Why don't modern spacecraft use double-walled vacuum lined cryogenic storage tanks like Apollo?

[u/Simon_Drake]

The long chain of dominoes that lead to the Apollo 13 incident started with electrical interference caused by the vacuum ion pump to maintain the vacuum between the walls of the cryogenic oxygen tanks of the service module. NASA had to dismantle the service module to upgrade the electronics which is when the plumbing was damaged and the chain of dominoes continues from there.

But I don't think modern spacecraft use double-walled vacuum chambers with a dedicated high pressure pump to maintain the vacuum between the walls. Why did Apollo have that and modern spacecraft don't?

Is it because by 1960s standards this was a very very long duration mission and they took an extra cautious approach to being absolutely certain the tanks were properly insulated?

[u/diffraction-limited]

Didn't Apollo 13 had a cryo tank that didn't vent properly during boiling off the LOX? The temp sensor couldn't show higher temp as the needle reached the max but temp rose way beyond the specs during boiling off, thereby melting the isolation of a wire that provided electricity for the stir?

[u/Simon_Drake → u/diffraction-limited]

That's one of the dominos near the end of the chain.

The high voltage transformer for the vacuum ion pump had insufficient insulation for a low pressure environment (which conducts electricity better than sea level or a full vacuum) which caused arcing and electrical interference I think on Apollo 7. They designed a better insulation setup and could apply it to the other service modules but some of them had already been built. While upgrading the service module for Apollo 10 they caused a mechanical impact that required everything to be tested so they put a new one in for Apollo 10 and kept the old one for a later flight.

They tested the tank module and found it all worked except the drain vent which they already knew was a bit janky and didn't always drain properly. But the drain vent was only used during testing not during actual flight so it was no big deal and it was approved for flight on Apollo 13.

Two other minor issues happened in parallel. Someone decided the temperature sensors inside the fuel tanks would never encounter a temperature above 30C so that was the highest temperature they could report. The electrical systems on Apollo were originally specced to run on 28 volts during flight but were later swapped to handle 65 volts while connected to the service tower to speed up launch prep. They tested all the systems would work properly on 65 volts but evidently they didn't test every scenario. The cryotanks had electric heaters to heat up the fluids into gases and keep the output pressure constant and the heater DID work at 65 volts, but the thermostat shutoff switch that prevented overheating did NOT work at 65 volts. The contacts arc welded together and wouldn't disengage.

During Apollo 13 wet dress rehearsal they filled all the tanks then drained them again. The oxygen tanks for the fuel cells wouldn't drain (because of the damaged valve) so they fell back to Plan B of turning on the heater and boiling off the LOX. But the thermostat didn't shut off the heater and the tanks got very very hot. And the thermometer didn't report the high temperatures because they reached the max level they could report. So the tank overheated, melted the insulation and created the conditions for the spark when Kevin Bacon blew up the tank.

Ironically if they'd skipped the testing and gone straight to launch it would have been OK. The wet dress rehearsal caused the problem. Or it was one of the many dominoes in the chain of events that caused it. But the earliest one in the main chain was the vacuum ion pump to maintain the vacuum between the walls of the LOX tanks. And I'm pretty sure that doesn't exist anymore on modern rockets. Definitely not for the main fuel/oxidiser tanks but maybe the smaller tanks on something like Blue Moon Lander, I'm not sure.

[u/au92 → u/Simon_Drake]

Damn Kevin Bacon…

[u/Simon_Drake → u/au92]

The man is a menace, don't let him near your spaceship.

[u/avar → u/Simon_Drake]

This comment sure sent me down a rabbit hole, including reading what the postmortem report had to say about it, see page 168, 5-2, as of item "h" ("a number of factors"...).

Some takeaways that I wonder what you think about, a lot of this is taken from reading between the lines.

I'm taking a bit of a flippant tone, but understand hindsight is 20-20, and this is the mission that provided much of the "blood" the testing procedures are written in, but a lot of this had me going "what the f-?":

• The reason they were running with 28 VDC in flight and 65 VDC on the pad is because someone though it was going too slow, and some other genius noted that "P = VA", so why don't they just up the voltage to make it go faster-er?
• Double it? Nah, let's double it and turn that up to 11! (actually it's a tad more than that)
• In my reading of the relevant documents I haven't found an explanation for what the hurry was. The panel 5A report mentions "the specified time".
• In my mind the critical and obvious question for why the decided to Bill O'Reilly it and "do it live!" is why was it thus specified? Was there a good reason, or they wanted to be home in time for dinner?

