Could a fairing vacuum protect the payload from acoustic loads?

Question

This is a weirder question, but here it goes:

During launch, a payload (let's say a satellite) undergoes intense shaking and acoustic loads. This is known from the launcher parameters and the satellite is adjusted to be able to handle these loads, undergoing environmental testing before. The payload fairing on a rocket is designed to protect the payload from some of that, besides the other thermal and atmospheric effects.

From answers to Pressure inside a launch fairing - falcon 9, we know that the environment inside a fairing is pressurized.

My question is why isn't it a vacuum instead? Wouldn't this negates most of the acoustic loads on the payload, and thus make the shaking the main concern during launch? I see that a good vacuum would be pretty hard to achieve inside a fairing, and there are lots of small problems with that, but I can't see what they could be.

I see this as a possibility for more delicate payloads, say a telescope mirror, but I don't know in that case whether the engine shaking is a bigger problem than the acoustic shaking generated, and whether that effect is as problematic as I'm thinking.

Might be too much for this question, but I'd greatly appreciate if someone could provide a reference on what's harder to design for in a launch (vibro or acoustic?).

Answer 1

I can't comment on the advantages, but there are certainly quite some disadvantages.

First of all, such a fairing would have to be structurally quite different from the ones you know. Normal fairings only have to cope with the dynamic pressure, which peaks at a little below 40 kPa for both the Shuttle and Saturn V (and likely in the same ballpark for other launch vehicles). Doesn't sound like much, but it's already an important design constraint.

Your evacuated fairing would need to handle additional 100 kPa of sea-level air pressure and would thus have to be at least 3-4 times stronger and heavier than a normal fairing. Given that current fairings already weigh several tons and you need to carry them all the way out of the atmosphere to be useful, this extra weight will significantly cut into your payload.

But it gets worse. Rockets normally fly a near-zero AoA, so the dynamic pressure acts as a head-on compression load. That's also why their shapes are very similar to gothic arches. But the air pressure would act on an evacuated fairing from all sides. An empty soda can will also easily handle loads from the top, but try squeezing it and it'll fail miserably. The fairing would thus have to have either a radically different (more spherical) shape, or additional bracing to handle the loads from the sides. It's also quite likely that you'd have to design most of the launcher around that weird fairing. You might have a hard time finding enough payloads that would appreciate the vacuum enough to pay for all that development.

Finally, it would be really hard to actually evacuate the fairing. Even though you'd probably need just a lousy vacuum (a couple kPa), evacuating something the size of a bus or a living room to that level isn't exactly easy. (Pressurizing stuff is trivial, you just need a pressure bottle and a valve, but the opposite requires real work as an unfortunate consequence of the second law of thermodynamics.) That's why we're all still stuck on trains instead of taking a Hyperloop to work every day.

You could evacuate it using ground equipment and just keep it sealed. But with all the shaking during the launch, just adding an O-ring (or two) won't likely be enough to keep it perfectly airtight. It only takes a tiny leak somewhere to completely ruin your vacuum and all of its advantages, turning your precious fairing into dead weight. Or you could add some pumps and batteries to actively keep the air outside during the whole launch, but given the volume involved, we're again talking about several tons of weight cutting into your payload.