I need help making a delta v map.

[Bluelogic32]

Hi, I'm trying to make a delta v map that better suits my purpose then the ones that I've found.

What I need help with is calculating how much delta v is needed to launch my craft into orbit. What I've got so far is a formula for orbital velocity, so I know what speed my craft needs to be going at the lowest stable orbit.

What I'm having trouble with is finding out how much delta v I need to add to account for the gravitation and atmospheric drag.

For example the lowest stable orbit around earth is 160 km, at this height you'll be traveling at a speed of 7.8 km/s. So it takes 7.8 km/s of delta v to orbit the earth at this altitude, but because of the drag from gravity and the atmosphere, a rocket will need an additional 1.5 Km/s to 2 km/s of delta v to achieve an altitude of 160 km.

I need to know how to calculate how much delta v I need to add to account for the drag.

That's really all I need help with, but there is couple of questions I need answered as well.

The delta v map I'm making is for the Real Solar System x6.4 config, I don't like the charts that are currently available for it so I'm going to make my own. When I try to do the equation for the orbital velocity I know the answer I get is wrong, for example the number will be like 900 when it should be a lot higher.

I'm guessing that the values are wrong, so what I'm asking, is how do I get the radius and mass for Kerbin in the RSS x6.4 config? What i've been doing is multiplying the values on the KSP wiki by 6.4 but it doesn't seem to be working.

And lastly I need someone to confirm my theory of how to make a delta v map because I don't actually know how to do it. These are the steps I'm taking.

1. Calculate the velocity needed for the lowest stable orbit.

2. Calculate how much delta v I need to add to account for the loss from gravitation and atmospheric drag.

3. Find out how much delta v is needed to do a hohmann transfer to the desired altitude (from 94 km(the lowest stable orbit for the RSS x6.4 config) to 100 km).

4. Add all the values together and get the required delta v to launch from Kerbin and apply the technique to other bodys in the Kerbol system.

So I just need someone to tell if that is the correct way to make a delta v map, and if it isn't will my way still work?

Any help I receive is tremendously appreciated, thank you

[mhoram]

What I need help with is calculating how much delta v is needed to launch my craft into orbit. What I've got so far is a formula for orbital velocity, so I know what speed my craft needs to be going at the lowest stable orbit.

What I'm having trouble with is finding out how much delta v I need to add to account for the gravitation and atmospheric drag.

For example the lowest stable orbit around earth is 160 km, at this height you'll be traveling at a speed of 7.8 km/s. So it takes 7.8 km/s of delta v to orbit the earth at this altitude, but because of the drag from gravity and the atmosphere, a rocket will need an additional 1.5 Km/s to 2 km/s of delta v to achieve an altitude of 160 km.

This is a tough question, mostly because this number depends on a lot of factors (ship-configuration, ascent-path).

So every ship needs a different amount of Delta-V to reach orbit.

But usually they are in a similar range: For Stock-KSP this value is around 4500m/s, for RSS I have not yet seen any numbers, but I also don't use it. have you tried assking in the RSS-thread?

I need to know how to calculate how much delta v I need to add to account for the drag.

You might try the approximation K^2 posted a while back: http://forum.kerbalspaceprogram.com/threads/46194-I-need-someone-help-me-do-some-math-for-launch-optimization?p=987663&viewfull=1#post987663

1. Calculate the velocity needed for the lowest stable orbit.

Have a look at Chapter 9.1.2 Orbital Velocities or google the Vis-viva equation.

3. Find out how much delta v is needed to do a hohmann transfer to the desired altitude (from 94 km(the lowest stable orbit for the RSS x6.4 config) to 100 km).

v1 = Orbital-Velocity while on 94km circular orbit

v2p = velocity at the periapsis while on the transferorbit with a periapsis of 94 km and apoapsis of 100km

v2a = velocity at the apoapsis while on the transferorbit with a periapsis of 94 km and apoapsis of 100km

v3 = Orbital-velocity while on 100km circular orbit

Delta-V = (v2p-v1) + (v3-v2a)

4. Add all the values together and get the required delta v to launch from Kerbin and apply the technique to other bodys in the Kerbol system.

So I just need someone to tell if that is the correct way to make a delta v map, and if it isn't will my way still work?

Sounds like a reasonable approach.

