External Tanks in Orbit

Participants:

• A. Marc Passy | passy@flex.net
• Tom Abbott | tabbott@intellex.com
• Frank Crary | fcrary@rintintin.Colorado.edu | CU Boulder
• Dave O'Neill | dave_on@hotmail.com, david.o'neill2@virgin.net
• Thomas Moore | t_moore@bellsouth.net
• Solinar | Solinar@NO.SPAM
• Gregg Germanin | gregg@sboat.harvard.edu, gregg@head-cfa.harvard.edu
• IanD | iand@ozemail.com.au
• pat | prb@clark.net
• Chris | chris@ecpccafe.com
• aerodesic | drjvhhill@eudoramail.com
• Jorge R. Frank | jrfrank@ibm-pc.org
• RACEnCART | racencart@aol.com
• Jeff Findley | jeff.findley@sdrc.com
• George Gassaway | gcgassaway@aol.com

Note that the individual authors may retain copyright to each of their respective statements. The goal of this page is to archive the Usenet discussion in a fashion which allows interested parties to become aware of previous discussions on this topic.

Pros and Cons

Marc Passy: I worked as an intern many years ago at Martin-Marrietta Michoud Division in their Advanced Technology section. Even they didn't propose refitting an ET. They wanted to store stuff in the intertank or in an aft endcone.

Tom Abbott: I would have to dig into my cardboardbox database to get them, but I have references from the head of the ET program promoting using ET's as human habitats. This was her personal opinion and not official Martin policy but that's good enough for me. The chief designer of the ET also had a very nice plan for turning an ET into a human habitat. One of the better studies I've seen, in fact.

Tom,
Have you ever built anything even moderately complex? Wired a house?

Marc Passy: Have you seen the Service Module, Lab, Node 1 or any ship or submarine under construction?

Tom Abbott: Yes, I have.

Marc Passy: Have you seen the simple tasks that are labeled as difficult when EVA? (extra-vehicular activity)

Tom Abbott: Yes.

Marc Passy: Or even when IVA? (intra-vehicular activity)

Tom Abbott: I would say IVA is much easier than EVA.

Frank Crary: Is that IVA in suits (pressure work) or IVA with a working life support system?

Tom Abbott: That would be IVA (internal vehicular activity) where the ET is pressurized with a breathable atmosphere, so one can work in their shirtsleeves. It would be just like working on Earth except you would tend to float around if not tied down to something.

Frank Crary: IVA with a working life support system is definitely easier than EVA, but IVA without a life support system and in a suit is worse than EVA. You have most of the problems of EVA work plus confined space and lots of things to get in your way.

Tom Abbott: Yes, I agree. The object is to get rid of the spacesuit, and that can be done as soon as the ET is pressurized, and before lifesupport equipment is installed.

Tom Abbott: I didn't say easy, I said easier.

Marc Passy: Did you watch the FD9 highlights from the STS-88? Sergei K and Nancy C were securing the deck and overhead hatches in the FGB and had to wedge both feet in the deck corners AND hold on, just drive a few bolts!!! Or the video of them removing the Current Converter Unit on the FGB the day before? Eight connectors, eight bolts, (on and off), two tie wraps, TWO HOURS! It's a 30 minute job on the ground!!!

Tom Abbott: I'll agree micro-gravity will be a small encumberance, but I recall a quote from one of the Skylab astronauts to the effect that: "if we can tie our feet down properly, we can do anything in orbit that we can do on Earth."

Marc Passy: A space vehicle, any vehicle for a harsh environment, is a horribly complex beast, and to postulate, even for one second, that an expended ET could be used for anything other that tankage (makes a great sanitary tank, though) is absolutely Science Fiction.

Tom Abbott: Oh, I wouldn't say so.

Frank Crary: Well, it is definitely not science fiction. I'd say it is a very demanding and difficult EVA/IVA job. Quite possible, but it would be hard and require lots of work. On the other hand, hard and requiring lots of work is a fair description of what building ISS will take, and I really like the volume an ET-based space station would provide.

Tom Abbott: In fact, ISS would be much more demanding, IMO, since most of the activity will be EVA, while most of the activity of converting an ET would be IVA, without the need of a spacesuit.

We've thrown away over 80 of them so far. Somebody ought to be mining these, if they are looking for space resources.

Marc Passy: Certainly the technology is there, that's not the issue. It's the expense and trouble. The can, the shell is the LEAST of the worries - look at TransHab. They're using a balloon!!!

Tom Abbott: Yes, but they have to pay to launch that balloon. The ET is practically free, and you can get one on practically any shuttle launch (well, maybe not during space station construction).

Frank Crary: Given the work required, I suspect a ET-based space station would require as much EVA/IVA work as ISS. But I also suspect that it would result in a much larger and more useful space station. I guess that means that ISS is a sub-optimal design, but that conclusion hardly surprises me.

Marc Passy: The ET idea is so prohibitive in terms of LABOR, the one thing technology can't do much to reduce.

