External Tanks in Orbit

Participants:

• Geoff S. Jones | geoffk_AT_sover.net
• Graham Nelson | graham_AT_gnelson.demon.co.uk | Oxford, United Kingdom
• Joseph J. Strout | jstrout_AT_ucsd.edu | http://www.strout.net
• Michael Martin-Smith | martin_AT_miff.demon.co.uk
• Frank Crary | fcrary_AT_rintintin.Colorado.edu | CU Boulder
• Joshua "J05H" Gigantino | gigantin_AT_shell1.tiac.net
• Tom Abbott | tabbott_AT_intellex.com
• Jorge R. Frank | jrfrank_AT_ibm-pc.org
• Cathy James | cajames_AT_alumni.princeton.edu | PPSEL, N5WVR
• Phil | pgf_AT_globalreach.net
• Bill Bonde | stderr_AT_mailexcite.com
• Chuck Stewart | zapkitty_AT_geocities.com
• John Schilling | schillin_AT_spock.usc.edu
• Peter "Kermitus" Schorn | peter.a.schorn1_AT_jsc.nasa.gov
• Rand Simberg | simberg_AT_interglobal.org
• Ian Stirling | 000035599C47_AT_mauve.demon.co.uk
• Mike Dicenso | mdicenso_AT_seds.lpl.arizona.edu
• Jabberwocky | jabberwocky_AT_v-wave.com
• Thomas Moore | tmoore_AT_hercii.mar.lmco.com
• George Herbert | gherbert@crl2.crl.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 is a fashion which allows interested parties to become aware of previous discussions on this topic.

OPEN PLEA FOR ACTION!!!

Geoff S. Jones

The ET is 153.8 feet long and has a diameter of 27.6 feet. The External Tank is larger than the shuttle or any of the space station pressurized components!

Beginning with the STS-6 mission, a lightweight ET was introduced. Although future tanks may vary slightly, each will weigh approximately 66,000 pounds inert. The last heavyweight tank, flown on STS-7, weighed approximately 77,000 pounds inert.

These statements should raise some taxpayer eyebrows! Why!! Why, are we throwing away this money and opportunity!!!

With private enterprise moving into the launch industry it is time to start providing stations that the private sector will need for industry, tourism, exploration, and colonization.

Please act on this opportunity. It is time for industry to provide the impetus in space development. Help provide the leverage they need to get past break even on their balance sheets. Help to provide business development sectors by providing the space for them to work.

How many more ETs will go to waste at the expense of the taxpayer dollar, and the People of Earth's dreams of space development/colonization...

Please add any comments or ideas to this draft and send it to me at my e-mail geoffk@sover.net

Discussion

Graham Nelson

But not one that had previously held fuel. Skylab was an extremely intricate manufactured object, entirely fitted out on the ground. Its greatest similarity to a Saturn V stage, in the end, was probably its physical dimensions.

Its hugeness is itself a problem. Since it is lightweight but has a very high cross-sectional area, it would re-enter the atmosphere extremely quickly without constant reboosting. This problem can be solved, but not without cost and logistical difficulty. An ET space station is not a free lunch.

There is nothing original about this idea, which has been extensively studied and debated.

This is absurd. Cost per pound to orbit is only measured for payload. At $15,000 per pound, an orbiting space shuttle is worth over 1 billion dollars. By your reckoning, NASA must be mad to bring it back down to earth! Just think -- they do this eight times a year! That's over 8 billion dollars wasted! Enough to double the whole manned space flight budget!

Bill Bonde

NASA *is* mad to bring it back down. The only justification for bringing a cargo space ship back down to Earth is if it's carrying something back other than people. Since we haven't found a thing in Space that is worth bringing back down to Earth, we shouldn't be doing it.

Graham Nelson

In short, if someone wanted to buy an orbiting ET -- nobody does -- and wanted to pay such a grossly large amount of money for it -- nobody in their right mind would -- perhaps you'd have a case.

