Optimal Ascent Profile

[shynung]

OK, here's my problem.

I have designed a 3-man capsule with only enough fuel for orbital maneuvers (typical space capsule), launched by an orange-tank-Mainsail core booster, augmented by 4 identical boosters in a 3-stage asparagus staging arrangement. If I were to launch using the straight-up-and-45-degree-after-10km ascent profile, I end up having to do a 900m/s² circularization burn. While carrying the core stage to apoapsis and using it for the circularization burn is possible, it makes the whole spacecraft hard to turn, and end up orbiting as space junk once the burn is over.

I need a shallower ascent profile that maximizes effective ÃŽâ€v while still in atmosphere to minimize ÃŽâ€v needed at circularization burn, so I could drop the core stage mid-flight. Can anyone explain?

Edited March 25, 2014 by shynung

[Kasuha]

Instead of full circularization, get yourself to orbit with your chosen apoapsis and periapsis at 20 km (anything below 23 km is fine). If you decouple your lifter stages there, they will disappear in atmosphere even if you leave them on rails and you'll have just a few m/s to spend to finish your safe orbit.

[mhoram]

One way to lower the dV needed for the circularization burn is pich adjustment.

Try to lower the pitch at an altitude of 20km to lets say 25° and have a look at the resulting circularizarion burn.

By the way there is no optimal ascent profile that fits all rockets.

Each rocket has it's own optimum that depends on TWR and dV of each stage.

You can find some discussion about ascent optimization in these threads:

http://forum.kerbalspaceprogram.com/threads/39196

http://forum.kerbalspaceprogram.com/threads/46194

http://forum.kerbalspaceprogram.com/threads/58531

[brdavis]

I'm not positive… seriously, I've only had this since like 0.22, so newbie here… but I thought the idea of a gravity turn is to let gravity do all the turning: go straight up until you start the gravity turn (by nudging your nose maybe 5° eastward), and then let gravity turn the heading as it needs to. In one sense that requires very little control (Newton is in the driver's seat), but it does require figuring out when to start it. Try experimenting. As far as not leaving space junk… use the core stage to *almost* due to circularization burn, then dump it, and supply the last tiny bit using the on-orbit capsule. Or, hey, with tweakables, remove fuel from the stages

[KvickFlygarn87]

Well, the optimal ascent profile would be ascending in such a way that you reach apoapsis the instant you circularize.

[capi3101]

You've got five Mainsails to lift a Mk1-2 command pod to orbit? Sounds to me like you may have too much thrust. Try Skippers. Follow the prograde vector and keep your gees down. Push comes to shove, add a couple of round RCS tanks and a block or two to help with the steering.

Any chance we can see a screenshot of your ship? Or perhaps the craft file?

[John FX]

Try not to go too fast, the air resists and you use more fuel than the saving from gravity. Get to 100m/s as quick as you can then make your TWR 1.4 and you`ll use less fuel than full throttle. (this is just a rough guide)

Also try to start your turn at 8km and be at 45 degrees by 20km, 20 degrees by 30km, 15 degrees by 40km, 0 degrees by 55km. then burn at 0 degrees until you have the desired Apo. circularise at Apo and you`re golden.

Without seeing images of your craft it`s hard to say what to do but this should be s good start.

[shynung]

Also try to start your turn at 8km and be at 45 degrees by 20km, 20 degrees by 30km, 15 degrees by 40km, 0 degrees by 55km. then burn at 0 degrees until you have the desired Apo. circularise at Apo and you`re golden.

Thanks, this is what I'm looking for.

BTW, here's the craft in works:

The Mk1-2 pod and upper stage is just a test weight of some sort. I was designing the lifter underneath. It was to be a general purpose lifter in subassemblies, so I don't have to make an entire rocket just to launch some newly-designed pods/sats/probes/whatever.

I'm planning for a 6-booster craft after this (either 7 Mainsails, or 6 Mainsails and a Skipper).

[Tex]

In my experience, it is always best to design subassembly lifters around the target payload weight. Design your station/probe/what-the-heck-ever first, and then develop the lifter to do the mission that you need it to. Once you know what it can do, then you know what you can do with it.

[cantab]

I need a steeper ascent profile that maximizes effective ÃŽâ€v while still in atmosphere to minimize ÃŽâ€v needed at circularization burn, so I could drop the core stage mid-flight. Can anyone explain?

You have this the wrong way round. The steeper your initial ascent, the more you need to burn at apoapsis and the less efficient you are overall. On the other hand too shallow and you'll be even worse as you waste loads of fuel fighting drag.

[shynung]

In my experience, it is always best to design subassembly lifters around the target payload weight. Design your station/probe/what-the-heck-ever first, and then develop the lifter to do the mission that you need it to. Once you know what it can do, then you know what you can do with it.

Much appreciated

[shynung]

You have this the wrong way round. The steeper your initial ascent, the more you need to burn at apoapsis and the less efficient you are overall. On the other hand too shallow and you'll be even worse as you waste loads of fuel fighting drag.

Sorry, I meant shallower.

[Renaissance0321]

I'm somewhat of a KSP noob as well but that rocket looks incredibly inefficient. That capsule doesn't weigh very much so why are you using so many rockets and tanks to lift it? Like someone said previously it is usually better to design a rocket for a specific payload and engineer mod really helps with this kind of stuff instead of trying to eyeball everything. I wouldn't want to design a rocket without it.

[MockKnizzle]

You have a huge TWR at liftoff with all those mainsails. You could replace every single one of them with a Skipper and it would still be overkill. Anyways, like everyone else has been saying, just watch your periapse as you get up nearly to orbit. Once it's up to 20km or so, just stage and drop your lifter.

[shynung]

You have a huge TWR at liftoff with all those mainsails. You could replace every single one of them with a Skipper and it would still be overkill.

That is intended. This was a lifter design; it was to lift heavier stuff to orbit. This early design is rather fragile; the finished one has more strutwork on it.