Is is practically impossible to leave Jupiter's atmosphere with just chemical propulsion. The deltaV requirements are too high.Just to get to Low Jupiter orbit requires over 33-12 = 21 km/s of deltaV compared to less than 10 for Earth.You need SF level nuclear propulsion.And as many mention it's easier to get propellant from Uranus.
Quote from: lamontagne on 03/16/2022 04:50 pmIs is practically impossible to leave Jupiter's atmosphere with just chemical propulsion. The deltaV requirements are too high.Just to get to Low Jupiter orbit requires over 33-12 = 21 km/s of deltaV compared to less than 10 for Earth.You need SF level nuclear propulsion.And as many mention it's easier to get propellant from Uranus.No, it’s not “practically impossible to leave Jupiter’s atmosphere with just chcemicql propulsion” and no, you don’t need “SF level nuclear propulsion.” An expendable Starship (shortened to increase thrust to weight) with a Falcon 1e as payload (and maybe a Falcon 9 first stage to get it going) could get about 1 ton to low Jupiter orbit.People who make such claims should learn about the rocket equation. Jupiter ISRU is impractical for the fuel ISRU purposes given, but if there was something extremely valuable that you could afford a couple hundred million dollars’ worth of expendable rocket per mission, and all the engineering required to get it there (which would be the expensive part), then it is most certainly possible.I agree about Uranus, of course, but you CAN just spam the logarithm with a huge mass ratio. And it’s not particularly hard to do IF you had a good reason for it.
Add more stages like I said. SuperHeavy +expendable starship + F9 first stage+ Falcon 1e first stage + Falcon 1e upper stage.I get about 21.5km/s (24km/s with a small payload). In reality, you’d be better off replacing the F9 and F1e first stages with some Raptor-based stages. And replace the F1e upper stage with a centaur. That pushes the delta-v above 25km/s (about 29km/s if you have a very small payload). You need more than just the bare minimum 21km/s because of gravity losses and aero losses. But the overall idea holds. Chemical expendable rockets can do this job, if you had a really good reason to do it and needed just a small payload.The hard part is getting that rocket stack there, along with the propellants.
Think about it this way. If you’re in an elliptical orbit dipping down only at perigee, you’re moving at almost 59-12= 47km/s. Exhaust velocity of chemical rockets tops out at a tenth of that. You have to make that propellant then match the velocity of your vehicle (and the energy is dissipated as heat). So you’re dissipating more inertia by capturing the propellant than that propellant will give you if burned. By an order of magnitude.
There's a lot of discussion on the impracticality of this due to the extreme Delta V costs in a large gravity well. While this is true, that's not exactly what I meant. The idea here was to only make one pass through the atmosphere, when the vehicle is approaching at interplanetary velocity. It wouldn't use its engines at all in the atmosphere - it would simply aim for a periapsis that would result in sufficient deceleration to get into orbit, without getting stuck in the atmosphere. A small burn could be performed if necessary, but I'm not talking about getting into orbit and then entering the atmosphere; I'm talking about simply opening a vent when you use the atmosphere to enter orbit. Ideally, the Delta V requirements for this would then be minimal.Also, I agree that other gas giants might work much better. Jupiter was just the first that came to mind, but Saturn's low gravity, decent proximity to Earth (compared to Uranus and Neptune, which are also decent targets), and interesting moon system may also be a good site for this maneuver.