And, spookily, nobody does.

If Congress believe arguments like the above, they should be compelled to resign.

Stockpiled? On earth, you mean? That's madness. Cheaper just to manufacture them and make a big pile.

Industry doesn't want to buy them. There isn't an industry utilizing pressurized orbital space, yet, anyway.

Why? Look at this way. Would you insist that NASA recycle the water vapor thrown out of the back of the Shuttle on every launch? Of course not. So why make a fuss about the small metal sleeve containing it? My point is that it's only a "waste" if it's worth something.

An OTV is a significant expense. Every time it's been studied, it has been concluded that it probably has no very useful mission.

Now this almost makes sense. It just might be useful to provide the ISS with one large-volume pressurized space, possibly. I can't think what it could be used for -- growing food, crew recreation, ... ? -- but yes, it just might be worth outfitting one single ET. The present ISS design couldn't possibly accommodate it, of course.

Joseph J. Strout

I agree that this is a tremendous waste, and they really should be kept on orbit.

This is true. So to understand the problem, we must answer this: *why* has no enterprise taken possession of these free 660-million-dollar prizes?

Frank Crary

As I understand it, NASA has not agreed to give away ETs: They will do so _if_and_only_if_ the new owner can provide station keeping or a controlled reentry. That is fairly difficult, and no one seems to have a way to do it at a viable cost.

Joseph J. Strout

It may be (probably is) that the market value of the tank is much less than its gross liftoff cost. Certainly, there isn't much market for material in orbit today, but we can expect that there will be in the future. But in the future, liftoff costs will be lower, so the value of the tank (as raw material) will also go down. As good-sized pressurizable volume, however, it will probably remain valuable for quite some time. So it looks like a good investment.

Bill Bonde

The market value of something can have no relation to its manufacturing cost. That's why they don't make 40meg hard drives anymore.

The problem is that like the taxes on a piece of land you are speculating on, there are costs to hold the items until they can be used. We can't even figure out who is going to keep Mir in Space right now. They are probably going to de-orbit it because they can't afford to keep a problematic though functioning space station aloft. Imagine what that means for an ET.

Joseph J. Strout

I suspect that the problem is the minor modifications needed to economically use an ET on orbit. NASA probably refuses to make the modifications, no matter how minor. That suggests a two-pronged attack: (1) lobby to convince NASA, or those gripping NASA's snarglies, to made the necessary modifications; (2) develop ways to economically stabilize and outfit the tank without any modifications.

Bill Bonde

Joseph J. Strout

That seems like a good idea. One OTV could ferry many ETs to more stable orbits. Questions that leap to mind include: how much delta-V is needed to stabilize the orbit? How much thrust will this require over a reasonable time scale -- and how much time do we have? Could ion engines or an electrodynamic tether do the trick, or will it take chemical rockets? What sort of orbit could we most cheaply reach?

Michael Martin-Smith

It was once suggested that 300kg of ion engine fuel could maintain ISS in orbit for 1 year, as opposed to 10 tons of conventional fuel a la Progress tanker. Certainly worth a look for ET?

Frank Crary

Well, the atmospheric density at 200 miles is 1.3E-11 kg/m^2. That would
produce a drag of about 0.2 N on an ET (assuming it stayed in the correct orientation to minimize drag.) A 30-cm ion propulsion system has a mass of about 50-100 kg, depending on the design and the power source, and generates 0.09 N of thrust at a specific impulse around 3000 sec. So you would need two or three of them just to compensate for drag. The fuel requirements for station keeping would be around 7E-6 kg/s or 215 kg/year. The change in velocity required to put an ET in a stable orbit would be quite modest. But building the thruster, as well as the required attitude control, navigation, etc. systems could get expensive. I would be surprised if it could be done for under $100 million.

Joseph J. Strout

To the last question, I think we can assume that the final ET orbit will at least be easily reachable by the shuttle, since they both have the same launch trajectory.

