Is there an optimal maximum speed per altitude for an efficient trip to orbit ?

[Minarkhaios]

It is something that seems to be hidden in the corner of my brain but I don't have the level of knowledge (or brain power) to find it out precisely (but I do know that I don't know, that's a start).

The question is basically : "is it better to be at full thrust all the way up ? Or is it better to not be over a certain speed at a certain altitude ?"

It always seems a waste of fuel to burn against the air resistance just to gain a small meter per second of speed.

Trajectory is important for a trip to orbit (gravity turn), but is there a similar detailed pattern for speed ?

(I dream of an answer like : "the optimal speed is 150 m/s + 1m/s per 100 meters, any m/s above this speed is just waste of fuel")

Edited December 30, 2013 by Minarkhaios

[Kasuha]

Depends on your notion of "better". If you have enough fuel and want to be in orbit as soon as possible, it's better to use full thrust. If you want to use as little fuel as possible, you should ascend at terminal velocity.

[Novastar]

The first table is the terminal velocity: KSP Wiki, Basic maneuvers

Be as close as possible to this values and you save as much fuel as possible (I think...).

[BlackBicycle]

"is it better to be at full thrust all the way up ? Or is it better to not be over a certain speed at a certain altitude ?"

Of course it is better to keep below certain speed when the atmosphere is "too thick for current speed".

MechJeb does it for you in its "Ascent Guidance" module, and there is also a table of "terminal velocity speeds" at different altitudes. Which basically marks the top speed above which you will have to exponentially increase your fuel flow to reach relatively negligible increase in speed.

P.S. Ninjaed - see message above : )

Edited December 30, 2013 by BlackBicycle

[Minarkhaios]

Many thanks for that !

Now it is time to do it.

[Kasuha]

Well, rather than adjusting your throttle to stay at terminal velocity (although MechJeb can do that for you) it is better to build a ship that will keep terminal velocity at full throttle.

And throw those mainsails out of the window and use engines which have decent ISP.

[Minarkhaios]

I'm more a "solid booster forever *" kind of guy, but with the new tweakables it might be possible.

* forever is not guaranteed for more than 2 minutes, a loud bang and some burning parts ** flying around

** parts includes booster, reactor, limbs, head, and other squishy objects

[BlackBicycle]

And throw those mainsails out of the window and use engines which have decent ISP.

I really want to see you do a manned Jool mission while "throwing those mainsails out of the window". IMHO, you really need to practice a few more dozens of missions before publishing "opinions" like that.

[TheDarkStar]

Mainsails are easy and cause lower part counts, so I use them for the first liftoff stages for medium to large rockets. For anything else, I use NERVAs for space travel, varying engines for landers, and sometimes air-breathing engines for Kerbin and Laythe. I also use aerospikes for the I_sp for things like Eve ascent attempts.

[BlackBicycle]

I see. Well, personally, I use my Mainsails for everything but the transfers planet-to planet, in which case I use nuclear for their efficiency. And the small fry, of course I use the small fry engines : )

[Kasuha]

I really want to see you do a manned Jool mission while "throwing those mainsails out of the window". IMHO, you really need to practice a few more dozens of missions before publishing "opinions" like that.

I think I have done enough missions to be able to afford my opinions. I have done quite a few Jool missions with no mainsails at all, too.

Mainsails and fuel effectivity don't go together well. That's all I'm saying. Sticking to terminal velocity to save fuel if you're using Mainsail is pointless because with Mainsail you're wasting fuel no matter what.

[Guest]

Mainsails have their place for heavier lifting. While a bunch of LVT-30s are better in pretty much all ways, they also require a ton more parts. When you combine that with stitching struts and asparagus staging, you're looking at a pretty complex rocket. It is also entirely possible to use mainsails and stick within the optimal launch speed graph that was earlier posted, you just have to have the right combination of engines/fuel/payload. In fact, I did one last night that was pretty close; brought 95 tons to orbit, think I started with a TWR around 1.4, maybe 1.3, hit 200-something m/s around 8km.

In all honesty, though, you really only need to worry about terminal velocity losses until you hit around 12km. I don't think I've ever got flames on my launches, even when launching satellites at a TWR > 2. For heavy launching with Mainsails aim for a TWR around 1.2 ~ 1.4 with a decent early delta-V (1500m/s or better), you should be just fine.