Should I just go throttle full until orbit?

[SpaceGoblin]

So I am playing unmodded, which might make a difference. Just pure original KSP.

When I launch rockets, I just go full throttle until I reach about 70k up and 1800 per sec sideways (surface speed). This doesn't put me in full orbit but I do another burn to make the orbit a complete circle, after checking my map.

I guess my question is, should I go full throttle the whole launch? I mainly wonder about fuel consumption. It doesn't seem to affect the Kerbal astronauts any. Going less than full makes me not hit my orbit height.

Is there a better way to do it for fuel? Or is this ok?

[Taki117]

As long as you don't hit Terminal velocity at all you should be fine going full throttle, I assume you are doing a gravity turn?

[Smidge204]

Less throttle means you go slower, spending more time in the atmosphere, which in turn means your Isp suffers longer and you spend more time fighting the air, which reduces efficiency overall.

You want to go as close to terminal velocity as possible, but not going over because then you're wasting fuel again. Get out of the atmosphere as soon as possible - which is why it's common practice to go straight up until ~10km just to get up to where the air thins out before picking up horizontal velocity.

So unless your TWR is too high for your craft and it shakes itself to bits, there's generally little reason to throttle back at launch.

=Smidge=

[Andrew Hansen]

Hey there,

If you go to the wiki and scroll down a bit you can see the terminal velocities at various altitudes for Kerbin. The most fuel-efficient launch stays close to those numbers. So in answer to your question, if you have overly powerful engines you'll need to throttle down to stay at terminal velocity (unless your not concerned about fuel efficiency). Or, if your engines are slightly weak, you'll find yourself keeping the throttle at 100% all the time.

For me, I usually end up with lower-than-ideal TWRs, and so I'm usually always at full throttle. But alas, keeping your speed perfect isn't necessary. You can afford to waste some fuel if you have enough Delta-V.

[longhornchris]

That will depend on your TWR. If you've got a really high TWR, and thus can get going really, really fast where drag losses become significant. As long as you stay below terminal velocity then keep the throttle maxed (or maxed so you don't overheat). KER's surface info tab has the current terminal velocity available.

Ideally, your TWR should be such that you just barely ride the terminal velocity line. Any more and you've got more engine than you need.

I use FAR most of the time. FAR's flight data tab provides equivalent information via the dynamic pressure (Q) term. Drag in FAR is (if I've read Ferram's implementation correctly) Q * Cd * A, where A is a reference area an Cd is the drag coefficient which changes with the angle of attack (angle to the airflow). Q is (1/2) * density * velocity^2. For FAR, keeping Q 'low' is equivalent to flying the terminal velocity profile - if I Q gets too big I throttle back until it starts dropping due to altitude and then raise the throttle.

[Murph]

Ideally, your TWR should be such that you just barely ride the terminal velocity line. Any more and you've got more engine than you need.

This, more or less, for me. I'm not adverse to having slightly excessive TWR for about the first 30-45s from SRBs + main, throttling up after SRB separation (not unlike the real Shuttle launches in concept), if I just want that little bit of extra dV without making the entire 1st stage taller. Other than that, the "perfect" rocket for me is one which sits at around 100-120% of terminal velocity from not far above the pad up to around 30,000m.

[SRV Ron]

Balancing of thrust to weight ratios throughout launch to orbit can make a difference of having a 760 ton rocket placing a payload of 108 tons in orbit, or needing 1500+ tons to acomplish the same goal. This is often where the varied selection of engines from mods, such as KW Rocketry and Nova Punch can help out with performance design.

Tweak-able SRBs are going to be a big factor in efficiency in the initial launch phase.

[EtherDragon]

SpaceGoblin,

Basically, you should do some experiments with different ascent profiles to find out what is most efficient for you. It will probably look pretty similar to other good flight profiles - but vary slightly on the way to fly.

Check out this video which is all about ascent profiles, and why certain things are done they way they are...

LAB - Ascent Profiles

[SpaceGoblin]

As long as you don't hit Terminal velocity at all you should be fine going full throttle, I assume you are doing a gravity turn?