Frank Crary

That's a bad assumption. The initial orbit would be identical, but you might need to boost the ET to a 500 mile altitude before it had a significant lifetime against atmospheric drag.

Joseph J. Strout

Yes, and not just manufacturing, but also entertainment (telepresence games, movie making, someday tourism) and perhaps other industries as well.

Bill Bonde

Having that large of a working space on ISS would be wonderful, of course.

Graham Nelson

Has anyone studied the possibility of a one-off Shuttle mission to ISS whose payload, in effect, _was_ a specially modified ET? That is, the Shuttle's cargo bay would contain nothing except some knick-knacks to attach the ET to ISS?

Note that I'm not advocating doing this with all, or even two ETs: just one, and for a specific reason.

Joshua Gigantino

Here's an idea: would it be feasible to build a station-keeping system that could use the residual fuel inside an ET? I'm thinking something launced separately, maybe on a K1 or Roton, that is strapped on or clamped on magnetically. Would this be possible, or practical?

Addendum: I've read about plans for a hydrogen solar thermal rocket, I think that Boeing is doing a feasibility study on them, and that Steve Mickler has done a lot of work with designs for STRs. Would it be useful to build the booster as a STR that used the residual hydrogen in the ET? I'm not sure how effective one would be out of direct sunlight half the time, but it would seem to be a lot cheaper to design and maintain than a ion or hypergolic engine.

Tom Abbott

Why not use a gaseous hydrogen/oxygen rocket engine, like the one on the DC-XA?

Joshua Gigantino

I know that STRs are still theoretical/prototypes, but, AFAIK, they have a higher Isp than normal chemical engines. The engine on the DC-XA burned gaseous hydrogen? Either way, I'm wondering how hard it would be to design a station-keeping system that could be strapped onto an ET that was already in orbit?

These are the requirements that I can think of:

Joseph J. Strout

I'd weaken the last requirement; salvaging the remaining fuel from the ET would be nice, but not strictly necessary if some other economical means of boosting the thing works better. For example, ion thrusters have much higher Isp than chemical rockets, and so a booster using such engines might be able to boost several ETs with just its onboard propellant. Or an electrodynamic tether might be able to do the trick, needing no propellant at all (just a hearty supply of electricity).

Joshua Gigantino

If I remember from discussions of sci.space.tech, and having purused the Tethers Unl. homepage, a tether can actually be used to change orbits by itself, if it is conductive. So, aside from an STR or ion thruster, a system designed around a couple of kilometers of Hoytether might be practical. It would still need to fulfill the other requirements listed. A tether-based system would be a very elegant, and partly off-the-shelf solution...

Joseph J. Strout

Otherwise, though, I think this is a first-rate idea. If someone could develop a booster or station/keeping system that can attach to an unmodified ET, then we have the potential to make use of these things with minimal impact on regular shuttle flights.

Joshua Gigantino

If the system could be made as a black-box package, it could (maybe, depends on USA and Congress) be flown in the Shuttle's cargo bay, and could be tele-operated into position once the ET was in orbit. The ET has to have hard points to connect to the Shuttle. I'm sure they get damaged during separation, but that shouldn't matter. The station keeping package could attach to undamaged hard points, could have a loop that slips around the tank and tightens, epoxy (does epoxy cure in vacuum?), there are all sorts of ways it could be attached...

Jorge R. Frank

It's not the only problem. An even bigger one, which I have yet to see mentioned on this thread, is the cost in terms of shuttle ascent performance to put an ET in a stable orbit.

The original post stated that the ET is discarded after having achieved 95-97% of orbital velocity. While that is true, making up that last 3-5% is far from trivial.

The shuttle post-MECO, pre-OMS-2 trajectory is an elliptical orbit with apogee usually around 160 nmi, and perigee below the Earth's surface. The orbiter separates from the ET and performs the OMS-2 burn at apogee to raise perigee at least above the minimum safe altitude (at least 85 nmi). The ET continues on the original post-MECO trajectory and re-enters the atmosphere long before reaching the (theoretical) perigee.

