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[ank]

well, first of all the less you accelerate the more you lose to gravity drag. if you can set all your engines to fire using a concentric construction, you will obtain a much greater efficiency from the same number of fuel and engines.

Gravity losses as a proportion of delta-v are minimised if maximum thrust is applied for a short time, or if thrust is applied in a direction perpendicular to the local gravitational field

So the trick is to find the sweetspot between gravity drag and air resistance?
What acceleration is optimal in the lower atmosphere?
And what about higher up?

and why doesn't the craft have an accelerometer?

[LoSboccacc]

pretty much. an intense burn at start, a conservative 150/200 m/s speed up to the 1/3 of the atmos, then burn as a madman.

if you have enough power, you should burn vertically to reach a 80km ap and then horizontally to reach orbital speed at the latest time;
usually you have to start horizontal burn early: if you can accellerate 3m/s^2 and orbital speed is 3000m/s, you will need 1000s just to reach orbital velocity, more if you're not burning horizontally*.

the trick is the balance of this.

btw, yes it has an accelerometer, it is the g meter on the right.


*I may have messed this up.

[ank]

Hmm, I just tested this using a small rocket.
test 1: Burn constantly at max power.
test 2: Burn until a velocity of 160m/s is reached, throttle down to maintain speed until 1/3 atmosphere(there darkest blue bar), then gun it.
test 3: My usual method. Burn at max until apoasis of 65km is reached. Then burn once outside of atmosphere.

Results:
test 1: 1394km altitude
test 2: 560km altitude
test 3: 630km altitude

It seems clear then, that gravity drag is a much higher factor than air resistance in KSP.

And we can conclude that burning burning everything at max thrust is the way to go.

[LoSboccacc]

yes, gravity drag is lower if you burn the most at the lowest altitude possible.

but you are not going up to reach orbit 

[sneakey pete]

I've been mucking around with SSTO space planes. Best so far has been 2200m/s orbit velocity at 70km. Soo close. Yet so far. Also landing stability leaves much to be desired due to weight concentration at the rear.

[ank]

yes, gravity drag is lower if you burn the most at the lowest altitude possible.

but you are not going up to reach orbit 

This was to get a straight up measurement, so I could compare.

In the case of getting into orbit you would ofc burn perpendicular once you reach 60km.
This test could also be done as most fuel left at 60km, but would show the same(though not as extreme differences).

The best method for reaching orbit is as far as I can tell:
Burn like a madman until 60km apoasis is reached, then burn perpendicular, throttling the engine only wastes fuel.
Maybe you need to go from prograde to perpendicular in a fluid motion.(seems to be the way NASA does it)

[LoSboccacc]

they do it gradually so gravity turns the craft instead of angling the engines, which wastes dV

gravity drag stops being relevant once you're in orbit, at which point you need not to burn anymore to compensate the craft dropping, so no, the nose up altitude test isn't relevant.

the fact that you need to burn at maximum as soon as possible is correct, but also depend on your craft. if you have enough booster to propel you at 200m/s or more, you can use the booster alone and use the fuel engine after boosting and at start when v < 100m/s

[ank]

they do it gradually so gravity turns the craft instead of angling the engines, which wastes dV

gravity drag stops being relevant once you're in orbit, at which point you need not to burn anymore to compensate the craft dropping, so no, the nose up altitude test isn't relevant.

the fact that you need to burn at maximum as soon as possible is correct, but also depend on your craft. if you have enough booster to propel you at 200m/s or more, you can use the booster alone and use the fuel engine after boosting and at start when v < 100m/s

I beg to differ. I'd like to see a test where this is the case.

The altitude test is appropriate because we can just imagine any thrust over 60km to be perpendicular instead. the thing is measuring your altitude is easier than remaining fuel.

The point is finding the method that leaves you with the most fuel at 60km, which is the minimum altitude you can get a stable orbit, after that, yes gravity drag is not an issue.

If the test was redone with an orbital manoeuvre in the end (with a bigger craft) I doubt the results would be different.

on top of that, I cannot recall a space launch that did not fire in on continuous burn(besides waiting for stages to separate).

going to make the same test with boosters and see.

EDIT: tested it with boosters bringing it up to exactly 200m/s with boosters alone, and it does work!
Craft went to 7600km with everything firing from the start, and got to 3000m/s at peak velocity.
But the one where the main engine was fired after the boosters, got out of Kerbins soi and reached a peak velocity of 3300m/s

so above some speed limit in the lower atmosphere it gets inefficient. I think this limit is above 200m/s though.

[alway]

Atmos drag is related to speed; gravity drops off with distance. The optimal speed in atmos for a craft is one such that increasing speed results in atmos drag which is greater than the decrease in a sum of gravitational pull from the decreased distance and where decreasing the speed results in the increase in force from additional gravitation being larger than the decrease in atmos drag.

That is to say, there isn't just one number, but you want to be as close to whatever it happens to be at any given moment for optimal launch.

[MarcAFK]

Atmos drag is related to speed; gravity drops off with distance. The optimal speed in atmos for a craft is one such that increasing speed results in atmos drag which is greater than the decrease in a sum of gravitational pull from the decreased distance and where decreasing the speed results in the increase in force from additional gravitation being larger than the decrease in atmos drag.

That is to say, there isn't just one number, but you want to be as close to whatever it happens to be at any given moment for optimal launch.

This is what happened to my head whilst attempting to read that sentence:

Spoiler (click to show/hide)

[LoSboccacc]

they do it gradually so gravity turns the craft instead of angling the engines, which wastes dV

gravity drag stops being relevant once you're in orbit, at which point you need not to burn anymore to compensate the craft dropping, so no, the nose up altitude test isn't relevant.

the fact that you need to burn at maximum as soon as possible is correct, but also depend on your craft. if you have enough booster to propel you at 200m/s or more, you can use the booster alone and use the fuel engine after boosting and at start when v < 100m/s

EDIT: tested it with boosters bringing it up to exactly 200m/s with boosters alone, and it does work!
Craft went to 7600km with everything firing from the start, and got to 3000m/s at peak velocity.
But the one where the main engine was fired after the boosters, got out of Kerbins soi and reached a peak velocity of 3300m/s
.

Told ya

Nice to have it tested, good work on that!

Earth launches are limited by load, I think. The profile nasa did for a 10g launch differs quite a bit and actually throttles engines.

I have the graph somewhere...

Edit: here http://www.projectrho.com/public_html/rocket/images/mission/accelerationProfile.jpg
And it was 4g.

[Sheb]

I can't access your link.

[Techhead]

It probably doesn't accept external referrals, but if you copy and paste the url, it works. At least, it worked for me.

On topic, the NASA profile is likely near optimal for real world conditions, but determining the acceleration profile for a specific rocket might be tricky. Not to mention, you can actually get a higher apoapsis if you level off earlier, due to the effects of your vector on gravity drag.

[ank]

Looking at that graph there is a sudden drop in G, followed by an exponential climb.
That looks a lot like the graph you would get from going full throttle, then dropping off solid rockets, then as gravity drag and air resistance decrease you get more G.
Mind you i don't know what rocket this is, or if it's the space shuttle or what.

In KSP I i think the game handles mass and drag with proportion, so that something weighing 2 tonnes have twice the drag of something weighing 1 ton.
If this is true, then the golden number should always be the same for all crafts, since surface area is not a factor.

And if it's the same for all ships, then it should be possible to find that golden number:
Onwards dwarves scientists! we must uncover this mystery!

[LoSboccacc]

It is a theoretical rocket with infinite twr climbing earth.

You get the 4g limit for humans to live and the drop is throttling to save fuel.