I'm new to KSP and don't know anything about rocketry in general, so I'm not sure what a gravity turn is. Forgive my poor descriptions I am about to use:

I get to 20k in altitude, before I start angling 45 degrees. From about 40k altitude to 70k I am slowly angling more toward 70 degrees. When I reach 70k altitude, I am angled almost 90 degrees and going about 1800/sec on the surface reading. At this point I stop engines, check my map, and look for the marker on my orbit path. I line up the yellow circle thing, and when I hit that mark I mentioned I throttle full until my orbit is a circle.

I've managed to land a Kerbal on the moon but he got stuck there. Right now my biggest problem is finding out how to add more fuel to my rockets. All attempts so far fail to reach proper orbit, so I seem to be at a weight limit for the science I have unlocked. I might do some simpler missions to get science points to unlock stuff.

[Pope_Gregory_IX]

Hi,

In that case, you need to look at gravity turns. There are some great tutorial videos on YouTube for KSP beginners that you may like to look at. If you have reached and landed on the Mun without a gravity turn and are stranded, you're not doing badly! Doing the gravity turn may give you enough fuel to get back with the same rocket...

Cheers,

Adam

[Soda Popinski]

When I was playing stock, when I hit 10,000 meters, I started gradually turning to East (Right / D key) to about 30 degrees by 30,000 meters (easy to remember). According to the Basic Maneuvers on the Wiki, you should be hitting about 260 m/s at 10,000 meters, and end at around 2000 m/s at 30,000 meters. At that point, you can probably be going full throttle (if not already), as atmosphere is negligible. I usually kept at 30 degrees until my apoapsis hits 100,000 meters, then I cut engines and coast up. I re-orient to the horizon, and burn again to raise the periapsis right before hitting the apoapsis (20 to 15 seconds before) - burning so the ETA to apoapsis doesn't go up or down too much (chase the apoapsis). Stop your burn when your periapsis climbs out of the atmosphere (70,000 meters) or when your circularize.

[MrPopcup]

I had the same question, so I did some tests!

what I found out is:

SMALLER craft won't waste fuel if you are on 100% full throttle ( you can reach 12km without wasting fuel, if you know how to do it well. )

MEDIUM crafts wont waste fuel if you are on 100% ( you can reach 9-10 km without wasting fuel, then do the gravity GRADUALLY! )

LARGE CRAFTS wont waste fuel if you are on 100% ( you can reach 7-9 km without wasting fuel )

this means->

SMALLER CRAFTS make gravity turn's later ( higher Kilometers )

MEDIUM CRAFTS make gravity turn just in between.

LARGE CRAFTS make gravity turn EARLIER

SIDE NOTE*: I normally do a full throttle until I reach 150-200m/s INSIDE the lower atmosphere ( the darkest colour ) then throttle down to about 1 TWR, say my speed is 160m/s, I throttle down to a point where my speed won't decrease, but not increase too much! so that I won't waste fuel fighting the atmosphere. and this actually works.

Edited December 18, 2013 by MrPopcup

[Murph]

I had the same question, so I did some tests!

…snip…

It's not related to the size of the craft. It's all about the TWR (thrust to weight ratio). If the TWR is excessive, 100% throttle will be inefficient due to the drag from significantly exceeding terminal velocity. If the TWR is just right (approx 2.0), 100% throttle will keep you on terminal velocity until you are out of the thick part of the atmosphere (approx 25–30,000m).

[SRV Ron]

It's not related to the size of the craft. It's all about the TWR (thrust to weight ratio). If the TWR is excessive, 100% throttle will be inefficient due to the drag from significantly exceeding terminal velocity. If the TWR is just right (approx 2.0), 100% throttle will keep you on terminal velocity until you are out of the thick part of the atmosphere (approx 25–30,000m).

That's what allows this huge design to place 100+ tons into orbit.

And beyond

[Skorpychan]

Personally, I go full throttle at launch, until the first couple of small asparagus-ed tanks have dropped off. That gets me to about 3-4k depending on payload, then I start and throttle back bit by bit, until I'm just about accelerating. Then I make a 45 degree gravity turn at 10k, and burn full throttle until my apoapsis is 75-80km. The main boosters usually run out and are dropped halfway through this stage, leaving me with the main engine and a full tank of fuel.

Then I cut the throttle and coast until 70-75k, turn, and burn prograde, cutting throttle and coasting as needed to maintain efficiency. The main engine and fuel tank are currently dumped after a stable orbit, but that's somewhat inefficient, and is pending a solution.