I haven’t run the numbers on this yet, but I was thinking about how to do reusable transportation on Venus recently. My previous Venusian Rocket Floaties blog post showed that existing upper stages, sealed off, could float at altitudes high enough not to melt their seals (though still roasty-toasty). My thought was that you could drop down to that altitude, deploy a balloon, and float back up to a safe altitude for recovery by another vehicle. But I got thinking about the reliability and risks of that approach, and it gave me a crazier idea (which as I said above I haven’t run the numbers on yet):
What if you designed a rocket with one of its tanks (the fuel most likely) actually a balloon with the fuel in gas form? Make the balloon big enough so that when the propellant tank is “empty” (and just some sort of buffer gas is in there to fill it), the whole stage is buoyant at a safe altitude. Leave the engine and oxidizer tanks at the bottom “normal”, but have a big balloon tank attached to them via some sort of truss structure.
- You’d most likely have to attach payloads to the side not the top of the balloon because you probably want the balloon at as low a pressure as possible when it reaches orbit.
- However you’d want it at around 1atm pressure of something when you come back in, so it won’t collapse at the 1atm pressure level.
- You’d have to parallel stage bimese style instead of the more traditional vertical stacking you do on earth, for similar reasons to those mentioned above.
- While a spherical balloon would be most mass efficient, to keep air drag down you’d probably want a cylindrical balloon.
- It might be best to pick a fuel gas that liquifies when chilled (methane or propane?). Then you could theoretically have a fan pull gas from the balloon, and run it through a heat exchanger with the LOX to liquify it before running it into the engine?
- Likewise, you probably want the gas in the balloon on reentry to be something light like GH2 or GHe…not sure the best way to transition between the fuel filling the balloon and this filling the balloon. Could be something carried in an onboard reinflation tank, or it could be something you do at an orbiting station?
- Due to the very large diameter you could get even with a cylindrical real ballon tank, I wonder if you could use that large diameter to wrap a fixed MAC coil around for both initial aerocapture, and maybe magnetoshell assisted aeroentry. Could you get the velocity low enough that your balloon can take the remaining heating without any special TPS on it?
- You probably don’t want to make the thing have to float when fully-fueled–you probably want the carrier blimp to support it until it is launched. That way your balloon volume is determined by the mass of the system at recovery, and you just fill the balloon to whatever density of fuel gas makes the most sense to provide the right amount of fuel for the stage.
I won’t have time to run the numbers on this for a week or more, but I wanted to toss this out there. It’s crazy, but if you could pull it off, it would enable fully-reusable Venus rockets with passively safe recovery. You’d still need a carrier vehicle to come out and fetch it, much like ocean recovery of capsules, but without flat, non-moving platforms to land on, this may be the best way of doing things.
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