A cryo tank within another cryo tank...is it a sound engineering concept?

Question

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.

Answer 1

...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.

Answer 2

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.

Answer 3

“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.