant accelerated to the front of the tank and afterwards was seen floating around in various sized globs. How do you propose to collect all these globs (or do you do that?). I'd expect the tank would have to be absolutely dry before you start putting any electrical equipment in it. Can you expect all the residuals to vent?

deesqrd@vespucci.iquest.com (Dave Dooling): The film shot you saw probably was from the S-IVB stage on an early Saturn IB launch. To my knowledge, the Shuttle ET has never had a camera inside (or outside) because propellant floating is not an issue here. It was for the S-IVB which had to restart to send astronauts to the Moon. As for the residuals in the ET, they are small and probably would sublimate rather quickly. Seems like the best way to handle the problem is to inject warm nitrogen (say, room temperature) into the tanks to encourage sublimation, then let them vent to vacuum. I would not want to trust any electrical or chemical heat sources in there. Just give nature a few days to work.

simberg@ix.netcom.com (Rand Simberg): I think you mean evaporate. The propellants are liquid--not solid.

Dave Dooling I am assuming that what does not boil off right away freezes when tank pressure goes to zero. Film cameras mounted in Columbia's ET attach points on early missions showed ice inside the propellant lines as the tank separated. It was large, fluffy, and loose, but ice. But, yes, the liquid stuff would evaporate/boil.

Mike Zeleski: That kettle is going to Boil for a while until we get to opening it. That is going to put the ET structure under a lot of pressure in a direction that it was not originally designed to resist. Outwards vs downwards... The O2 tank does not worry me as much as the H2 tank does, as it does not have a simple vent-purge valve like the O2 tank does and it also boils at a much lower temp.

gherbert@crl.com (George Herbert): The LH tank can be vented by just exposing it to space for a long enough period of time. As heat seeps into the tank the residual liquid hydrogen will boil off fairly quickly. It helps to have occassional small thruster burns to keep it from floating in globs away from the walls, that makes it take longer.

The oxygen tank will probably take longer, due to greater heat capacity. You may want to seal it and keep the residual oxygen, too. It's useful.

Tom Abbott: According to a study undertaken at the direction of the External Tank Project Office, Marshall Space Flight Center, NASA, Huntsville, AL, "ET Inspection On-Orbit," and I quote:

"For manned ET inspection, the pressure in the tanks is decreased to less than 5 psig. There are three ways to accomplish this:

The first method, through the fill and drain valves, is recommended. The second method has the disadvantage that the vented hydrogen could affect the engine unfavorably. The third method requires modifications to the ET.

Depressurizing through the fill and drain valves reduces pressure to a satisfactory level within the short time of 20 minutes. However, the torque generated by the effluent gases and liquids requires the RCS to offset this torque. This 20 minute time limit applies even for the maximum total residuals (34,000 lbs) considered. Sufficient offsetting torque is available from the aft RCS, with the RCS propellant requirements of 1,066 lbs for the baseline case.

Dealing with the Foam Insulation

The External Tank is covered with orange Spray-On Foam Insulation (SOFI) to insulate the ET, and the supercold liquid H2 and O2 while on the launch pad and during the heat of ascent. There is some concern that this material could erode once in orbit, causing annoying and potentially dangerous debris.

Tom Abbott: The External Tank is covered with orange Spray-On Foam Insulation (SOFI) to insulate the ET, and the supercold fuel it contains, from outside temperatures. After the External Tank reaches orbit, it can be held in a 170 mile high orbit while the SOFI is scrapped off. One study predicts it would take less than a week to strip the SOFI, and the debris would deorbit in from hours to a couple of days, depending on the size of the piece. Another option is to leave the External Tank in a 160 mile high orbit for about a month and all the SOFI would oxidize off of it.

Mike Zeleski: This is a very critical problem as this popcorned foam is the Space equivalent of Hazardous waste on Earth. Can you say Micro-Metorite? Scatter enough debris like this in LEO and even the Shuttle will need its tiles replaced every time it is launched.

Tom Abbott: From the Air Force Institute's "ASSET" ET study: "One way Spray-on insulation (SOFI) can be removed without adversely impacting the EVA budget is to use an automated SOFI removal system. Using the same rail truss that the electron beam cutter rode to cut the LO2 (oxygen) barrel, another electron beam device could move over the surface of the ogive sections, spraying a defocused electron beam. This defocused beam would remove the SOFI in 22 hours. This operation would produce a great deal of very fine SOFI debris. Estimates show, however, that this debris would deorbit on average in half a day."

