SpaceX Falcon 9R ascent path

[Jhonny007]

Did anyone else notice, that The ascent path was really steep? They where at 600km altitude with only 450km ground distance... Isn't that incredible inefficient?

[N_las]

I think there was the intention to get to a 750x615km x47° Orbit with a single burn of the second stage. The satellite deployment can now happen very early. A more traditional ascent path to that altitude would involve long coasting.

[cantab]

IIRC they want to fly the jettisoned stage back to the launch site. A steep ascent profile will make that more easy.

[sojourner]

The ascent path was at the request of the payload provider.

[Mattasmack]

It was also a very light payload for the rocket -- around 1000 kg, when it is capable of lifting over ten times that amount to orbit. So they certainly had some leeway to take a less-than-ideal trajectory. (I am curious about how they handle such light payloads. Do they have to add ballast? Do they launch with less than a full fuel load, or does any unneeded fuel act as ballast? Do they have to throttle down more than usual to stay within acceleration limits?)

[DonLorenzo]

It was also a very light payload for the rocket -- around 1000 kg, when it is capable of lifting over ten times that amount to orbit. So they certainly had some leeway to take a less-than-ideal trajectory. (I am curious about how they handle such light payloads. Do they have to add ballast? Do they launch with less than a full fuel load, or does any unneeded fuel act as ballast? Do they have to throttle down more than usual to stay within acceleration limits?)

I reckon none of that is needed. The payload is such a small fraction of total rocket mass already that an order of magnitude reduction in that doesn't change things dramatically for the first stages. I'm sure the upper stage was calibrated and throttled to fit the payload and target orbit, but that's something that they surely always do

[Mattasmack]

I'll admit, I wrote that last message without actually looking for numbers. Looks like you're right. Using the numbers from http://www.spaceflight101.com/falcon-9-v11.html, for a light payload like this one the second stage would have to throttle down significantly to keep the acceleration reasonable (under 5g's, say) if they run the tanks close to dry, where it wouldn't be necessary for a heavy payload. On the other hand, there's enough extra deltaV in the second stage for a light payload that if they start with a full load of propellants, it should finish its burn before running close to dry. I thought the first stage burn was shorter on this flight (MECO around 2:43, where SpaceX's website says the first stage burn is 180s), but looking back at a couple of previous flights on Youtube it looks like that's the normal MECO time as well. So, never mind!