Here's the idea: A large spherical LH2 tank is placed inside a spherical LOX tank.

The reason I thought of this is so the inner tank doesn't need to be insulated. It doesn't need to be a double-walled, or "dewar-flask" tank. The outer tank full of LOX keeps it insulated. Therefore, the LH2 tank would reduce its weight by 50%.

Or at least, that's the idea. Would this actually be good insulation for the inner tank? If so, would it actually cut the LH2 tank weight in half? And even then, are there insurmountable complications with this?

In other words, is the tank within a tank a sound engineering concept for rocket stages?

Some possible complications I can think:

  1. Both tanks might have to be filled at the same time.

  2. Plumbing from the LH2 tank to the engine would have to go through the LOX tank too, at first.

  3. The LOX might be cooled even more than normal, changing its density and maybe other properties important in the pumps that feed it to the engine.

AFAIK these are not bad/expensive enough to be impossible. But I want to hear what you think.

EDIT: Yes I'm talking about large cryo tanks for large rocket stages that carry payloads to orbit. It could be any stage, although I'm not aware of any sole first stage that uses LH2/LOX (with no other parallel boosters), so probably it would be the 2nd or 3rd stage. These can still be pretty large for, say, Saturn 1B or Saturn V.

EDIT: It looks like LOX would not be a great insulator, because the temperature difference between LOX and LH2 is very great. I thought that, with both of them exchanging heat through conduction, the LOX would also go down or near to LH2's temperature.

  • 1
    $\begingroup$ Ariane V and Delta IV first stages use LOX/LH2, as did STS. $\endgroup$ Commented Jul 20, 2015 at 20:30
  • $\begingroup$ @OrganicMarble my bad I meant sole first stage. $\endgroup$
    – DrZ214
    Commented Jul 20, 2015 at 20:35
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    $\begingroup$ Delta IV medium doesn't have any boosters. $\endgroup$ Commented Jul 20, 2015 at 21:52
  • $\begingroup$ @OrganicMarble Yep I just found that out just now. Had no idea it was out there. AFAIK it's the only orbital carrier rocket that uses LH2/LOX in its first stage without any other booster help whatsoever (such as solid rocket boosters). $\endgroup$
    – DrZ214
    Commented Jul 20, 2015 at 21:59
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    $\begingroup$ "the LOX would also go down or near to LH2's temperature" LH2 boils at 20K while oxygen freezes at 54K. $\endgroup$
    – Lex
    Commented Nov 28, 2018 at 0:51

3 Answers 3


...is the tank within a tank a sound engineering concept for rocket stages?

I take this to mean that you are not talking about pressurant bottles or other small devices submerged in the propellant tanks. Instead you mean the large primary propellant tanks.

Then, No, this is not a good idea.

  1. It doesn't need to be a double-walled, or "dewar-flask" tank.

    Boosters don't have double-walled tanks, so there is no need for a design to replace this. Boosters can't afford the weight of double walled-tanks. This problem is mitigated by fueling the booster as late as possible in the launch count. The cost of boiling off a little prop is way less than the penalty you'd pay by having an vacuum-insulated tank. Plus..don't forget...for upper stages, really soon you are going to be in a vacuum anyway.

  2. The outer tank full of LOX keeps it insulated.

    Insulating LH2 by wrapping it in LOX is like insulating an ice cube by putting it in a hot frying pan. The temperature difference between LOX and LH2 is tremendous. In fact, Centaur-G, which had a common bulkhead between the LOX and LH2 tanks, had a special insulation scheme specifically to prevent heat transfer between the LH2 and LOX.

  3. A large spherical LH2 tank is placed inside a spherical LOX tank.

    Booster propellant tanks are not spherical. They are cylindrical.

  • $\begingroup$ Well wrapping it in LOX has to be a better insulator than wrapping it in room temperature air, doesn't it? Or am I mistaken due to air not having as much heat-sucking power as LOX since it is way less dense? Also, I know most tanks are cylindrical. I was just trying to consider an optimal case at first. But I understand the first point, yet nevertheless am surprised that cryo tanks aren't insulated with double-walls. What about the Saturn IVB stage that had to wait a few hours in space before re-igniting to send Apollo to the Moon? $\endgroup$
    – DrZ214
    Commented Jul 20, 2015 at 18:50
  • $\begingroup$ Again....the SIVB stage was in vacuum for its coast phase so there was no need to insulate the outer walls. However, there was internal insulation in the LH2 tank on the common bulkhead with the LO2 tank...because of the huge delta temperature. LO2 is not a help for LH2 it is a problem. books.google.com/… $\endgroup$ Commented Jul 20, 2015 at 19:15
  • $\begingroup$ Ok I think I'm starting to understand. It sounds like a better idea is to find the best lightweight reflective sunshade we can and put it around the hydrogen tank. Since it already has vacuum insulation from space, less sunlight would yield less boil-off. $\endgroup$
    – DrZ214
    Commented Jul 20, 2015 at 19:30
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    $\begingroup$ Is it an advantage to have a tank of boiling LH2 inside a tank with freezing LOX? The volume of LH2 is much bigger than that of LOX, the outer layer of LOX will be thin. $\endgroup$
    – Uwe
    Commented Nov 12, 2018 at 14:47

Yes it is, and in fact, this is being done, to some extent, already. It does have a number of risks, however, and is typically only used for small items as a result of that risk.

