Chasing Ap

[RizzoTheRat]

Had another brief play with this at the weekend.  My original ship was launching at a TWR of about 1.8, but with a fairly short burn first stage (a pair of slightly detuned hammers).  Dropping the TWR to 1.5, and therefore not pitching as steeply in the initial turn, resulted in it hitting orbit with less dV than the 1.8 TWR launch.

The trouble with this is the launch profile is very dependant on the craft design, I think a lower TWR at launch would need a longer burning first stage, or a more powerful second stage, which means it's hard to compare profiles, and it's near impossible to base it on pre-launch dV as the first stage ISP is going to change so much as you ascend.

Edited December 16, 2019 by RizzoTheRat

[Dan Kerman]

I've follow Scott Manley's and others way of launching and gravity turn. 1.5 TWR at liftoff. At 100 m/s tip over 10 degrees east and let the prograde marker catch up. Then lock prograde and let it go. At 10 km altitude, should be about 67 degrees...  at 20km 45 degrees. then slowly throttle back to maintain 45 secs to Ap...  until you are at the lowest throttle setting at which point, about 60 km,  you let the time to Ap run out all it wants. When you hit your target Ap, coast and circularize.

[Laie]

On 12/16/2019 at 10:25 AM, RizzoTheRat said:

Dropping the TWR to 1.5, and therefore not pitching as steeply in the initial turn, resulted in it hitting orbit with less dV than the 1.8 TWR launch.

 

It seems that you're optimizing for delta-V. In that case, you want to fly as fast and as low as you possibly can get away with. You want a needle shaped-rocket, a TWR of 2+ to begin with, and possibly also want to tilt the rocket on the pad for a faster turn after lift-off. Just check out any of the payload fraction challenges we had in the past.

[RizzoTheRat]

On 12/12/2019 at 6:00 PM, Laie said:

I'd say: go for it.

Provide a ready-made rocket and ask people to optimize the launch profile for this particular thing. There may be only a few participants, but I for one would like to give it a try. Depending on how unhappy I am with your LV, I may swap it with one of my own design after a few tries... it would be good if you gave a few requirements for the LV. Probably lowest cost, but does it need to be reusable? Tech level limitations?

Finally got round to trying it as a challenge

 

[CBase]

Things I learned while doing the challenge with MechJeb and therefore fit better in this thread

• Using intermediate altitude and hold AP features in MechJeb Gravity Turn did not save dV if the turn is somewhat close to optimal
• I modified MechJeb to pitch somewhat horizontal instead of throttling down more aggresive if time to AP reaches the limit: no improvement as well
• in classic ascent profile it was easier to adept profile to the given rocket and harder to screw it up