Comment by jlgreco | Hacker News Reader

Comment by jlgreco | Hacker News Reader

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Grasshopper is a test vechicle, they plan on doing this with the F9R. My understanding is that the F9R will burn once to de-orbit (well, the first stage will be suborbital to begin with), aerobrake without a parachute for a short period, and then burn again for the final landing. I may be wrong about the aerobraking step, but they aren't planning on using a parachute regardless.

IIRC they are expecting a 10% drop in total cargo to LEO when the F9 is used in the reusable configuration. It takes far less fuel to get down than it does to get up since they only have to decelerate the lower stage (not the upper, or the cargo) and since the lower stage will be mostly empty at that point (much lighter). Furthermore the initial decent burn will be at a much lower outside atmospheric pressure (near vacuum?) resulting in a higher efficiency.

Also, the rocket will be falling at its terminal velocity, having lost the rest of its speed to air drag. Even without parachutes (and they don't intend to carry any), that's only ~120 m/s - the upper stage and payload are gone, the 1st stage has big tanks but only a little fuel left, so its density is low. The final burn for propulsive landing only has to cancel that last little bit.

Yup, air resistance works with you on the way down, not against you, and it is courteous enough to start small and work itself up when you are on the way down.

Doesn't it result in a lit of heat from the friction? Or is that only a problem at much higher speeds?

There will be heat; even the suborbital Mercury-Redstone missions needed heat shields.

I don't know how SpaceX is planning on handling that. Maybe the trajectory they put themselves on is going to be slow enough to survive reentry using just the main engine nozzles as make-shift heat shields?

Can't speak to the plan for the second stage (which will do a once-around and de-orbit if I understand correctly), but the first stage will not be following a ballistic trajectory and re-entry like a normal suborbital launch. Its goal is to return to the pad a few minutes after launch.

I suspect they will still do a single burn somewhere around apogee of the first stage; if I understand correctly they are well into their gravity turn by the time it separates so they'll have some horizontal velocity they will need to get rid of.

Maybe they are going to push the gravity turn later into the flight so that the first stage doesn't participate in that as much?

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