1.0.2 - Fuel Efficient Launches

[Hamburglar]

Hey there,

Just wondering what the take on throttle control for fuel efficient launches is in the new version?

I may be totally wrong here, but it used to be that if you saw mach effects, you were going too fast and wasting fuel battling against drag. Or at least that's what the tutorials taught.

However, some limited testing in 1.0.2 suggests that the optimal launch is simply wide open throttle from the start, regardless of speed or mach effects. Anything less just uses more fuel for less AP height assuming a straight up sub-orbital trajectory. It seems the only reason to throttle back is to prevent rocket flips from too much wind pressure while gravity turning. I may be totally wrong here? Any thoughts? I'm not talking ascent curve, just throttling and speed management. It appears to have become moot (or perhaps it always was?)

Thanks!

Edited May 6, 2015 by Hamburglar
spelling and clarity

[Taki117]

Throttle and speed management were important when terminal velocity was a set value (due to how drag was calculated). With the new aerodynamics terminal velocity is still a function of drag, but because drag has been recalculated and now depends on rocket shape terminal velocity has gone up significantly. (From what I understand playing with pre 1.0 FAR) So much so as to not matter unless your rocket is extremely over powered. This means that as long as you are not getting reentry flames on ascent you should be fine (as terminal velocity gets up to orbital velocity rather quickly with a well designed rocket).

[magnemoe]

Note it depend a bit on your design too, if you have to launch something who is not aerodynamic you should climb up to 6-7 km before starting turning, the rule about turning slowly still stands and is critical as this rocket is not aerodynamic stable and might tumble, this is especially dangerous then you drop the long first stage and activate the stubby upper stage

[falconek]

In vertical flight, terminal velocity is by definition achieved when rocket's TWR is equal to 2 plus enough surplus to acclerate to new terminal velocity (on higher altitude). Personally I try to keep my TWR between 2.1 - 2.5 during the atmospheric part of ascent.

Of course, with lower drag, it is less important to keep your speed below terminal velocity, so you may just want to keep your TWR relatively high, to decrease losses from gravitational drag. Overall, like Taki117 said, as long as you don't see flames, you are probably fine.

[technion]

wide open throttle from the start, Thanks!

Wide open at the start definitely makes sense because, no matter what your TWR, you aren't going to go from 0 to "too fast" in the first few seconds. This is also why small SRBs seem to never be a bad thing.

It's definitely important however to manage your speed beyond that. If not for efficiency reasons - it's because going too fast makes it very likely to flip over once you start to turn.

[Blu_C]

As a rule of thumb I find that with the current aerodynamics a TWR of around 1.5 works pretty good, and that a TWR of 1.8 is nearing the threshold where things are overcooking. You can go to a TWR as low as 1.3 if you need to, but things will be less efficient as you cannot accelerate as fast so you'll have significantly more gravity losses early on.

Rockets with weaker TWR's also need to have a more vertical ascent than ones with a higher one (by contrast a higher TWR allows you to safely take a flatter ascent which in my experience is generally more efficient so long as you don't flatten out too much before 10k). I've got a completely unguided launch system that gets a smallish payload into orbit and is at a pitch of probably 25 or 30 degrees at 10k, but by that point the TWR 8 or something absurd like that as the SRB burns itself out.

[falconek]

Wide open at the start definitely makes sense because, no matter what your TWR, you aren't going to go from 0 to "too fast" in the first few seconds.

Yes, optimal TWR at the moment when acceleration starts is infinity

[maccollo]

Higher TWR (like 1.5-2 rockets tend to be more efficient both when it comes to the payload fraction and the deltaV. However now that engines are much more expensive than fuel they are probably not the most cost effective rocket you can build.

[Hamburglar]

In vertical flight, terminal velocity is by definition achieved when rocket's TWR is equal to 2 plus enough surplus to acclerate to new terminal velocity (on higher altitude). Personally I try to keep my TWR between 2.1 - 2.5 during the atmospheric part of ascent.

Of course, with lower drag, it is less important to keep your speed below terminal velocity, so you may just want to keep your TWR relatively high, to decrease losses from gravitational drag. Overall, like Taki117 said, as long as you don't see flames, you are probably fine.

Thanks! It makes sense to me and I always used to follow the previous "rules" of aiming for specific speeds at specific altitudes on the way as seen here: http://wiki.kerbalspaceprogram.com/wiki/Atmosphere#Terminal_velocity

Just as an experiment, I put a 3 part rocket together. Mk1 pod + big orange tank + mainsail. I launched it straight at full throttle with SAS on, no gravity turn and reached an AP of 16,400,000m with Mach effects, flames and everything. Basically, everything they say not to do.

If I use the same rocket but throttle down when the mach effects start and then gradually throttle back up enough to keep mach effects at bay, I only reach an AP of 6,300,000m.

Perhaps I'm missing the point entirely, but it seems the old "terminal velocity" rule with regards to speed no longer exists. It's full throttle all the way and the only logical reason to back off is to keep your lumpy rocket stable through the dense 5k-15k altitudes and permit early gravity turns. Right now it almost seems cheaper on fuel to just blast through to 20k as quickly as possible and then start turning than it does to gravity turning early (and have to slow down). This is just based on some pretty rudimentary tests but flames with regards to fuel economy appear to be a good thing. I don't quite understand.