Frank Crary: This is incorrect. The difficulty of EVA work is largely a result of the space suits used. Those suits are 1970s technology, and not (in my opinion) a good design. New suits designed with modern technology and a better design would greatly reduce the labor required for EVA work.

Tom Abbott: They did a feature on CBS this morning about the new spacesuit gloves used on this latest shuttle mission. It seems the designer of the gloves is the daughter of one of the astronauts and he told her this morning on TV that they had worked very well. I think NASA needs to put a lot more money into spacesuit upgrades, but they are making some progress it seems.

Tom Abbott: Well, let's think about this a minute. Let's say I manage to get an ET in orbit and get an airlock attached to it. Once I fill the ET with a breathable atmosphere, I will be able to stay alive for five months inside the ET, without using *any* lifesupport equipment whatsoever. Because the ET is so large, it will take five months for the atmosphere in the ET to become saturated with carbon dioxide.

Space station life support equipment is modular in design and is built to conform to the shape of the ISS module. Life support equipment could also be built to conform to the size of the ET's airlock, so I could push it through the airlock, seal up the ET, and bolt the life support equipment to the floor and turn it on, and I would have five months to accomplish this task, although it should not take more than a couple of days. What's so labor intensive about that?

Dave O'Neill: See other comments on working in zero gee, and noting you need the airlock fixed, the power supply on, the systems set up and installed before you can get out of the suit. I find it highly unlikely.

Tom Abbott: Building modular life support and other equipment to fit through the ET's hatch is certainly doable, and it can be designed so all that is required once it is onboard is to bolt it down and plug it in. Since this work will take place inside, no spacesuit is required, and since no spacesuit is required, anything that can be done on Earth can be done inside the ET with about the same degree of effort, IMO.

Dave O'Neill: Yes, but plug it into what?

The ducting has to be installed, the wiring has to be installed, plumbing, controls, communications equipment, computer control systems... Then you have to commission the damn thing. I don't know if you have ever commissioned process plant, it's a lot simpler that a life support system, but even a fairly simple pumping system can take weeks on Earth in ideal conditions...

Side issue, how do we maintain the stability of the ET?

Don't get me wrong, I like the idea of using ET's, but until we have a shed load of more zero gee and construction experience it sounds like a non-starter.

It certainly isn't a trivial problem...

No you won't, because life support is not just a matter of breathable atmosphere. In a 0-g environment, you need equipment to move the air around so one doesn't suffocate on their own exhalations.

Tom Abbott: I think we can squeeze a fan through the ET's airlock without too much trouble.

Dave O'Neill: I think his issue is that it's not a "fan" it's a network of fans to ensure the air is circulated around the whole interior. That's quite an air-conditioning job.

Marc Passy: You need equipment to remove the condensation that would otherwise accumulate on every un-insulated surface.

Tom Abbott: Yes, we would, and that equipment would be part of the life-support equipment that would be installed inside the ET. The hardware that does this on the ISS is self-contained and plugs into the rack inside ISS. Condensation would not be a problem for the short time it takes to install the hardware.

Dave O'Neill: What do you mean "short" time? You need to install the necessary ducting. We're not talking about a small module either, we are now talking about a dehumidifier for a large contained area.

Marc Passy: To POWER that equipment, you need some means of providing power.

Tom Abbott: Yes, you do.

Marc Passy: How about wiring, breakers, distribution? Solar arrays are your only realistic renewable space power

Tom Abbott: I agree and solar arrays are what is used in all the ET space station designs I've seen. The wiring can connect to the ET's interior through the airlock, and breakers and distribution would be modular units which would require bolting to the deck and connection of the wiring.

Dave O'Neill: So we have to attach solar panels to the exterior of the ET before we can start. That means this is a specially modified ET. And, you still haven't mentioned how this airlock is going to be attached. I noticed that later you say you aren't going to weld. How do you propose to attach it?

Marc Passy: And need sealed penetrations to get the power from the outside to the inside. Where do those come from?

Tom Abbott: Well, that would depend on the particular design, but the best way would be to have all wiring routed through the airlock/docking adapter and into the ET's interior.

Dave O'Neill: So we have a large hole in the side of the ET with a docking/power adaptor. Right, so now we have to attach the solar panel array, run the cabling to the door and hook it up. In terms of the ISS that's 2 missions right off for a couple of truss segments and the solar paneling.

Marc Passy: Oh - I forgot to mention heaters!!! It's Damn Cold out there, and the ET, though insulated, starts with a REAL BIG cold soak. So you need heaters.

Tom Abbott: The ET reaches an equilibrium temperature of around 40 degrees F. a few hours after the propellant is purged. You might need a jacket, but you won't freeze to death inside it, and the real problem in space is getting rid of excess heat, but heat radiators would be some of the first hardware installed along with solar arrays.

Dave O'Neill: Excellent idea, although do you have the thermal equilibrium calcs, I'm curious how the ET once purged deals with the cyclical loading. How do you propose getting the waste heat to the radiators? I'm assuming we're going to have some more plumbing installed for this on the airlock block (starting to sound like a whole shuttle launch in itself), then we've got the ducting to connect the waste heat system up, with fans, controlers etc...

