Delta-V for LKO

[Gooru]

I havn´t played KSP for about 1,5 years and much has changed now.

I was used to build my rockets with ca. 4500 dV to reach LKO and just did not think much about that.

But yesterday one of my rockets reached LKO with 1200 dV left, so it just used 3300 dV.

I reduced the fuel and retried to reach LKO with 3300 dV and it worked indeed.

I have no idea, why this thing is 27% more efficient than other rockets.

did they change kerbin gravity or something?

[legoclone09]

With the new atmo, the Delta-V to orbit is much less, you only do need ~3300 m/s of Delta-V. With the kid souposphere, it did take 4500 m/s. I would try to look at some recent guides if you want to build planes as well.

[Gaarst]

Yep atmo is what changed everything.

Now you actually have to make a somewhat aerodynamic rocket and watch your ascent profile. I think some people got to LKO with as little as 2800 m/s with great optimisation. A bulky rocket with a inefficient ascent profile can need up to over 4000 m/s for LKO.

But you'll generally be between 3000 and 3500 m/s with reasonable rockets.

Edited September 5, 2015 by Gaarst
Corrected dV value

[Renegrade]

Now you actually have to make a somewhat aerodynamic rocket and watch your ascent profile. I think some people got to LKO with as little as 2500 m/s with great optimisation. A bulky rocket with a inefficient ascent profile can need up to over 4000 m/s for LKO.

2500 is physically impossible - it would take 2,556.386 vacuum delta-v to reach a 70km orbit assuming Kerbin had no atmosphere and the rocket infinite TWR.. :/

[Gaarst]

2500 is physically impossible - it would take 2,556.386 vacuum delta-v to reach a 70km orbit assuming Kerbin had no atmosphere and the rocket infinite TWR.. :/

OK, maybe 2500 is a little bit optimistic, I'm correcting my post to 2800 m/s which is more accurate.

Thank you for correcting me.

[brettuzzi]

OK, maybe 2500 is a little bit optimistic, I'm correcting my post to 2800 m/s which is more accurate.

Thank you for correcting me.

I don't mean to be a jerk, but I'm so sure that 2800 is as equally unrealistic as 2500, that if you can demonstrate a stock parts rocket reaching LKO for under 3k ÃŽâ€V (vacuum) I'll donate $50 to a charity of your choice.

(edit; $50 Cdn)

[Val]

I accidentally made a less than 3k dV ascent with one of my entries for Stock Payload challenge.

It's not 100% confirmed, but KER and MechJeb thinks so. There's video and craft file plus some discussion in the following posts.

[Panzerbeard]

I don't mean to be a jerk, but I'm so sure that 2800 is as equally unrealistic as 2500, that if you can demonstrate a stock parts rocket reaching LKO for under 3k ÃŽâ€V (vacuum) I'll donate $50 to a charity of your choice.

(edit; $50 Cdn)

Does my bank account count as a charity?

[SanderB]

I don't mean to be a jerk, but I'm so sure that 2800 is as equally unrealistic as 2500, that if you can demonstrate a stock parts rocket reaching LKO for under 3k ÃŽâ€V (vacuum) I'll donate $50 to a charity of your choice.

(edit; $50 Cdn)

75km LKO for 2,930 m/s.

Considering the previous poster didn't specify any charity, I suggest this one:

http://www.churchmilitant.com/donate

[brettuzzi]

Interesting approach, very specialized and not what I was anticipating (I meant a simple rocket) but I definitely didn't clarify that... nice work. I'll give Gaarst a couple weeks to see if he can duplicate it, if he doesn't then you win.

edit: Regardless of the results of this: "Indeed. MJ is showing 2999m/s of vacuum dV for Val's rocket on the pad, KER gives me 3067m/s of vacuum dV in the VAB." I'll honor it if Gaarst can't, but I can't donate to everybody's charity and the challenge was initiated at Gaarst.

Edited September 5, 2015 by brettuzzi

[Gaarst]

Interesting approach, very specialized and not what I was anticipating (I meant a simple rocket) but I definitely didn't clarify that... nice work. I'll give Gaarst a couple weeks to see if he can duplicate it, if he doesn't then you win.

I'm on it !

