Ascend profile - wacky or working idea?

[heng]

Trying to get to Minmus with a top-heavy launcher, not very aerodynamic (only have 1.5m fairings)...

So, the general consensus seems to be a pretty flat profile, turning more or less immediately after launch - not easy, see above.

Can anyone who knows some numbers tell me, what the effect would be of the following? delta-v wise, i mean...

Do no gravity turn _at_all_ inside the atmosphere.

- just burn straight up(*), until you AP is at roughly 150-200 km or higher?

- resulting AP should still be way above 100km after leaving the atmosphere

- do _not_ circularize, just raise you PE to 70km

- at PE start your escape burn towards Minmus

Granted, you would have to time the launch just right to encounter Minmus...

pro:

: minimum of atmospheric influence

: no stability problems due to drag

: high AP makes for an efficient burn to raise PE to >70km

: low PE makes for an efficient burn to Minmus

con:

: did not find any reference to it, so i assume it can't be the optimal ascend...

--

(*) yes, i know, burn downwards. you know what i mean :-P

Edited May 21, 2015 by heng
typo

[Streetwind]

The higher your apoapsis is, the less dV you need to invest in order to raise your periapsis above the atmosphere, yes... but, at the same time, the more fuel you need to invest to get the same amount of dV. This is called the Oberth effect, which says (simplified) that the deeper inside a gravity well you perform a burn, the more efficient it will be.

The sideways velocity needed to stay in orbit is higher than the upwards velocity required to leave the atmosphere... actually, in real life this is far more pronounced than in KSP with its tiny planets, but it still applies. Therefore, you're performing the largest part of your orbit insertion burn at the most inefficient point.

Additionally, when you burn straight up, you are fighting gravity. For every second of acceleration, gravity accelerates you with roughly 10 m/s² in the opposite direction. But on the other hand, if you burn perfectly sideways, towards the horizon, there is no such opposing acceleration. That means your engines effectively produce 10 m/s² less acceleration upwards than they would produce sidewards - that amount of acceleration is wasted on just hovering and preserving the status quo, while only the excess actually contributes towards lifting you towards space. Rockets turn sideways very early because the less time they spend burning downwards, the less fuel is wasted on fighting gravity.

So although your idea isn't bad in principle, you're probably going to lose more fuel to inefficient burns than you get back from exploiting favorable orbit shapes.

Though, if you wanna figure out the exact thruth, you could grab MechJab and run a series of tests...

Edited May 21, 2015 by Streetwind

[n0xiety]

I actually posted this in another thread but this looked perfect for it.

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What you see here is a Fuel refinary and a science jeep with basicly 50000dV and can go anywhere fast on minmus. Collected all the science in 30 minutes and visited atleast 20 sites for crew reports which was my mission. (I know 50k was a bit op. Ended up using 15k for everything on Minmus) The Lander works as a refueling station for the 4 ion engines at the bottom of the jeep + its the way back home leaving the jeep at minmus. As you can see the minmus bus is not symetrycal and this causes alot of problems with drag. I have to force it to stay straight because of the uneven drag forces. Mechjeb provides me with the easy solution to this = Precision which is humanly impossible. You can set the angle of attack against the prograde so that it never turns more degrees then you set before the prograde. In my case i use 3 degrees. I noticed most rocket flips happen when the rocket wants to turn more than 5 degrees infront of the prograde and then the atmosphere drag hits the side so hard it creates a chain reaction that flips the rocket. If i wanted to launch the Minmus Bus manually im guessing i would go for 25km straight up and then start the turn which would cost me alot of dV.

[ArchGaden]

With the new aerodynamics, nobody can really give you accurate numbers without actually flying your particular vessel through different launch profiles. I've played a lot of launch profiles on some pretty shady vessel designs I've resorted to before the right parts were available. I generally plan for 600 d/v extra if I'm doing a late gravity turn to avoid aerodynamic issues. I always circularize before travelling elsewhere as I find it simpler than trying to time my launch precisely. I expect the d/v cost of a late turn is about the same though, as you'll have to gain the orbital velocity eventually.

[Requia]

Maaaaybe. For ~5500dv, which isn't too insane. The problem here is the efficiency of that initial launch to a high AP. The faster you come up, the lower the dv, it might be possible with a really aero friendly rocket, but with heavy non aerodynamic load you're gonna have more drag, not much stability issues, just force straight down. terminal velocity is still in fact a thing... and low to the ground its about 400m/s for my relatively aero-friendly rockets. You need to hit something around 1600m/s as quickly as possible. Trying to bust through TV will just add dv, same as the soup always did.