On non-rendezvous flights, the OMS-2 burn usually circularizes the orbit. But the vast majority of future missions are ISS assembly flights (rendezvous flights), and on these flights, OMS-2 is considered a rendezvous burn. Depending on the phasing requirements, the post-OMS-2 perigee could vary from 85 nmi to ISS' altitude (220 nmi). On the last four Mir flights (which would be at least fairly representative of ISS flights), the perigee has varied from 85 nmi to 138 nmi.

To put the ET in orbit, you would, at least, need to keep the ET attached to the orbiter through the OMS-2 burn. So OMS-2 will cost more propellant, due to the extra mass of the ET. On flights that have low perigee, you must also raise perigee enough to prevent the ET from dragging too quickly. The total cost would range about 1000-2500 lb of propellant.

The propellant itself is cheap, of course. The expensive thing is its effect on ascent performance. Most ISS assembly flights are fully loaded, and launching to a decidedly non-optimal 51.6 degree inclination. Performance margins are tight. The number of ISS flights with 1000-2500 lbs of performance to spare to put the ET in orbit can be counted on the fingers of no hands.

Bill Bonde

Or you could just delay one flight that was going to go to the normal ISS infrastructure and use it without much cargo (you could bring lots of lab rats) to get the ET exactly where you wanted it. The ET doesn't weight more than the normal cargo on an ISS flight, does it?

Jorge R. Frank

Of course, I'm sure the ISS folks would be happy to submit to yet another redesign in order to shave enough weight to accomplish this worthy goal. And I'm sure the designers of the SLWT would be happy to remove some of that unneeded structure from the ET to contribute to the cause. Not like it actually contributes to safety or anything. Assuming that the SLWT even meets its original weight targets... the first one is 400 lb overweight, which would also translate 1-to-1 into ascent performance loss. And we haven't even begun to discuss the "minor" modifications to the ET needed to actually use the thing once it's in orbit, the measures needed to counteract drag to keep it in orbit, and the problem of maneuvering ET's launched into different orbits so that they can be assembled.

Phil

You're also all forgetting one basic fact: Using ET's in orbit would require on-orbit assembly and/or outfitting. Which violates basic physical law; it was disproven in the early design work done on ISS.

Jorge R. Frank

At least you now use a smiley when you misrepresent my position. I guess I'll take any improvement I can get!

Cathy James

Is there a reason not to use the residual propellant in the ET? This would require on-orbit restart of the SSME's -- are they rated for that? Is the residual propellant likely to be sufficient for OMS-2 with an ET attached?

Chuck Stewart

(The kitty puts on his Crary-Herbert worksuit, exits the shuttle, and holds a welders sparklighter up to the SSME's... )

Yes.

Not only are they not rated for it, they are incapable of it. The ignition devices are left on the launch pad.

Bill Bonde

The ignition devices are left on the launch pad!?

Chuck Stewart

Kidding :)

The SSME has electrical ignition devices in quadruplicate or some such...

It's just that there is this pyrotechnic device that starts throwing sparks on the pad just before SSME ignition, and many people think it's the igniter. Actually it's a safety device to burn off stray fuel when the engine pumps spin up.

(I've been in a weird humor since a car crash last week... feeling my mortality mayby ? :)

But to put it in Cathy's phrase: The SSME's are not rated for on-orbit restart. As soon as they are shut off a variety of valves and switches go into a fail-safe mode to ensure that _nothing further_ occurs with those engines. I'm sure it could be re-designed... but at what cost?

Bill Bonde

Given that this comes up here about every third day, I sure hope so.....

Kermitus

But it says right here at the end of S0007:

"Ha, ha, just kidding. All you need to do is:

"Well, we were drunk, and we had some dynamite..."