The Falcon 9 is the rocket that I am aware of that does this. There are a number of helium tanks, known as COPVs. These would need to be insulated more if they were not in the LOX tank, as the linked question hints.

Why has this not been done for a larger tank? These pressure vehicles are already quite difficult to manage. It becomes even more difficult with additional size. You have to be able to fuel them, and run the fuel. Also, having the fuel and oxidizer in the same tank means even a very small leak is virtually guaranteed to explode, a very bad thing. It also would mean that you would need a hole large enough to put the smaller tank inside the larger one, potentially causing more issues to maintain pressure. The helium tanks work because they are small, and inert, any issue with the tank will just result in an overpressure event. The Falcon 9 has has at least one issue where a tank ruptured on the ground, which delayed the flight by a few months, but caused no serious damage. I personally suspect this was the cause of the previous failure. If it was a non-inert gas, then there would have been an explosion.

Overall, I think it sounds like a good idea, however, there are some serious risks. If these risks could be overcome, it might save a bit of weight. Still, I don't expect this to be significantly changing things anytime soon.

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    $\begingroup$ For Falcon 9, the clue is in their name (Composite Overwrap Pressure Vessels). Their base is a thin and light-walled tank that is later wrapped around with carbon fiber (similar to an embroidery sewing thread wrapped around a tube) to give it structural strength and prevent further expansion. They're not like vacuum / Dewar flasks, there's no insulating space between these two layers. $\endgroup$
    – TildalWave
    Commented Jul 21, 2015 at 21:01
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    $\begingroup$ Worth mentioning that by "any issue with the tank will just result in an overpressure event." you mean 'can cause the entire rocket to be destroyed, costing near enough to half a billion dollars in damages' as spacex has so recently discovered. $\endgroup$
    – NPSF3000
    Commented Jul 22, 2015 at 3:17
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    $\begingroup$ Both Falcon 9 and Saturn V's 1st stage used cold helium to pressurize the RP-1 tanks. Not liquid helium. The cold helium tanks were located in the LOX tank to keep them cold. But helium was not used to pressurize the LOX tank, GOX was. $\endgroup$
    – user8269
    Commented Dec 4, 2017 at 20:35
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    $\begingroup$ @caInstrument ironic coming from you, since all your posts are reference-unsupported 'trust me, I'm an expert' type posts. $\endgroup$ Commented Aug 3, 2022 at 13:22
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    $\begingroup$ The Aquarius was an expendable SSTO with little mass budget for safer approaches. It also allowed for 33 losses per 100 attempts. Occasional explosions were deemed an acceptable tradeoff for reduced cost...look at Astra to see how that's working out. And only limited small scale tests were ever actually done. (web.archive.org/web/20160303210804/http://www.dtic.mil/dtic/tr/…) $\endgroup$ Commented Aug 3, 2022 at 16:19

“is it a sound engineering concept?”

Either way, it’s a bad business concept.

Fuel tanks, and their vehicles, operate in the real world. Real machinists turning real stock into real stages, that need real processing/field operations, for real payloads demanding real schedules. That field experience has taught us that, given the stakes (multimillion-$/¥/£/€/whatever, not that there’s a spare sat lying around anyway), it’s not worth it. The loss involved with a tank flaw is so high, and the cost of inspection so low in the overall sequence, that an uninspectable tank is JUST NOT GOOD PRACTICE. A submerged tank is not literally uninspectable, but the extra work involved- it’s not worth it.

There are exceptions… real world, there are always exceptions. Ariane 5 ECA had a semi-submerged LH2 tank. But that stage was derived from Ariane 4 (itself from Ariane 3, and so forth…). Between Arianes, and over >twenty years, the LH2 tank was worked out pretty thoroughly. That, and it wasn’t that big of a structure to begin with, certainly in the modern era of launchers, and the greater aero industry. It was the LOX tank that had a giant hole, where the LH2 tank is poking out a bit. But LOX is less likely to seep than hydrogen (what isn’t?), and again, management felt it was within current technology and practice. There were also some missiles that did things with their tanks, but that‘s the missile world. Lots of things you can try when the politicians can be scared into funding your tries.

  • 1
    $\begingroup$ The Ariane 5 ECA core just has cylindrical tanks with a common bulkhead. The bulkhead is domed as is typical for strength, and actually has the oxygen tank protruding into the hydrogen tank. The ESC-A upper stage uses the LOX tank and lower structure from the Ariane 4's third stage, with a new larger-diameter LH2 tank wrapped around the top. This is not a common bulkhead or any kind of "submerged" tank, the tanks are physically separate: twitter.com/Mausonaut/status/771431952507428869/photo/1 $\endgroup$ Commented Aug 3, 2022 at 15:56
  • $\begingroup$ I did get ESC fuel/ox backward, but the larger, upper tank is an odd shape that extends down the sides of the lower tank. Whether or not the separation consists of single-wall or double-wall is semantic- it’s a load-bearing structure because of pressurization alone. $\endgroup$ Commented Aug 4, 2022 at 14:03

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