Marc Passy: Which need power. Oh yeah - the heated air needs to get moved around, which means more circ equipment, and, you guessed it, MORE POWER!

Tom Abbott: Every ET space station design I've seen included hardware to produce the required electrical power.

Marc Passy: Oh yeah - you have to connect all of this equipment to some sort of central control system/computer to coordinate its action. Which is more wiring, more power.

Tom Abbott: It would require about as much connecting as was done on this last ISS space mission, and demonstrates the feasibility of this work. The "central control/computer would be modular and would require nothing more than making a few electrical connections. This is easily accomplish if one is working inside in shirtsleeves. If one wanted to eliminate this work, these components could be incorportated into the airlock/docking adapter before launch. Certainly the guidance, navigation and control will have to be up and running as soon as the ET gets to orbit.

Dave O'Neill: I strongly beg to differ. You are talking a significant amount of extra work. And surely this equipment will also have to be fitted to the exterior of the ET and in the right places, what system for guidance and attitude control are you proposing?

Marc Passy: And, if you want to talk to home, you need comm equipment, which is more penetrations, more power and more wiring.

Tom Abbott: This can be preintegrated into the airlock, although installing it in the ET's interior is also fairly easily done.

Marc Passy: This presumes ALL controlling is done from orbit, and the ground CAN't help. If you want to control this stuff from the ground, that's alot more wiring, and alot more complicated equipment - a remotely operated breaker is alot more complex than a locally operated one.

Dave O'Neill: How can you keep saying this! We've now got the wiring for:

Then you've got:

Tom Abbott: I don't see this as a problem since the airlock/docking adapter can be configured with this equipment. Just imagine attaching an FGB to the docking port of an ET and you will get the idea.

Dave O'Neill: But this docking port isn't a feature of the ET and how do you make it one without compromising the ET's primary function... ok this probably isn't insumontable, but I'd hardly call it easy. I'm just not sure how you manage it without having to do some structural work when in orbit. Either way the ET is now a significantly costly piece of hardware and not a piece of standard waste.

Marc Passy: This is before ANYTHING gets added to accomplish anything. Like GN&C equipment. Vaccum supply for experiments. Cooling for experiments. Fridges. Freezers. More data. More comm, or lots of data storage, so your experimenters can get their data.

Tom Abbott: Guidance, navigation and control would have to be preinstalled, either on the ET itself, or in a separate airlock/docking adapter which attaches to the ET after it reaches orbit. As for installing the other items you mention, I don't see them as complicated at all. Each item is prepackaged and just plugs in to the rack when used on ISS and there is no reason the same can't be done inside an ET. The ET even has an advantage in this since there is plenty of room for all the hardware and it doesn't have to be squeezed into a small space (other than sqeezing through the airlock initially).

Dave O'Neill: I think the problem he's stressing is that, unlike a predesigned space station module, this will need to be wired up in situ. I'm not sure how you can pre-instal the racks for the equipment without screwing up the ET's primary function as... well, an ET.

Marc Passy: Oh - and you can't WELD anything. Temp gredient too big for good adhesion, can't consume O2, or scrub the contaminents that go airborne.

Tom Abbott: Outside the ET you mean. I don't contemplate requiring any welding when converting an ET.

Tom Abbott: Because the ET is so large, it will take five months for the atmosphere in the ET to become saturated with carbon dioxide.

Marc Passy: I suspect your 5 month number is for all O2 to be replaced with CO2 by normal respiration. You need to compute the time to 17% 02, or the time to PPCO2 of 10 mmHG, for long term exposure, presuming 12.5 psia nominal pressure. And I can tell you from personal experience that less that 18% O2 (at 15 psia) SUCKS. You can't really exert yourself without sucking wind.

Frank Crary: Well, maybe you can't, but I don't have any problems with it. Here in Boulder, the atmospheric pressure is around 0.8 bars. So I and millions of others live and work with an oxygen partial pressure 20% lower than what you consider acceptable. Personally, I don't have any problems below an altitude of 11,000 ft above sea level, which is roughly the 0.65 bar level.

Tom Abbott: The five month figure is the time it takes for the carbon dioxide levels to reach levels dangerous for one person. Five persons could last one month without lifesupport inside an ET.

Tom Abbott: Wiring? All it would amount to is plugging the device into the electrical outlet. The device itself should be designed so it needs no further assembly in the ET. As for ducting, there is not much need for ducting but what is required is easily installed. There are no space constraints.

How do you easily "install" ducting? I can understand that space is not a problem but I can only assume you've never tried to instal ducting in 1 gee let alone zero gee.

Marc Passy: And where are these bolt holes? Oh yeah - your tinkertoy decking system. Which has to be put in before the airlock, or it's in LOTS of LITTLE pieces.