Mike Zeleski: Find a insulator that will not popcorn in a vacuum. It would have to be just as easy to use as the original spray on foam.

rewinkel@maf.mobile.al.us: The strongest argument against it (ETs) that I've heard is that the insulation outgasses Horribly! Not a good idea when one of your biggest plans is to use optics and other sensers that would get covered with vapor deposited gases.

mheney@access.digex.net (Michael K. Heney): All spacecraft outgas. The solution to that is simple - wait 6 weeks before opening the covers on your optics. (This is what the EOS-AM1 satellite will do).

pusch@mcs.anl.gov (Gordon Pusch): What I wonder is: granted that the insulating foam is necessary to limit hydrogen boiloff and the cascade of liquified atmosphere while the Shuttle is sitting on the pad, is it still necessary to retain the insulation on the way up?

Might it be possible to enclose the ET in a lightweight foam ``sabot'' that could be jettisoned moments before ignition? This strategy would not only solve the ``insulation-removal at orbit'' problem, but reduce the mass needed to be boosted into orbit (not a trivial consideration --- remember what merely eliminating the ET's coat of white paint did to the Shuttle's payload capacity!)

Another possiblity would be to surround the ET with a lightweight envelope that could be pumped full of water-based ``soapsuds'' shortly before the ET is filled. Upon filling the ET, the suds would freeze, forming a lightweight but excellent layer of ``sno-foam'' insulation(tm). Most of the ``sno-foam'' is likely to shake off on ignition, of course --- but since it will have a low density, it should not damage the TPS tiles on the Shuttle's underbelly nearly as badly as solid ice would have. Any remaining ``sno-foam'' will sublime once the ET is in orbit, automatically solving the ``popcorning'' and removal problems...

Gaining Access

dpearce@hcds.net (KDavid): Let's take a quick look at what must be done in space to convert an E.T. to an inhabitable space station.

These are some of the systems that have tank penetrations that currently do not exist.

Tom Abbott: All you have to do is use the existing penetrations of the ET, such as the access hatches or the 16-inch-diameter fuel line. As for the water storage tank, inflatable bladders would also work.

Patrick Patriarca: Are there were access port to the interior of the tank?

Tom Abbott: Thirty-six inch ports are standard docking ports. I don't know if they were made that way on purpose but they will sure work!

Andy Reynolds: The access holes may indeed be 36 inches in diameter, but they are basically manhole covers sealed using a naflex seal and dozens of bolts, nothing standard about that when you're talking about docking adapters. The US hasn't flown a "standard" docking adapter since Skylab. Just what he means by "standard" is anybody's guess. The access port on the ET isn't stressed in the local area for anything except ground use. Slapping a docking adapter onto it would only be the beginning of your troubles.

Tom Abbott: Tests have shown a space suited astronaut can remove the bolts securing the access hatches on the aft ends of both the hydrogen and the oxygen tank of the External Tank and can move into and out of it freely. What we're going to do is make it easy on ourselves and fasten an airlock to the ET just as is done on the Alpha spacestation or the Space Shuttle.

On the subject of the access hatch area of the ET being too weak to support a direct airlock connection, the loads on the interior portion of the airlock can be transferred to the main ringframe just inside the access hatch, and the exterior loads can be carried by the "strongpoints" on the outside of the ET, where the space shuttle and solid rocket boosters attach. It would only be necessary to "clamp" the ET's endcap wall, around the access hatch to form a seal; the ET endcap would carry a minimal load.

Darrell Holloway: While the LO2, intertank, and LH2 tanks are all interconnected by bolts, the aft closure domes of the two tanks are fusion welded to the cylindrical portions of the tanks. By the way, getting access to the bolts in the intertank is a B**** due to the curvature of the tank domes, getting to those hundreds of bolts on orbit, inside of the intertank (a cramped space even on the ground), in a EVA suit, borders on the insane. Thousands of hours were spent in designing Hubble so that the Astronauts could efficiently service it in orbit, and I've yet to hear of any interview with an astronaut involved who didn't credit their success to that design. The ET was not designed with any of this in mind, 36 inch access covers don't make up for that. No handholds, no tie downs, no reference markers, no access panels, nothing, this thing was never intended to be taken apart here on earth, let alone in orbit.

