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Ferram on Ascent Profile and TWR

Chris Lockfort edited this page Dec 2, 2015 · 2 revisions

Here are some guidelines that I've worked with:

Initial TWR 1.2: Vertical until 100 m/s, ~2-5 degree initial pitch, follow prograde (SLS w/ Pyrios Boosters, Saturn V) Initial TWR 1.6: Vertical until 50 m/s, 5 degree initial pitch, follow prograde (Delta II 7925)

Try to hit 1 km/s when your velocity vector is headed down the 45 degree mark, usually out of the densest part of the atmosphere (somewhere above 20 km, but below 40 km).

Don't hit 1.5 km/s until you're above ~30 km. If you do, you're probably taking too shallow a trajectory or your initial TWR is too high. This won't waste fuel, but it does risk heating melting your rocket apart.

Look to have at least 2.5 km/s of velocity and an apoapsis of >90 km at first stage burnout if your second stage has an initial TWR of 0.9 and will provide most of the dV to orbit. If you're going to need to burn a complete third stage to get to orbit, make sure that your second stage has an initial 1.2 at this velocity.

You can get away with an initial TWR for a second stage (that gets into orbit) of 0.4 if your first stage burns out at ~4.5 km/s - 5 km/s. Generally, in this case you're sending something to another planet, the moon, or to GEO.

You want an orbit of ~190 km for lunar and interplanetary missions, and ~300 - 400 km if you're sending something to LEO. The higher apoapsis gives you more time to circularize.

Don't be afraid to burn upwards slightly and not follow prograde once you're out of the atmosphere. It can help buy you time to get extremely heavy payloads into space. Also, don't be afraid to circularize after apoapsis; most rockets do this actually, since the lower thrust engine allows the stage to weigh less and get more dV.

If you need practice getting into orbit, build a Titan II or a Falcon 9; both have abnormally high-thrust upper stages that will make getting into orbit easier, and then you can go for the lower-thrust upper-stage rockets where piloting is more difficult.

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