Kerbin Orbit & Delta V inconsistencies

[Dead Astronaut]

The Kerbin orbit tutorial in the game uses a ship with about 2,300 delta V, yet the delta V map online states it as 3,400. Why such the big difference? I'm still stuggling with getting into orbit and this is one of the things I think I need to resolve first.
https://wiki.kerbalspaceprogram.com/images/thumb/7/73/KerbinDeltaVMap.png/600px-KerbinDeltaVMap.png

[OHara]

KSP simulates how rocket engines perform better in vacuum than in the atmosphere.  The delta-V display in-game will show you numbers appropriate for thrust in the atmosphere or in space, letting you select which by clicking on the Δv button.  By default, the in-game display shows you the smaller numbers for use in the atmosphere -- I guess because new players will first need it to check the thrust-to-weight ratio at launch.

The online delta-V maps estimate how much vacuum delta-V you need to get to orbit, roughly allowing for the lower efficiency of the commonly-used engines for the brief period when the atmosphere is thick enough to make a difference.

[Superfluous J]

@OHara nailed it, but I'd like to add that while the charts use vacuum numbers for consistency, you can't use any old vacuum engine in thick atmosphere. A Terrier could look great by vacuum numbers but be garbage on the launchpad.

Edited November 13, 2020 by Superfluous J
swypeos

[Dead Astronaut]

12 hours ago, OHara said:

KSP simulates how rocket engines perform better in vacuum than in the atmosphere.  The delta-V display in-game will show you numbers appropriate for thrust in the atmosphere or in space, letting you select which by clicking on the Δv button.  By default, the in-game display shows you the smaller numbers for use in the atmosphere -- I guess because new players will first need it to check the thrust-to-weight ratio at launch.

The online delta-V maps estimate how much vacuum delta-V you need to get to orbit, roughly allowing for the lower efficiency of the commonly-used engines for the brief period when the atmosphere is thick enough to make a difference.

Thank you, that resolves that confusion. I have a rocket fairly similar to the orbit rocket used in the training (I did my best to copy it exactly, but it's based on old notes) and it's around 4,000. While I'm here, do you know the best TWR? I heard it should be at least one, but maybe stop at 2. This would probably change at various altitude levels, but it's something I could fine tune my rocket for now that I know how to change the altitude in the calculator.

 

[AHHans]

1 hour ago, Dead Astronaut said:

While I'm here, do you know the best TWR? I heard it should be at least one, but maybe stop at 2.

Well, a TWR of less than one means that the engine generates less thrust than the force of gravity on the craft. I encourage you to try out what that means for a rocket that rests on the launchpad.

Another issue is that atmospheric drag is a thing. So you don't want to get too fast too deep in the atmosphere where the air is still dense, that would mean that you spend most of your energy just pushing air around. (I don't know exactly, but 50% of the mass of air is in the lower 10 km or so, so you don't need to get all that high to avoid the bulk of the resistance.) In addition high air resistance means also high aerodynamic forces - which can make your rocket hard to steer or even flip out - and lots of aerodynamic heating, which may become a problem. Conventional wisdom has it that a TWR at launch of 1.3 to 1.8 is good, and that you want to avoid a TWR of larger than 2 while still in the atmosphere.

Random facts:

• Once you are well on your way to space and don't need your engine thrust to counteract the force of gravity you can live with a TWR less than one.
• When designing a rocket I don't check the TWR at different altitudes. Except for the first stage I only check that the total vacuum dV is enough and all stages that run while still in the atmosphere have a TWR of at least around one.
• Once you are in space both very high and very low accelerations are viable, but both have their own set of issues. (Try making fine adjustments to your orbit with an acceleration of 5g, or a transfer burn of 1000 m/s with an acceleration of 0.1 g.)
• The optimal launch profile (how much to pitch when during the launch, how much - if at all - to throttle down at what time) for a rocket depends very much on the specifics of the rocket (how much drag it has, it's TWR at different stages, etc.) and even on the target orbit,so there is no simple "correct" answer.
• The Saturn V left the launchpad in "slow motion" because it had a TWR of just above one. In reality the dry mass of KeroLox fuel tanks is very low (in contrast to KSP), so the engineers could just add as much fuel as the engines could possibly lift without significantly lowering the TWR of the rocket with empty tanks, and thus increasing the dV a bit more.