Tom Abbott: The External Tank can be welded in some places and there are other areas which should not be welded. Since welding in space is potentially dangerous, most designs which outfit the External Tank in orbit rely on bolting components together rather than welding them. If the External Tank is outfitted on the ground before launch, as is proposed for NASA's Option C space station or the California ET space station, then welding is not a problem (as long as the thinner areas are avoided).

The most complex part of the assembly will be attaching the airlock to the ET: the support structure for the airlock will have to be placed in position by astronauts while wearing spacesuits. Once the airlock is in place and the ET is pressurized the work will become much easier. There will be much more interior outfitting required for the ET spacestation as compared to Alpha but 90 percent of this part of the conversion can take place after the ET is pressurized. Interior modifications to the External Tank can be made without the necessity of wearing spacesuits because the interior can be pressurized with a breathable atmosphere after the airlock is attached. Working in shirtsleeves instead of spacesuits greatly simplifies and speeds the modifications.

Heating and Cooling

There hasn't been an extensive discussion of this topic, but it remains an important one for any space station.

Mike Zeleski: A Heating and Cooling system with a circulator fan also would have to be provided. If that has to be added first then adding a air scrubber is just another small step... Think... It is just as difficult to work in Parka & Gloves or wear thermal protection as working in a space suit would be.

Tom Abbott: I'd have to look it up, but if I remember correctly, the External Tank reaches an equilibrium temperature of something like 44 degrees after just a few hours of exposure to sunlight in orbit.

Outfitting the Station

dpearce@hcds.net (KDavid): You must add internally all the computers, control systems, air purification systems, ventelation, eating, sleeping, experimental facilities, etc. and they must fit through the aft end of the external tank or the hatch system. Plus, if you want any direct optical viewing capability, that must be in a seperate, non-tank module.

Tom Abbott: This is true, and the study has already been done on how many packages this would require, and how much it would weigh. One space shuttle (capacity 25 tons to low-Earth orbit) could put it's expended ET in orbit and could carry ALL the equipment required to outfit it for 12 people, in the shuttle's cargo bay. Also, installing all this equipment inside will be done in shirtsleeves instead of spacesuits which makes the work much easier. The spaciousness of the ET is also a plus for on-orbit assembly.

A few hours after the ET reaches orbit it attains an equilibrium temperature of about 44 degrees F. Once the ET is filled with a breathable atmosphere, two astronauts could work inside for two months, without using any air purifying equipment, before carbon dioxide levels built up to unacceptable levels. The ET's size gives us a large safety margin even though it shouldn't take more than a few days to get the air processing equipment for the ET, up and running.

Mike Combs - mikecombs@ti.com I seem to recall reading about a study on projected human productivity in space. They concluded that on the one hand, zero G was a tremendous productivity booster. On the other hand, the encumbrances of a pressurized space suit reduce productivity. The two factors tend to cancel each other out, so the conclusion reached was that workers in space would be only about as productive as workers here on earth.

But if the workers were inside of an ET pressurized with a breathable atmosphere, and working in their shirt-sleeves, I would expect them to be more productive than on Earth.

Tom Abbott: Complete ground integration of a space station, such as was done with Skylab, is preferred but this requires a heavy-lift vehicle to put the space station in orbit. Without heavy-lift the ET must be outfitted after it reaches orbit. Of course, we now have a perfect compromise in the $4.7 Geode ET space station, since it can be ground-integrated by adding a 20-ft long aft-cargo carrier (ACC) to the bottom of an External Tank, which carriers all the guidance and control and life support, and it can be launched on a conventional shuttle launch; no heavy-lift required.

If you're going to turn that empty ET "can" into a space station, it won't stay empty for long. An External Tank space station's final mass would be equivalent to the Alpha space station's mass, since we're talking about using almost all the same equipment, the only difference being we'll use one big space station module to substitute for 5 or 6 smaller space station modules.

An External Tank space station outfitted in orbit can have "modules ready to go," too, can't it? Attach an airlock, habitation module or an FGB-type vehicle to the ET's hydrogen tank hatch, pressurize the ET with a breathable atmosphere, and we can outfit 90 percent of the ET space station in our shirtsleeves without the encumberance of wearing a spacesuit.

Ground modifications to get an External Tank into orbit where it can be modified requires the adding of one wire to the safing system so it can be deactivated from the space shuttle after it reaches orbit, and for convenience, we would add a 3 cu. ft. helium tank to the intertank area for use in purging the leftover fuel from the ET. That is the bare miniumum ground modification required to process an ET on-orbit, regardless of application.