[Th3F3aR]

as i learned this several days ago i give advice as far as i can

vDrag results from v0/Apg

Apg = A/g

v0 - Escape or Orbit dV

A - acceleration

g - Gravity of Planet

That should be quite correct although you should everytime keep in mind that this is so theoretical that you never can rely a 100% on it

dGravity is calculated by GM(1/o1-1/o2)

GM is a Gravitational Constant for Kerbin it is 3,53E+12

o1 is your starting Orbit (600km / 600000m) for Kerbin on Waterlevel

o2 is your destination Orbit (Kerbinradius+Destinationorbit)

Calc for a craft i made

v0 = 2100,60 m/s (200km Orbit)

Apg = 1,44 (a is 14,13m/s at start)

dvDrag = 1458,16 m/s

vGravity = 1212,78 m/s

dV-Total = ~4600 m/s

This is quite realistic and with a test i proved that it is also quite accurate

But there is also another Method that gets you "worse" values, like calculating a Hohmann transfer from 0x200km Orbit (off-perigree)

TO the delta-v map, im doing one for myself, and i think you are on the right track with your idea is a do it quite similar

Hohmann to Mün encounter for example then hohmann for mün orbit and i got that somewhat around 846,5 m/s from 400km Kerbin Orbit to 100km mün orbit, which is also quite precise as i found out.

hope i helped you

[Alshain]

Ship configuration has nothing to do with delta V unless your using FAR/NEAR. In stock, DeltaV is always the same regardless of how the ship is built. The only thing that matters is your ascent/piloting skill really. So assuming you pilot every ship the same way, which in stock is also possible, and you always target the same starting TWR, then it will always be the same amount of delta V required.

Edited October 22, 2014 by Alshain

[Streetwind]

Not really true. Your ship also needs to be able to fly a certain ascent to achieve a certain dV value. And not all launchers are able to follow the most optimal paths due to TWR or control issues. So it does in fact depend on your configuration.

[Alshain]

Not really true. Your ship also needs to be able to fly a certain ascent to achieve a certain dV value. And not all launchers are able to follow the most optimal paths due to TWR or control issues. So it does in fact depend on your configuration.

In stock they are. If they have the same TWR and you fly it exactly the same, you will always reach the same point at the same time during the ascent. Even with slight human error, its close enough to not matter as far as how much delta V you need. Unless you have an outside factor that is dependant on the ships shape and size, this will always be the case (that factor is aerodynamics)

[Th3F3aR]

In stock they are. If they have the same TWR and you fly it exactly the same, you will always reach the same point at the same time during the ascent. Even with slight human error, its close enough to not matter as far as how much delta V you need. Unless you have an outside factor that is dependant on the ships shape and size, this will always be the case (that factor is aerodynamics)

If the crafts are equal i matters of TWR, yes. But i assume you are not flying the same craft everytime? So drag varies for all crafts not having same TWR / acceleration, therefore a slight dragV change is possible.

[Alshain]

If the crafts are equal i matters of TWR, yes. But i assume you are not flying the same craft everytime? So drag varies for all crafts not having same TWR / acceleration, therefore a slight dragV change is possible.

Not the same craft but most people target the same TWR in their build. It doesn't even matter what TWR you target, most people will target the same one on every craft. That is why the deltaV maps work. Slight variances won't change much, if you Target 1.5 and get 1.6, it's not going to make a big enough difference to matter.

In FAR the deltaV maps don't work quite as well. If the payload is bigger or less aerodynamic you will end up spending more deltaV.

Edited October 22, 2014 by Alshain

[Th3F3aR]

Not the same craft but most people target the same TWR in their build. It doesn't even matter what TWR you target, most people will target the same one on every craft. That is why the deltaV maps work. Slight variances won't change much, if you Target 1.5 and get 1.6, it's not going to make a big enough difference to matter.

In FAR the deltaV maps don't work quite as well. If the payload is bigger or less aerodynamic you will end up spending more deltaV.

As we both said now, same TWR = same dVDrag, i just wanted to give the explanation cause i have varius crafts that don't get the same TWR (like i don't have the standard TWR of 1,5 or 1,7 for every craft).

For example i have the ones limited to 1,5 TWR, 1,2 TWR or 1,9 TWR, even 2,2 TWR even with 2,5 TWR (although everybody says your engines will die or something like that, dunno what that means )

That everything changes with FAR / NEAR is okay That's why i hope someday better physics will get stock

[mhoram]

So assuming you pilot every ship the same way, which in stock is also possible, and you always target the same starting TWR, then it will always be the same amount of delta V required.

Note that the ascent-path does not only depend on the starting TWR, but also on the TWR-distribution during the whole ascent. So your statements are a bit of a generalization.

In FAR the deltaV maps don't work quite as well. If the payload is bigger or less aerodynamic you will end up spending more deltaV.

Delta-V Maps are mostly about reaching other planets and Moons and the Delta-V for atmospheric ascent is only a very little part of them. So in general Delta-V maps work for FAR when no atmosphere is involved.

@Th3F3aR

Thanks for putting this together.

What exactly do you mean by "v0 - Escape or Orbit dV"? I assume you mean escape velocity or orbital-speed while in a circular LKO.

[Th3F3aR]

@mhoram

Yes with v0 i meant Orbital-speed (Sqrt(GM/(Orbitheight+Planetradius))) or escape-velocity (Sqrt(2*GM/(Planetradius)))

Edited October 22, 2014 by Th3F3aR
Darn Typos!