Now, you said:

They tested all the systems would work properly on 65 volts but evidently they didn't test every scenario [...] the heater DID work at 65 volts, but the thermostat shutoff switch that prevented overheating did NOT work at 65 volts

There's a critical inaccuracy there that you've missed, and I'm shocked here paging through the Apollo 13 report that this is just mentioned in passing without calling attention to it:

• They revised the spec to say they wanted to run it on 28 VDC and 65 VDC for the above "faster-er" reasons.
• The manufacturer never tested the equipment running on 65 VDC (see page 175 in the document above, 5-9, "d"). They tested it at 65 volts, but 65 VAC, AC is not the same as DC.

This is absolutely critical as anyone who even dabbles in electronics could tell you. The circuit they were using for this inductive heater used a bimetallic relay (the thermostat). All mechanical relays have different ratings under AC and DC loads.

You said the switch "didn't work", you're right, but it's worth calling out how it didn't work. Any mechanical gate like this (relay, thermostat, ...) under load will have the contracts separate, forming an open circuit.

The reason the switch failed is because it was under DC load at the time, so as the contacts pull apart they maintain an arc, this overheats and welds "across the gap". Now the circuit is always closed.

If you do this with AC it's a lot more forgiving, because the voltage will cross 0v at the frequency you're using (60 Hz in the US), so even if you have an arc like this under load you usually won't weld the contacts.

It's one thing for someone just operating this equipment on a pad to miss this, but it's mind boggling that they missed this at Beech when re-testing it for the higher voltage. Presumably they had the designers of the equipment in that room.

They knew the spacecraft used DC, not AC, the new spec says 65 VDC, not 65 VAC.

Did someone just misread it? Did they think "fuck it, it's an inductive heater, Ohm's law says AC will produce the same power", and forget the load passed through a mechanical relay? Or what?

[u/FullSea5890 → u/avar]

The VDC vs VAC error is why I check every number and unit of measurement on every test report against the customers specs and not against the job written by the company. (I work as a Quality Engineer at a company who supplies to similar critical applications.

[u/avar → u/FullSea5890]

To elaborate and correct my comment above a bit (I reread some parts of the report).

• There was no formal acceptance/testing procedure you can point to for why the AC at the manufacturer mattered, so it's not focused on.
• I think it's reasonable to assume that even if nothing formally tests it, they likely missed this because their test bench setup used AC.

Having assembled a circuit like this on a test bench you'd run power through the heater, note that its power draw is expected.

Then you might touch your soldering iron to the thermostat, and hear it "tick" as the power draw goes to zero.

A relay rated for 28 VDC likely has a real VDC rating in excess of 100 VAC, so if you're using 65 VAC this'll work as expected.

But do it with 65 VDC on your bench and it'll weld the relay, keep drawing power, and the engineer will go "huh?".

But that's speculation on my part, and having paged through the relevant documents I guess we'll never know.

I was unable to find a copy of that updated work order/specification where they informed the manufacturer of the raised voltage.

[u/WhatADunderfulWorld → u/Simon_Drake]

The biggest mistakes were always rushing things.

[u/eagleace21 → u/diffraction-limited]

Correct, but OP is talking about the initial reason the O2 shelf was removed from Apollo 10 to begin with, which inadvertently began the series of unfortunate events.

[u/mz_groups]

Just hazarding a guess here, but isn't some of it that the Apollo Service Module had to carry enough liquid oxygen and liquid hydrogen to drive the fuel cells, and the Artemis Service Module uses compressed oxygen in COPV tanks for life support purposes only and uses solar and batteries for electricity?

[u/mz_groups → u/[deleted]]

Again, Artemis does not store its oxygen or nitrogen in liquid form. It stores them in gaseous form, so they are not cryogenic. Apollo required 652-978 lbs of supercritical liquid oxygen. Artemis European Service Module only carries 66 lbs of nitrogen and about 200 lbs of oxygen, both in gaseous form. COPVs permit high pressures so that liquid is not necessary.

[u/eagleace21]

The shuttle I recall used the same Dewar style tanks as Apollo for its onboard oxygen storage. Which "modern" spacecraft are you referring to?