Tom Abbott: There are preexisting bolt holes in the ringframes of the ET and it would be quite easy, IMO, to install a floor (a grill-work floor like Skylab used for instance), just like you said: "in tinktertoy fashion. I would bet I could install the entire floor by myself in a day or two, using tools no more complicated than a powered socketwrench. A couple of people working together would make short work of it.

My particular ET space station design preference would be to install a main deck right down the center of the ET lengthwise. This would give a deck 27 ft wide and 96 ft long with a 13 ft high "ceiling," with room for a second deck "below" the main deck Of course, it would not be necessary to install this entire main deck to accomodate the space station hardware required to make the ET viable, a small percentage of this "floor" space would suffice. In fact, it could all be shoved over into one little corner of the ET, which would leave plenty of room for space basketball. :)

Marc Passy: You obviously don't understand exactly how LITTLE the EVA crew on 2A did, in Earth terms?

Tom Abbott: Well, I think I do.

Marc Passy: A few dozen electrical connectors, a few thermal covers, a toolbox, lots of pictures.

Tom Abbott: The EVA the astronauts did on the FGB/Node 1 this week would be equivalent to the EVA required to initially outfit an External Tank in orbit. I would expect the exterior ET conversion to go as well as the FGB/Node 1 connection/construction went.

Marc Passy: All of the major assemply was accomplished by one Orbiter thrusting to attach PMA-1 to the FGB and a bunch of pre-installed motors being run from the ground latching the two together. And each assembly mission is similar. The FGB flys into the SM, auto dock, all electrical and fluid connector make automatically (with a couple of exceptions commanded by the ground later). The remaining US assembly is about like the N1 install.

Tom Abbott: I don't see that this has much to do with converting an ET.

Marc Passy: If you were just working IVA, maybe your scheme might hold water.

Tom Abbott: As I said above, I don't expect the EVA requirements to hookup electrical and heat radiator connections to the ET would be any more difficult than the tasks performed by the astronauts on the FGB/Node 1. The astronauts demonstrated their abilities and capabilities quite well.

Marc Passy: The volume is certainly attractive.

Tom Abbott: It certainly is.

Marc Passy: But the key is that a whole lot of infrastructure is required to build a human habitable space, in the first place. It's a whole shitload more complicated than slap on an airlock and add air.

Tom Abbott: Not really, NASA just makes it look more complicated than that.

Marc Passy: And ALL of this infrastructure installation must be accomplished on the ground, or EVA.

Tom Abbott: True to a certain extent, but most of the EVA work can be eliminated by outfitting the airlock/docking adapter with the required equipment before launch, then all that's required is to route cables inside the ET and connect them up.

Dave O'Neill: Ok, all comments aside. Here's my work plan for your proposal. (I may forget things and putting times against jobs can be difficult)

This looks like 2 flights to me, with 2-3 EVA's.

If I had a team of fitters doing a fit out of a similary sized piece of plant on Earth I'd guess at 2 weeks from start to finish assume no hitches before you have the basics of a habitable shell, and that would be working damn hard to meet targets. I also can't see there being sensible life support for at least a week or 2.

So that's working in the cold and dark for a week or more installing cabling and lighting. That's before we get a chance to get proper air circulation et al.

Reading your post I get the impression that the ET interior will look like Node One looked on the TV the other day.

I may have missed some steps out of here. None of it is out of the question, but I do not believe it is easy nor more practical than what we are doing. Maybe when we have some more experience.

Question and Answer

Thomas Moore: Gee all these questions sound familiar. I once asked Tom the same things. I think the biggest thing that always turned me off of the ET designs was that they always visually portray the ET as a single unit able to be turned into a station when it is actually two tanks, and the Intertank that connects them is not a pressure vessel. However I've since played with a few ideas while debating this issue with some collegues.

Q): How do you intend to attach the airlock to the ET?

A): You don't. All you need to do to the ET is build it with a docking port on it in the place of the hatches (manholes) that are already in the tanks. So how do you get the airlock up there you ask? It rides up in the payload bay of the shuttle and gets attached in orbit. Actually I wouldn't even attach an airlock at first. ISS doesn't need one at the moment, and won't have one until after it is crewed. So instead of an airlock, we bring up a duplicate of Unity (lets call it U2). If you rig the ET to seperate into its 3 main parts, (H2 tank, O2 tank, and intertank) you can then connect the H2 and O2 tanks with U2. Attach a PMA to the bottom of U2, and you have a connection point for the shuttle. Throw away the intertank, its not incredibly useful. The U2 node being a duplicate of Unity provides all sorts of functionality and readymade racks to support the life support of the O2 and H2 tanks as they are converted. Not to mention you have lots of room inside of the Unity node to operate shirtless even before you pressurize either tank.

Q): What cutting technique are you going to use to put the hole in the side of the tank. It's still got to do it's primary function hasn't it. And when the hole is push through, how do you fix the airlock in place, and how do you ensure a "safe" and reliable seal?