[Renegrade]

OK, maybe 2500 is a little bit optimistic, I'm correcting my post to 2800 m/s which is more accurate.

Thank you for correcting me.

Eh, no problem. I do imagine 2800 is possible, given a really well-flown, pointy, 3.75m beast.

In case you're wondering why I'm so fast with that figure, I've been itching to play an alternate career with a Kerbin with no atmosphere heh. Part of my prep included calculations of delta-v to orbit. Another part of my prep is trying to figure out (on paper, as it were) to land an SRB-powered capsule flight without parachutes..

[Sharpy]

And he probably could have saved a couple m/s, by making it a non-returnable vehicle. No airbrakes (their drag while folded is low but non-zero), and the trajectory wasn't entirely optimal, plus it was 75km orbit, not 70km.

BTW, *technically* I reached the orbit on less when I circularized at 30km. Of course that was not a sustainable/stable orbit due to atmospheric drag and I burned far more than would be an optimal amount to climb to 70km from there, but technically, it *was* an orbit...

[Yasmy]

*Technically* you reached orbit as soon as you lifted off the pad. Just not sustainable due to degeneracy pressure. But it was an orbit.

[Superfluous J]

*Technically* you reached orbit as soon as you lifted off the pad. Just not sustainable due to degeneracy pressure. But it was an orbit.

Actually you're *technically* in orbit ON the pad. You're at apoapsis, with a free (cheaty?) 1g of constant radial thrust.

[Ripper2900]

You're not in orbit untill you trajectory does not intersect the ground. Neither of the last two posts are right. Those are SUB-orbits.

You can be in orbit right off the pad IF you accelerate horizontally to orbit velocity. Your orbit will decay immediately, but for an instant, you'll be in orbit.

[SanderB]

Technically you *could" not be a jerk when being overly *technical*. Come on, get real here, playing with semantics hardly ever really does anything for anyone unless they're a damned politician.

Anyway,

Due to constructive criticism arguing the technical points of how to improve my attempt at low dV LKO, I've revised it.

Also, a <2,800m/s dV attempt LKO for all the "technical" people :

[Superfluous J]

For the record, I was using "technically" the same way that Yasmy did: As a synonym for "not."

And yes you can be in orbit while deep in atmosphere, but practically speaking you're just flying really fast.

[cantab]

A few things to bear in mind:

It depends on aerodynamics. A rocket with poor aerdynamics, for example because you're launching an oversized payload, will require more delta-V. Less obviously a smaller rocket will require more delta-V because drag mainly relates to areas not volumes.

If you measure by vacuum delta-V then you're influenced by how the engine's Isp falls off in atmosphere. A Reliant will show less vacuum delta-V than a Swivel with the same fuel, but actually give more delta-V at sea level. When designing I use sea-level dV and TWR for the first stage and vacuum for all the rest. I think MechJeb might be able to record actual delta-V during the flight.

Minimum delta-V to orbit is irrelevant anyway, except as a challenge for the sake of it. Payload fraction is also irrelevant, but correlates better to things that matter. Things that are relevant include cost per kilo of payload in career mode (and in Real Life), parts per kilo for lag reduction, fuel per kilo for reusable launchers on Kerbin or elsewhere, and even mass to orbit in a given diameter stack for common booster core designs.

[SanderB]

Getting to orbit in a reasonable time frame might also be considered, which makes small launchers and slow launching spaceplanes especially inefficient in some cases.

[Sharpy]

Actually, you can achieve a stable orbit as low as 22km above Kerbin surface. It will require a trick though:

You establish an orbit with 22km periapsis and a somewhat higher periapsis.

You drop a stage containing the root part of the rocket. You may allow it to burn, explode, land, crash, whatever. Probably causing a self-destruction mere moments after separation (separatron with exhaust directed into that part? Making it crash into the rocket?) would be optimal.

Then just go to KSC or allow the orbiter exit the phys bubble, whatever - to put it on rails.

It will keep orbiting Kerbin at 22km for as long as you don't switch to it nor approach it less than 2.2km with an active vessel. Even then it could contain more droppable stages "on path to the (discarded) root part" so if you switch to it, just burn to account for momentary loss due to drag, and "abandon ship" to return it to the rails. Or move it to a 70+km orbit.