Jorge R. Frank

Besides the engine restart problem, you'd still need an OMS burn to settle the propellants in the aft end of the tanks so they could be pumped (the ET lacks the He pressurization system used by the OMS and RCS to force propellant flow in microgravity). There's also the minor problem that the amount of residual O2 and H2 can't be guaranteed to be at the right mixture ratio for the SSMEs, which could result in a burn that is fuel-rich or oxygen-rich enough to damage or blow up the engine.

Rand Simberg

Two. One is that it's not designed for zero-g propellant acquisition (no screens, etc.)

That's the other one. They aren't. Last time I asked Rocketdyne (several years ago) what it would take to requalify the hot box (among other things), they said a couple years and a couple hundred million (grin).

Dunno--you'd have to run a sim.

John Schilling

More to the point, the residual propellant is propellant you can't count on being able to use at all. Either it won't feed into the plumbing, or it is there only because of some uncertainty as to how much propellant was really needed to begin with.

If you *could* count on being able to use that propellant, the Shuttle's operators either *would* have counted on being able to use that propellant to carry a bit of extra payload, or not bothered to load it in the first place to save a bit of weight. Or, at the design level, not bothered to build a tank quite that big in the first place.

In order to rely on "extra" fuel in the ET, you'd need to reduce the mission payload accordingly. Which is not impossible, of course, but will constitute a substantial marginal cost to getting an ET into a stable orbit.

Ian Stirling

Random thought. Ion engines need very little fuel. Isn't the vapor pressure of the residual fuel high enough to fuel a ion engine? Of course, hydrogen is not exactly the ideal propellant, due to it's low mass, and oxygen plasmas are somewhat corrosive AIUI.

Has any research been done into oxygen as a fuel?

Now, fueling it from AL/LI might be a outside possibility, though likely need too much development time.

Joseph J. Strout

Well, it may be that the next-generation launchers will also use an external fuel tank, and they may have been designed with the foresight to leave a usable ET in orbit on every launch. But I agree, the STS ETs are quite good for this purpose (though better with a few minor changes) and should not be wasted.

Act how? This has been discussed for years; some think it's a good idea, some don't, but clearly nothing has come of it. Without being a high-up at NASA, what can we do?

I suggest you tone down the draft; remove all double or triple exclamation marks, and most of the single ones as well. Your tone is fine for a mass-mailing brochure, but letters to Congress or industry heads are expected to be more calm and rational.

Jorge R. Frank

An SLWT weighs 57 klbs (theoretically...); typical ISS cargo is ~40 klbs. I doubt the ISS folks would be too thrilled about diverting flights from assembly. Schedule overruns cause cost overruns, etc. You'd be better off confining yourself to Columbia's ETs... Columbia will never be used for ISS assembly.

Joshua Gigantino

There is another advantage to using Columbia's tanks instead of the other shuttles: for good or ill, there is a chance that it is going to become a commercial space craft. It would then be a matter of business, not government, and it would become a transaction. This would be more expensive than NASA giving away an ET, but it also presents a potential opportunity.

Chuck Stewart

(the kitty takes a look at temptation...)

True. It doesn't have the proper docking adaptor. Even so... it may get detailed to just drop off parts if the ISS schedule really gets backed up.

Jorge R. Frank

Not just the lack of a docking adapter (that could be fixed at the next OMDP, if NASA were so inclined). The real problem is that Columbia would need to go on a 7000 lb diet to carry any useful payload to 51.6 degrees inclination.

Mike Dicenso

That may no longer hold true if what I read in the May 4-10 Spacenews is correct. Spacehab is working on a plan to modify their modules to carry a docking adaptor. This would place the adaptor in a "sweet spot" CG wise, allowing the OVs to carry more payload, and more performance during reboost ops. According to the article the improvment may be so signifigant, that even Columbia may be usable for resupply/reboost missions!

Chuck Stewart

But Columbia doesn't get a vacation even so. Columbia gets assigned all the science missions that the other shuttles can't do because they're ISS'ing. So Columbia looks to be booked solid as well.