A): You don't. The tank already has "manholes" in it. What you have to do is engineer it so that the manhole covers can be removed in orbit, and that the manhole is placed inside a docking port. This method requires less engineering then building in an airlock. Also it is smaller, so adding such a port at the base of the tank should be doable without changing the location of the SRB mount points or building modified SRB's.

Now some of the rest of the questions I've seen and ones I predicate from here.

Q): How do you fill the tank(s) with air?

A): By supplying it from the Shuttle. Since setting up the inside of the tanks to be used as a shuttle is going to take a lot more time then it makes little sense to attempt to work on both the H2 and O2 tanks at the same time. The first shuttle flight should be able to also carry enough air to provide enough of an atmosphere in the O2 tank in order to allow work in it. Later flights can bring air supplies for the H2 tank.

Q): How do you deal with humidity?

A): The nice thing about using a Unity style node to connect the 2 tanks is that it is loaded with equipment racks. These racks can be initially set up to carry the dehumidifier as well as air recirc equipment.

Q): How about power??? How are you going to get that into the tank?

A): Well I would attach the solar panels to the Unity style node. Hey if it works for ISS, it could work for this setup. Your second flight to this station could deliver the power mast and panels. As for feeding the power into the tanks, if you design the docking port correctly, you could feed the power in the same manner that it is done in ISS. No need to cut any holes at all.

Q): What about the engine fuel feed pipes? How are you going to seal them?

A): Well that would most likely require a design change on the ground. I would picture that the pipe would have to have a valve in it that could be closed in orbit, and the inside fitted with a "plug". The pipe should be able to be mostly if not completed disconnected so as to not cause unwanted protrusions.

Q): Well what about reboosting the station? You going to build a reboost module into the ET?

A): The name of the game is keeping it simple. You would need reboost, but a Zarya style node that is refuelable, or a copy of the Interim Service Module or the Russian Service module would be needed. Since a second Zarya was started, that path could be the cheapest. Although since most of us prefer a low inclination orbit, I would also prefer that the Zarya clone be launchable from another launcher if suitable facilities for the Proton cannot be located. The Zarya clone could plug into the base of the ET tank in a second docking port. Long cables from the Zarya clone could provide the Unity clone with temporary power until a power mast is attached... The same way its done on ISS, but with longer cables.

Q): All the shuttles are building ISS, so how could this be done.

A): Actually all the shuttles are *not* building ISS. Columbia is capable of carrying the heavy payloads to high inclination orbits, but if a station was in low inclination, then Columbia could carry the weight. Granted, it would require at least 2 flights before serious power was available, so that means 7-8 months from the first launch until the power mast is available since only Columbia would be available.

Q): 2 flights a year is pretty pitiful isnt it?

A): Well for crew rotation it isn't bad. For resupply its terrible. It would be nice if a Roton, or Kistler K1 or other re-usable was available for resupply. In fact cost wise thats preferable. Contrary to popular belief on my opinions, I think Roton's or K1's are perfect for resupply.

Q): Shouldn't we re-use all the tanks?

A): Logistically, this is not feasible. Under this scheme you would need at least one connecting node for each tank. Carrying a connecting node on each flight wouldn't allow for other parts like solar arrays. Additionally, even though a single connecting node can link the tanks, you would need at least one more node to attach the new linked pair of tanks to the growing station.

Q): How will you spin this ring station?

A): The ET ring station is one of those designs I frown on for displaying the ET as a single tank. It is also difficult to reboost while in construction due to being unbalanced. As a picture, its nice to look at. As a reality, the difficulty factor increases its cost. I would imagine the station to grow as a row of tanks side by side, and to be non-spinning.

Q): Won't it be difficult to juggle the pieces of the tank while assembling it?

A): Yes it would. Its not something that we couldn't overcome though. Maybe a second robot arm, or just a clamp arm to hold an extra piece would be required.

Other Items

Instead of separating the tanks and throwing the intertank away, why not use the intertank SRB attach points to connect tanks side by side as the station grows? You can still put the unity modules at the base of the H2 tanks, and have tunnels between the unities. Doesn't the intertank also make a good base for solar arrays and radiators?

Thomas Moore: You can still utilize the attach points that are one the H2 tank. Additionally the connecting node, if a unity style, allows for a tunnel connecting the tanks so that you still can add tanks side by side. The intertank is not designed to add things like solar arrays, but a unity style node is. Unity on ISS will have the power truss attach to its "top".

Solinar: The intertank area also provides a nice "shelter", or "tool shed", where objects can be stored without threat of escaping. The intertank need be modified only to the extent that an access door be added. (although, I don't know if it would be safe for a suited astronaut to go crawling around in there).

Thomas Moore: So does a unity style connector. Only a unity style connector is pressurized and provides for a work area for the astronauts while building the facilities inside the tanks.

Doesn't the O2 tank have enough residual O2 to at least partially pressurize the H2 tank after that tank has been vented? Or perhaps even better is to recombine the O2/H2 through a fuel cell, there by draining the tanks, providing interim power, and supplying water all at the same time.