Nevertheless, 70km is necessary for a stable orbit only if you plan to control the probe. You could easily drop a scansat or a RemoteTech relay at 22km orbit and enjoy all its benefits as if it was in LKO for the rest of the game.

Of course not burning up at that altitude at the orbital speed is a whole different matter. OTOH it could mean a whole family of quite small Rapier-based probes that could be launched e.g. into Kerbin polar orbit, without need to climb all the way to 70km before circularizing.

Getting to orbit in a reasonable time frame might also be considered, which makes small launchers and slow launching spaceplanes especially inefficient in some cases.

"Especially inefficient" only if you count delta-V as the ultimate measure, neglecting ISp that earns you that delta-V. For example, spaceplanes are extremely efficient because despite using some 4000+m/s of delta-V to reach the orbit, they achieve more than half of that on extremely fuel-efficient jet engines; replace the jet engines with, say, aerospikes, and the spaceplanes appear to have some 2000m/s of delta-V instead of the apparent 5000 they achieve with aid of the jets.

[SanderB]

Actually, you can achieve a stable orbit as low as 22km above Kerbin surface. It will require a trick though:

You establish an orbit with 22km periapsis and a somewhat higher periapsis.

You drop a stage containing the root part of the rocket. You may allow it to burn, explode, land, crash, whatever. Probably causing a self-destruction mere moments after separation (separatron with exhaust directed into that part? Making it crash into the rocket?) would be optimal.

Then just go to KSC or allow the orbiter exit the phys bubble, whatever - to put it on rails.

It will keep orbiting Kerbin at 22km for as long as you don't switch to it nor approach it less than 2.2km with an active vessel. Even then it could contain more droppable stages "on path to the (discarded) root part" so if you switch to it, just burn to account for momentary loss due to drag, and "abandon ship" to return it to the rails. Or move it to a 70+km orbit.

Nevertheless, 70km is necessary for a stable orbit only if you plan to control the probe. You could easily drop a scansat or a RemoteTech relay at 22km orbit and enjoy all its benefits as if it was in LKO for the rest of the game.

Of course not burning up at that altitude at the orbital speed is a whole different matter. OTOH it could mean a whole family of quite small Rapier-based probes that could be launched e.g. into Kerbin polar orbit, without need to climb all the way to 70km before circularizing.

"Especially inefficient" only if you count delta-V as the ultimate measure, neglecting ISp that earns you that delta-V. For example, spaceplanes are extremely efficient because despite using some 4000+m/s of delta-V to reach the orbit, they achieve more than half of that on extremely fuel-efficient jet engines; replace the jet engines with, say, aerospikes, and the spaceplanes appear to have some 2000m/s of delta-V instead of the apparent 5000 they achieve with aid of the jets.

What I meant was how much personal time it takes from the player to achieve orbit. Spaceplanes and re-useable rockets usually take a lot more personal time than non-reuseable rockets.

[Superfluous J]

Minimum delta-V to orbit is irrelevant anyway, except as a challenge for the sake of it.

Very much this. Optimizing for dV is like that trick car companies use sometimes where they say their car goes farther "on a tank of fuel" than the competition... not mentioning of course that their car has a 3-gallon larger tank.

[GoSlash27]

What I meant was how much personal time it takes from the player to achieve orbit. Spaceplanes and re-useable rockets usually take a lot more personal time than non-reuseable rockets.

I don't know about "a lot", but it definitely takes more time. If you're just putting a chunk of cargo in orbit, then yeah. But if you're actually docking with something else, the difference in total time spent is pretty small.

Best,

-Slashy

- - - Updated - - -

Very much this. Optimizing for dV is like that trick car companies use sometimes where they say their car goes farther "on a tank of fuel" than the competition... not mentioning of course that their car has a 3-gallon larger tank.

Agreed. Higher payload fraction, minimum stage mass, and minimum cost are worth optimizing for. DV isn't.

But still and all... people do need a good guesstimate of how much DV they need for the job for design purposes.

Best,

-Slashy

[SanderB]

Well, getting spaceplanes and reueable rockets to space isn't necessarily a lot more time consuming, but getting them back is and that I hope is finally what I meant to say.