Tom Abbott

Columbia would still be able to orbit its ET if the mission has enough margin. The shuttle's total lift capacity is not used on every shuttle mission.

Jorge R. Frank

This is true. I think STS-87 might have had enough performance margin to orbit its ET... but if it had, it would not have had enough propellant to retrieve the SPARTAN satellite when the deploy went awry.

There are a few flights with the necessary 700-3400 lbs of margin, but not many. And none of the ISS assembly flights have enough.

Tom Abbott

If you want a construction hangar in orbit, a space shuttle External Tank is 28 feet in diameter, and you could probably talk NASA into giving you one on-orbit, if you could convince them you can control the ET so it doesn't fall on someone's head.

Ian Stirling

Hmm, do NASA need to agree? As shuttle launch orbits are known, is there a reason why you can't simply take your own little launch vehicle up, rendevous with it in the 30 mins or so between ET separation, and re-entry, and start pushing? It was going to burn up anyway.

Tom Abbott

I think the ET would be "salvage" after the shuttle released it.

Jorge R. Frank

Personally, I wouldn't mind if someone else came up with a way to salvage ETs if it could be done without costing shuttle performance margin. But I suspect there might be trouble with the Outer Space Treaty, which makes the US responsible for any damage caused by the ET even if someone tries to salvage it. The treaty doesn't seem to provide a mechanism for transferring that liability. Another good point in favor of revising the treaty.

Jabberwocky

From the little I do know about the shutle. I bellive that the ET's of the shutle breaks up on re-entery of the earth atmosphere. They are not made to stay in orbit. They would then become a navigation hazard for the shutles.

Ian Stirling

It re-enters because when it's released, it's on an orbit that will take it deep into the atmosphere on the "other side" of the orbit, the shuttle applies thrust to raise it's orbit, so it misses the atmosphere, the ET does not. I was meaning, grapple, apply magnets, or grab it with velcro, and then push it into the orbit you want.

On the issue of residual gasses, can't you just drill a couple of small holes in the tank, (explosive cutters?) and leave it for several weeks to air?

Thomas Moore

You know for the longest time after I first heard the "Re-use the tanks idea" it appealed to me. But as I learned more about the makeup of the shuttle, I learned that the visible tank is only an aeroshell. There are really 2 tanks inside that shell and they make up part of the structural integrity of the entire ET. So you cant just cut open the ET and pull out the internal tanks... and as for cutting open the internal tanks to leave the structural integrity intact... I dont know about you, but cutting into a tank that most likely contains residual oxygen is not my idea of smart. You introduce a spark into that mix and you will be looking at the inside of a fireball. So in order for the ET's to really be usable for this sort of thing they would need to be redesigned so that they can be re-used in such a fashion. Hell its probably way cheaper to grab an ET aeroshell, throw some structural braces in to replace the tanks, mount a couple SSME's on the bottom and mate it up to a real ET/Booster stack. (I know its not that easy, the comment is just for arguments sake)

George Herbert

It looks like you've gotten a somewhat mangled version of the structural design details of the ET. It is not an aeroshell with two tanks within. That would imply void space between the outer shell and the tank wall, which is not there.

There are two tanks. The lower, liquid hydrogen tank, is cylindrical. The upper, liquid oxygen tank, forms the curved part of the nose of the tank. They both have convex eliptical ends, facing each other, and there is an intertank structure that's cylindrical that has void space that spaces the two tanks out and also includes the crossbeam that forms the top attach points for the SRBs and shuttle. Except for that intertank structure, both the LOX and LH2 tanks have only one wall, are internally stiffened and externally insulated, and the external spray-on foam insulation is directly exposed to the outside air.

The only launch vehicles I can recall with such an aeroshell and internal tank configuration are the Transtage, the Delta's old liquid second stage, and Agena. I'm probably missing something but the rule is, tank walls are an integral aeroshell, not seperate.