Thomas Moore: I don't know the answer to that. I'm not sure that anybody does. As for utilizing the remaining O2 and H2 for a fuel cell. You wouldn't be able to enter the tanks until after they ran dry, otherwise you instantly vent them thereby contaminating/ending your fuel source. Venting the H2 tank and then pumping excess O2 from the O2 tank into it might be possible, as long as you are SURE there is no H2 left in that tank. Frankly I think venting the O2 tank down to a suitable shirtsleeves level and mixing with normal air is preferable. Its less dangerous.

Keeping the tanks together means not having to have another node for the Zarya. That can be mounted on the opposite port of the Unity.

Thomas Moore: You don't need another node to connect a zarya style tug to the ET. What you need is either a port on the ET the size of the zarya port, or a PMA to allow the 2 ports to connect. Thats the method used to connect the real Zarya with the real Unity. Without the PMA, Zarya and Unity can't mate... thier ports are different sizes. Carrying a unity style node plus two PMA's is within the capability of the shuttle... thats what we just did in order to link Unity and Zarya.

Gregg Germain: I wonder how useful/simple it would be to use an expended External Tank as a very simple workshop/repair garage?

Instead of fixing things in the vacuum of space where you have to use clumsy space suits, it would be nice to haul it into the garage, shut the door, pump in the atmosphere, and repair the gizmo in a shirtsleeve environment. You still have to deal with weightlessness, but you at least get rid of the bulky suits. If you lose a tool or a bolt you have a chance of finding it.

It wouldn't require any sophistication at all: just a door, airlock, atmospheric controls.

Chris: Imagine how long it'd take to pressurize and re-pressurize everything, wouldn't it take even longer? Also, moving stuff around more than necessary, ie detaching it from X to put it in Y and back again is just asking for something to get hit along the way, I'd of also thought things like hubble, or perhaps other things are too big (wide?) to put in, and wouldn't NASA want it out in the open to test things on it perhaps?

Ian D.: You could probably get by without the atmospheric controls. Just open the door, manoever it over the object in need of repair, close the door, and pump in some gas. It wouldn't be 'shirtsleeves', but all you'd need is a simple o2 rebreather and a suitable warming/cooling garment. It'd save you worrying about circulation, dehumidifying, heating, etc.

At the end of the job, pump the gas back into some storage cylinders (through a moisture filter), open the door, haul the satellite out, and off you go. It'd be no worse than working as a commercial diver - probably a lot easier. Just removing the suit pressure differential would have to help a lot.

I guess the hassle would be in moving the garage around from satellite to satellite. I'd probably end up cheaper and easier just to do it the hard way.

Gregg Germain: Well I was thinking of moving the satellite to the garage. ;^) So you'd have to design and build a "space tug" for that purpose alone.

pat: Before you completely piss all over Tom, I would suggest you look at the Option C station concept, the Option D "Geode" concept proposed by JSC and the history of the Skylab developement effort. SKYLAB was originally supposed to be a wet workshop with On-Orbit outfitting, before they went to the Dry workshop configuration. Also during the OConnor commission review of Freedom, JSC seriously proposed an Option D ET based station design.

I find it interesting that you aren't even aware of work done by your own employers. I find tom's fixation with Option C and D rather silly, given the NASA structure, but they were technically meritorious.

Dave O'Neill: I've got to be honest I've no real problem with converting ET's, what I have a problem with is them being put across as a simpler option than what we're going for now. I'd love to have seen a Shuttle derived HLLV and ET wetlabs as a part of the ISS.

However, I don't personally see how it cuts down on risk, time or a significant amount of expense on what is being done now. That's just my opinion of course, but I can't see ET conversion being a goer until we have a crew on hand to do the work.

Then, we have something.

aerodesic: I think you should keep the tanks of all flights, and all "waste" nearby when building this. Just because you cannot use them right away does not mean you can't use them eventually, when there are more people there. They make a nice boneyard of spare parts and in any case can be placed Easterly in the orbit and act as a crude meteor bumper, like in the 1950's version of space stations.

Jorge R. Frank: ETs are extremely draggy, and won't last a week in orbit without active reboost capability.

aerodesic: too true the ET's are draggy. That is also what makes them good buffers for the shrapnel on orbit. They can be arranged to be more aerodynamic while awaiting use and attached to the centre mass of the station wantabe. The extra propellants needed for the delta Vee needed to keep on station is worth saving the structure for.

Arcjet type ionic thrusters would use less mass and would find uses later when there are more people working in Zero Gee. Any styructure in orbit will need such protection eventually and it would help to have the material at hand.

RACEnCART: Correct me if I'm wrong, but its my understanding that the E.T. isn't a hollow object. It contains other tanks inside of the E.T. shell. If this is the case, you would have to either use them as they are or dismantle them and remove them.