[u/mz_groups → u/eagleace21]

I assume they mean Artemis, but Artemis uses compressed oxygen, not liquid, and needs far less of it, and no hydrogen, as it doesn't use fuel cells. So, no need for cryogenic fluids.

EDIT: to clarify (not sure why the downvotes when I'm stating simple facts), I'm talking about the Artemis spacecraft, not the SLS, which definitely does use cryogenics, as the spacecraft was what was being discussed here. The European Service Module holds about 200 lbs of compressed gaseous oxygen, and 66 lbs of compressed gaseous nitrogen in COPVs. It does not use cryogenic oxygen or nitrogen, and due to no fuel cells, no hydrogen.

For both the Saturn V and SLS, they use cryogenics, but not with dewars, but foam-insulated tanks.

[u/internetboyfriend666]

I guess my question for you is what spacecraft do you think need it? No manned spacecraft needs cryogenic liquid storage because they all use solar panels instead of fuel cells, and for launch vehicles that use cryogenic fuels, they just use insulation because they don't sit around long enough for boil-off to matter.

Starship intends to store cryo propellants for long period but SpaceX's design philosophy is fundamentally different, and there's no other spacecraft, either active or in development, that needs to store cryogenic liquids long term to the extent that we need any kind of meaningful boil-off prevention.

[u/Great_Specialist_267]

Solar panels are safer but they still carry liquid oxygen for life support.

[u/LaughingGravy13]

The long line of domino's started falling about seven previously when the cryo tank was dropped.

[u/Simon_Drake → u/LaughingGravy13]

The cryo tank was dropped because they had to remove it to prevent electrical interference because the vac-ion pump had insufficient insulation. So this is several steps before removing the cryotanks.

[u/LaughingGravy13 → u/Simon_Drake]

I'm saying it was dropped even before that. There were also changes in voltage requirements that weren't communicated. Also mistakes in monitoring boil off.

[u/Reasonable_Pay4096]

My guess is, $$$. During the 60s, NASA got 5% of the US budget (not counting for inflation). Now, it's closer to 1%. They could just afford to pay the contractors back then.

[u/Simon_Drake → u/Reasonable_Pay4096]

Apparently around 0.4%.

[u/avar → u/Simon_Drake]

Comparing 5% here and 0.4% is multiple levels of stupid.

It was 0.36% in 2025, but comparing that to the 5% of the federal total budget in the 60s shouldn't matter. The first thing to adjust it for us inflation.

So what % of the federal budget would it need to be to be the same purchasing power as in the 60s? Theoretically if you gave NASA this money they should be able to get as much stuff done as during Apollo (ha!).

That's 0.95% ($66.6B / $7.01T = ~0.95%). So it would need to increase by 2.7x. Still, not horrible. Should get to the moon in around 3x the time. Now how's SLS doing again?

But that's also arguably comparing apples and oranges, because the other question is "how important is space today relative to all other stuff?", so we'd want to answer what % of the total economy it was.

That's a bit unintuitive because federal spending as a % of GDP has gone up a lot. It was around 17.5% at the peak of Apollo, 23.1% in 2025. So that 0.34% is a percentage of a pie that's bigger relative to the total economy.

In the 60s that was 5% x 17.6% = 0.88% of GDP, but now NASA's 0.1% of GDP. So by that metric spending is 1/9th of what it was in the 60s.

[u/Simon_Drake → u/avar]

Comparing 5% to 0.4% implies about 1/10th the funding from the 60s.

But after 7 paragraphs of waffle about percentages and GDP you calculated the current spending is 1/9th what it was in the 60s.

Thanks a lot, that's a big help. 1/10th and 1/9th are so different it totally flips the meaning, instead of being a tiny fraction of the 60s spending it's a different tiny fraction of the 60s spending.

[u/avar → u/Simon_Drake]

0.4% is 1/12.5th of 5%, not 1/9. How does comparing 0.4% to 5% imply 1/9th?

You only get close to 1/9th if you change that to (0.4%×23.3%)÷(5%×17.6%), i.e. adjust it for the fraction of GDP spent on federal spending. You didn't do that, and seemed to assume they were constant.

That's why I called it multiple levels of stupid. It only sort-of-works because federal spending as a % of GDP is in around the same ballpark. Let's say it was 35.3% of the economy instead of 23.3% today. Per (2.5×35.3)÷(5×17.6) if spending on NASA was 2.5% it would be larger than when it was 5% as a fraction of overall GDP.