[Dead Astronaut]

On 11/13/2020 at 9:28 AM, AHHans said:

Well, a TWR of less than one means that the engine generates less thrust than the force of gravity on the craft. I encourage you to try out what that means for a rocket that rests on the launchpad.

 

I know that well from experience. I made a rocket and didn't realise the TWR was too low and it didn't go anywhere.

I'm getting better with orbits now I know that the rocket is designed for the task and I'm not second guessing it's capabilities. I guess if my Ap is high and my Pe is too low it's because I didn't pitch enough? It seems that higher speeds makes the time to reach Ap higher as that puts energy into the Ap. Though there needs to be horizontal thrust to make the initial orbit more curved and to raise the Pe.

Does there happen to be a mod to guide the ship towards a more optimal path? With all the variables, meaning all ships have a slightly different launch profile, it would take a lot of the guess work out especially with a new rocket that you're not used to.

[Spricigo]

3 hours ago, Dead Astronaut said:

Though there needs to be horizontal thrust to make the initial orbit more curved and to raise the Pe.

To get into orbit you only need horizontal speed, and quite a lot. So it is where most of your thrust needs to go.  You vertical thrust is only for getting high enough to where drag* is not an issue and to counter the action of gravity while your horizontal speed is not  high enough.

 

3 hours ago, Dead Astronaut said:

Does there happen to be a mod to guide the ship towards a more optimal path? With all the variables, meaning all ships have a slightly different launch profile, it would take a lot of the guess work out especially with a new rocket that you're not used to.

It don't take that much practice to get the hang of it. Launch the rocket and briefly after give it just a nudge west; watch your time to apoapsis, it should be increasing slowly. (If is decreasing than you pitched to much/too early, If is increasing quickly then you pitched to little/too late) For rocket with higher TWR you need to pitch more/earlier, for rocket with lower TWR you pitch less/later.

In any case, whatching autopiloting mods like MechJeb or Gravity Turn in action is indeed a way to learn.

*collision with terrain is just a special kind of drag for that discussion.

[vv3k70r]

On 11/12/2020 at 7:29 PM, Dead Astronaut said:

Why such the big difference?

ISP in atmosphere is different from vacum. So You using different engines in space and different in atmosphere.

TWR will grow when tanks get depleted so it shoul get higher on altitude where drag is not an issue. If it looks good - it should fly well.

On 11/15/2020 at 1:37 AM, Dead Astronaut said:

I know that well from experience. I made a rocket and didn't realise the TWR was too low and it didn't go anywhere.

Call it a static test of engines - propably everyone realise this on this beggining that kN of thrust have something to do with mass of the vessel.

 

On 11/15/2020 at 1:37 AM, Dead Astronaut said:

I guess if my Ap is high and my Pe is too low it's because I didn't pitch enough?

Oposite - You go to above the drag altitude of 10 km slowly heading to around 45 deg and then You continue accelerating gradualy almost flat to apoapsis where it is plainly flat. If rocket is to heavy You go flat as soon the drag is away and trying to prolongue Your the fall untill You miss the planet, if it is to light You can just wait to manouver on apoapsis and circle the orbit (but it means it was to powerfull). Normally it is balanced so the engines run continuosly stage after stage (but because of realistic reasons that not trubling us in the game).

Orbit is about go fast and flat to horizon. Not high.

On 11/15/2020 at 1:37 AM, Dead Astronaut said:

Does there happen to be a mod to guide the ship towards a more optimal path?

You can read that from navbal - see the raise and fall of ap an pa in angles beetwen the prograde and radial before and after the apoapsis. There is math behind it but its easier to grasp it by watching Youself what this sine and cosine and tangent are about.

When You get to da Mun drag is not an issue there - so You can try to make an orbit beetwen the mountains. Atmosphere is a problem resulting in what we call gravity turn.

For sure there are mods, but it is a lot of fun to control rocket by Your own. Rocket that is poorly balanced because we have tech that we have and cargo that we must.

On 11/15/2020 at 1:37 AM, Dead Astronaut said:

it would take a lot of the guess work out especially with a new rocket that you're not used to.

Lern them - get back to vab - soon all of Your rockets would be better. You lern what is easier to control, and what is difficult. Then You get to designs that turn corectly just because of COM.

On 11/15/2020 at 5:50 AM, Spricigo said:

*collision with terrain is just a special kind of drag for that discussion.

Drag in higher density.

Edited November 17, 2020 by vv3k70r