Jeff Findley: You're partially correct. The ET is made up of the hydrogen tank, intertank, and the oxygen tank. The walls of the hydrogen tank and the oxygen tank are also the external walls of the ET, with the exception of the intertank. The intertank joins the bottom of the oxygen tank to the top of the hydrogen tank. Within the intertank are the bottom dome of the oxygen tank and the top dome of the hydrogen tank. The intertank is the ribbed looking structure just after the nose cone (top of the oxygen tank). Everything below that is the hydrogen tank and everything above it is the oxygen tank.

The hydrogen tank is so big, you could open up the access hatch (bottom of the hydrogen tank) and use it as living space, ignoring the oxygen tank altogether. The hydrogen tank has a volume of 53,518 cubic feet. The oxygen tank "only" has an internal volume of 19,563 cubic feet. Compare this to a typical ISS module and you'll see that the internal volume available inside the tanks is *huge* by comparison.

The above figures taken from a hard-copy of a NASA web site describing the shuttle components (the external tank section). If you search for and take a look at this site, it gives a good description of the ET including information like operating pressures (up to 22 psi for the O2 tank and 34 psi for the H2 tank). These things are *not* delicate structures. They are big, heavy, pressure vessels which should make good space station modules considering sea level pressure is only 14.7 psi.

RACEnCART: I like the ET idea, but it just isn't a viable piece to use in such a manner. One could make one here one earth and send it into orbit, just pull out the ol' reliable Saturn V booster and blast that sucker into orbit.

Jeff Findley: Why not? It's built to be a pressure vessel and to carry launch loads. Used as a station module, it would be subjected to much less pressure and would not be subjected to the launch loads of the SRB's and the shuttle.

This is nearly impossible, even with enough money. You're talking about building something that hasn't been built in about 30 years. You can't find parts, materials, or even labor that knows how to put it all together. Furthermore, the infrastructure at KSC for the Saturn V has been modified for the shuttle. ET's, on the other hand, are launched nearly to orbit every time the shuttle flies.

An ET based station would use an existing structure and attach hardware to it that's similar or identical to hardware already planned to be used on ISS. The way things are assembled changes, but the hardware doesn't need to change much. Much of the ISS hardware is already designed to be replaced in orbit and moved through hatches so that equipment that's obsolete or broken can be replaced.

The best proposal I've seen for the ET have involves using an aft cargo carrier to carry equipment into orbit along with the tank. This is just an extension attached to the bottom of the hydrogen tank. This approach allows things like docking equipment, solar panels, reaction control system, life support equipment, etc. to be pre-integrated with the ET while giving easy access to the existing hatch on the bottom of the hydrogen tank. The shuttle takes this combination into orbit, releases the ET then docks with the aft cargo carrier and work begins. No tricky EVA's required.

RACEnCART: The Saturn V part of my reply was a basic figure of speech, I know they would have to redesign everything and there isn't parts available to make them again. It was just how they did Skylab and I was making a comparison to how it was accomplished. (for those who remember) BTW, thanks for the replies, I didn't know the internal structure of the ET and am glad to get the knowledge you guys have passed on to me.

You don't. All you need to do to the ET is build it with a docking port on it in the place of the hatches (manholes) that are already in the tanks. So how do you get the airlock up there you ask? It rides up in the payload bay of the shuttle and gets attached in orbit. ..... ......[snipping]...... .....If you rig the ET to separate into its 3 main parts, (H2 tank, O2 tank, and intertank) you can then connect the H2 and O2 tanks with U2. Attach a PMA to the bottom of U2, and you have a connection point for the shuttle. Throw away the intertank, its not incredibly useful.

I can't believe this...the intertank is one of the most opportune ways to make use of an ET. All the complications to make the O2 and H2 tank parts separate then join them BACK with something else? No. Instead, modify the Intertank to have a docking port/hatch on the side (the intertank would have to be modified to begin with to allow the H2 and O2 portions to be separated i nthe plan quoted above). Have a pressurized tunnel that leads from the Intertank's port/hatch to some kind of hatch in the top dome of the H2 tank. There is very little space between where the H2 top dome and O2 aft dome come close to each other in the middle, but along the intertank sides there is some significant room where such a tunnel could be run.

The H2 portion of the ET alone would be a lot to make use of in some way, if there were actually a practical job for the ET. Although there could possibly be some use made for the O2 portion of the tank (off the bat...reclaim residual Oxygen. Perhaps fill it with a bit more Oxygen for that mission so there's more left after MECO, unelss the tank's already at maximum capacity). Just I see it as not nearly as much of a problem to add a openable hatch to the top dome of the H2 tank as to add one to the bottom of the O2 tank where there is the pressure from all the cryogenic O2 on top of it at launch.

One extremely unglamorous but possible use for the ET would be to just house waste from the ISS. Surely the ISS is not going to literally tinkle on itself with water dumps as the shuttle does? But I'll assume that whatever solid and liquid wastes that are generated by ISS will be taken back on the shuttle during the regular crew exchange missions, so it probably isn't worthwhile to do that.

At any rate I don't see that there's a practical job for the ET that isn't as hard or harder (or expensive) to do than the current ISS. But I did want to point out if there were, that the intertank is not as useless as some people think. It could be a great starting point for a ground-modified ET. Long as nothing involving safety was compromised. That's the biggest drawback to many of these plans.