If it's possible you need to burn as hard as possible off the launchpad. All this nonsense FAR players have been spreading about TWRs of 1.3 to 1.7 in nustock will KILL this ascent profile. Burn to supersonic then drop your TWR to about 2.2 (I'd not try this without Kerball Engineer Redux to actually tell you) and hold, when you hit 20km throttle up to maximum (in design, remember every second you shave with higher TWR is less fuel you need) till your AP is in place. The orbital burn is nominally 1960 (at 150km), but pack more because that'll be inefficient for how sharp the angle is gonna be, the faster you can execute it the closer it'll be to 1960, so TWR continues to be a serious need right until the end.

For all that, your PE velocity is gonna be fairly low I'm afraid, you really are throwing away 2000 dv on this, it might be better to burn to 20km straight up then gravity turn once aero forces aren't as big a deal, less efficient than normal for sure, but a good middle ground. Or that'll rip your rocket apart. In which case yeah try this.

[heng]

That means your engines effectively produce 10 m/s² less acceleration upwards

ah, yes, i see... forgot to take that into account :-(

What you see here is a Fuel refinary and a science jeep with basicly 50000dV ...

woah... nice. 50k?!? that's overengineered :-P

but see, that is exactly my problem you described... the usual gravity turn just kills me. or rather, my kerbals and the ship.

i do not use mechjeb (please no pro-con discussion in here) and i just can't manage to keep it stable... so no alternate value to compare.

guess i'll have to live with spending more dV than necessary... thanks anyway for all your explanations!

I expect the d/v cost of a late turn is about the same though, as you'll have to gain the orbital velocity eventually.

that's the difference Streetwind mentioned: dV vs. fuel

the same amount of fuel will result in different amounts of effective dV, depending on how when where you burn...

[n0xiety]

Well guys while surfing youtube found this

. I think this answers everyones questions about efficiency about the gravity turn vs drag. I must say this guy isn't even using an aerodynamic design. It's just an orange fuel tank with bare top taking all the drag.

[heng]

Well guys while surfing youtube found this...

awesome find! thanks.

[ArchGaden]

the same amount of fuel will result in different amounts of effective dV, depending on how when where you burn...

Yeah... I always look at vacuum d/v and judge my costs by that number. Burning fuel earlier results in a greater vac d/v cost of course. It is just one of the many conditions at play though. While in the atmosphere, you experience drag that also eats d/v, which goes at odds with lower engine efficiency. Additionally, burning straight up is wasteful as you really need orbital velocity, which is towards the horizon during launch and prograde when in orbit.

The overall result is exactly what others have told you, turn soon after launch. Obviously turning is difficult with unwieldy vessels. In the worst cases, you'll have to shoot straight up and then turn fast around 35km (rough point at which aerodynamics don't matter much). This costs d/v though as your fuel has been spent sending you straight up instead of contributing to orbital velocity. I've found that I waste as much as 600 d/v in my worst cast 35km turn profiles, so I've tend to budget that in right from the start when I know my vessel will be unwieldy.

As far not circularizing goes. It is more efficient to launch directly into the plane of your target right at the launch window and do the full burn with circularizing. It doesn't save much fuel and requires some more precise piloting. Go for it if you want though. A perfect launch is a beautiful thing.

[Laie]

For unwieldy payloads, it's advisable to clear the worst atmosphere quickly. You don't have to go straight up, but start your turn later than normal and don't turn as quickly. Keep an eye on apoapsis altitude and time to apoapsis (1)(2). Once the projected apoapsis goes above 15-20km, you may want to reduce throttle and kinda-sorta "coast" -- as long as the apo keeps getting higher and time-to-apoapsis doesn't decrease, everything is fine. Once you're above 30km, you may throttle up again and start pulling the nose down rather aggressively.

(1) watching time-to-apoapsis is generally a good idea. If that figure is running away too quickly, you're going too steep and/or have excessive TWR.

(2) if you have no data display tools, open the map view and click the apoapsis marker.

[Yasmy]

The higher your apoapsis is, the less dV you need to invest in order to raise your periapsis above the atmosphere, yes... but, at the same time, the more fuel you need to invest to get the same amount of dV. This is called the Oberth effect, which says (simplified) that the deeper inside a gravity well you perform a burn, the more efficient it will be.

Same amount of fuel spent = same dV. The Oberth effect says that the faster you are moving along the direction of the burn, the more specific orbital energy you gain or lose from the same dV.

initial specific orbital energy: 1/2 v^2 - 1/2 G M / r

final specific orbital energy: 1/2 (v + dv)^2 - 1/2 G M/r

dE = final - initial = 1/2 (v + dv)^2 - 1/2 v^2 = v dv + 1/2 dv^2

This is the Oberth effect: dE/dv = v + dv

It's nothing more than the statement that kinetic energy is quadratic in velocity, and therefore changes, dE, are linear in dv.

N.B: Specific orbital energy is total energy divided by mass. Since your mass has nothing to do with the shape of your orbit, it is easier to just remove it from the equation. Literally.

And of course, in the time it took me to write this post, I've been multiply ninjaed!

Edited May 21, 2015 by Yasmy