Thomas Moore: Lot more complicated then the other method. The intertank is vented, has a thrust beam running directly from one SRB mount point to the other SRB mount point through the centerline of the ET and the intertank has piping from the O2 tank running through it. All these things would have to be redesigned and rebuilt. Additionally, you would have to investigate the flex on the system to see what stresses would be placed on your tunnel. The approach of disengaging the tanks and reconnecting them involves much less redesign of the ET.

George Gassaway: Admittedly there are other things taking up space in the intertank such as the thrust beam and O2 line. But still there is room that could contain a tunnel between a docking port or hatch in the side of a modified intertank. And no I wasn't talking pressurized intertank either, only the H2 tank, some kind of hatch in the top from of the H2 tank, a short tunnel and docking port or hatch.

Thomas Moore: Most ET enthusiasts want to use both the H2 and O2 tanks. The O2 tank is still quite large in terms of volume and is useful. The whole idea that they espouse is to reuse as much of the ET as possible in order to keep costs low. Also of concern is limiting the actual changes made to the ET on the ground in order to minimize impact on its primary functionality. In actuality, neither a tunnel or connecting node are really required. By placing docking ports both on the top of the H2 tank and the bottom of the O2 tank, these two tanks could be directly linked after being disengaged from the intertank. However that introduces operational issues, such as the need to re-pressurize the H2 tank and do initial operations in it before you can even get to the O2 tank unless you place a docking port at the top of the O2 tank, which would require changes to the tank, and as I already stated, we want to keep design changes to a minimum. A port in the top involves tank penetrations that weren't already in the design. So some kind of external access in the vicinity of the intertank is desired. Building it into the intertank requires redesign of the ET as a whole. Replacing the intertank once in orbit with a docking node is less invasive on the overall ET design.

Additionally, the idea of a "tunnel" doesn't really allow for racks for equipment. By connecting to a ground built outfitted node, you get equipment racks for things like life support, dehumidifiers, etc. This allows you to start work on modification of the tanks a lot easier.

George Gassaway: Who says racks of equipment are supposed to go inside of an access tunnel? You do not find racks of equipment inside the shuttle airlock, you find only what' s needed to do the job to get from point A to point B, the equipment being at the other end of point A or point B.

Thomas Moore: The equipment I described, life support, dehumidifiers etc *are* required to do the job. While you can place all this equipment into racks inside the tanks, those racks would have to be built once upon orbit. However, in order to work inside the tanks to build those racks, we want the life support equipment etc operational. A preconfigured connecting node allows for a working support space while reconfiguring the tanks.

George Gassaway: Racks of equipment could go into yet more unused space in other parts of the intertank. Or sure go connect up a ground built node, but something really simple and small that is attached to another side of the intertank, a sort of "plug and play" module rather than this concept of having the ET come apart into separate pieces, throw out the intertank, then rejoin the O2 and H2 tanks with some node that replaces the intertank. Or that external node could be attached to the single docking port/hatch that the intertank could have on its side. As well, other equipment that might be desirable could be made narrow enough to fit into the tunnel to be brought inside the H2 tank and attached (all those stringers, stiffeners and slosh baffles inside to attach things to without touching the skin of the tank).

In any case I don't think it's practical to do something with the ET so I don't promote the general concept, but if there were a practical need it sure would not have to involve something as complicated as mentioned in several other postings. Anything that has to be done in orbit is hard, the more pre-fabricated the better. And redesigning the ET so the intertank can be replaced in orbit is not exactly a small change nor a simple procedure to do in orbit.

Thomas Moore: Actually its simpler then it sounds. We have plenty of experience in getting components to "destage" from each other. The Intertank is not built as part of the H2 tank or the O2 tank. It is a seperate "stage" that connects them and assists in the translation of thrust from the SRBs to the ET stack.

George Gassaway: Actually one of the things I seem to recall from early concepts was a problem of using an ET in orbit was as mundane as the foam insulation flaking off over time, making the ET dirty, anything connected to it dirty, and almost literally producing a haze or cloud of foam particles all around it. Sort of the orbital version of an oil slick. OK, so either then some protective layer or wrap would have to be applied after reaching orbit to prevent that from happening, or come up with some other kind of insulation that would not have that problem yet work well enough to prevent ice and work OK ablatively too. All without adding much significant weight since this involves thousands of square feet of insulation at minimum 1" thick, more in many places. Never gets easy or simple, it seems.

Thomas Moore: Agreed. This is definitely a problem to overcome. In my recent discussions on the topic I heard of solutions such as new insulation, a protective layer over the insulation, or a layer under the insulation that would allow the insulation to be "peeled" and discarded.

I also agree that ET tank stations will probably not be done, but thats more do to a lack of interest, a lack of funding, and a lack of logistics for a venture of this nature then it is do to the technical problems. But just because something won't be done doesn't mean we can't have fun speculating on how it *could* be done.