[Bluelogic32]

vDrag results from v0/Apg

Apg = A/g

v0 - Escape or Orbit dV

A - acceleration

g - Gravity of Planet

dGravity is calculated by GM(1/o1-1/o2)

GM is a Gravitational Constant for Kerbin it is 3,53E+12

o1 is your starting Orbit (600km / 600000m) for Kerbin on Waterlevel

o2 is your destination Orbit (Kerbinradius+Destinationorbit)

Thank you so much this is just what I needed

[Th3F3aR]

Glad I could help

[Bluelogic32]

@The3F3aR Sorry I just noticed something, how did you calculate the acceleration? What was the value for time?

Edited October 22, 2014 by Bluelogic32

[Th3F3aR]

a can be calculated using g*(TWR-1)

You may ask, why no time in this equation?

Short answer: As your mass reduces your TWR rises.

Long answer: Due to use of fuel ur mass lowers, your TWR rises. Your TWR influences the acceleration and you have per stage a linear growing acceleration(i think it was linear). The funny thing on the calc is now, that you caused by that mass loss, should calculate your a per every second to get an accurate value of dragV. Thats basically why you just use a = g*(TWR-1).

I personally calculate the a per stage to get more accurate values but if you want to imply a buffer you should stay with a0 from start

TWR by the way if you don't know that is calculated Thrust/(massCraft*g)

[Bluelogic32]

a can be calculated using g*(TWR-1)

So then I'll have to aim for a specific TWR for every rocket and stage that goes through an atmosphere?

[Yasmy]

Hi, I'm trying to make a delta v map that better suits my purpose then the ones that I've found.

What I need help with is calculating how much delta v is needed to launch my craft into orbit.

<...snip...>

The delta v map I'm making is for the Real Solar System x6.4 config, I don't like the charts that are currently available for it so I'm going to make my own.

I understand why people want to calculate their own delta-v maps. I've done it myself. But I don't understand your post.

But what kind of map do you want? Why do the normal delta-v maps not suit your purpose? What do you not like about the currently available charts? What would your map have that the x6.4 map doesn't have? I am quite curious.

As a side note, calculating delta-v to orbit from a body with an atmosphere is just an approximation. Expect it to be wrong by as much at 10%. The only way to get a good answer is to simulate it, either with a custom integrator, or with one designed specifically for this problem (for example, Kerbal Space Program makes a good KSP simulator.)

Edited October 22, 2014 by Yasmy
korect speeling

[Pecan]

Which is all to say, of course, "In theory, theory and practice are the same. In practice, they aren't".

Given the vagaries launches (and descents) through an atmosphere it might be easier to just launch a sample ship a few times, average the deltaV used and label the planet with that figure.

[Yasmy]

My preferred phrasing is "Theory and practice are closer in theory than in practice."

But I'm still left with the questions as to why the current maps don't suit his purposes. It is inaccuracies in the delta-v values or is there something else that he wants to see on the maps, presented in a new way? All we know is that he doesn't like the current maps.

[mhoram]

But I'm still left with the questions as to why the current maps don't suit his purposes. It is inaccuracies in the delta-v values or is there something else that he wants to see on the maps, presented in a new way? All we know is that he doesn't like the current maps.

Well here is a collection of stock Delta-V maps for ideas about how to present the infos.

• A more accurate delta-v map in a simplified form
  
• http://wiki.kerbalspaceprogram.com/wiki/Cheat_Sheet
  
• http://forum.kerbalspaceprogram.com/threads/96985-WAC-s-Delta-V-Map-Chart-UPDATED
  
• http://forum.kerbalspaceprogram.com/threads/25360-Delta-V-map
  
• http://forum.kerbalspaceprogram.com/threads/20993-Request-Delta-V-maps?highlight=delta (incomplete)
  
• System Delta-V Maps by JellyCubes V0.18.4
  
• A Delta-V Map for simple flightpaths
  

Edited October 22, 2014 by mhoram

[Th3F3aR]

@mhoram, nice maps

@Bluelogic32

Basically yes, you should aim for a TWR that is 1 < TWR < 3 (some may say less than 2,5)

If you are trying to create Lifters without payload you should always keep in mind the TWR change by the payload.

The TWR of the stages is (most times) proportial because you stack the stages and therefore your stages get smaller in size and thrust which leads to the same TWR like the stage below

An example of my 24T lifter, he got (without payload) TWR s1 3,88 / s2 3,90 / s3 3,88 / s4 6,53 (basically s3 and s4 are the same stage but in is in atmo one in vac so i splittet that a bit)

[Bluelogic32]

@Th3F3aR

Okay cool, there is just one more thing I'm having trouble with though, with this

dGravity is calculated by GM(1/o1-1/o2)

GM is a Gravitational Constant for Kerbin it is 3,53E+12

o1 is your starting Orbit (600km / 600000m) for Kerbin on Waterlevel

o2 is your destination Orbit (Kerbinradius+Destinationorbit)

When I do this a get a ridiculous number that I thing is wrong

Heres what I'm doing(note that the numbers are for the rss 6.4 config)

gm=1.3833139*10^24

01=3840km

02=3940km

1.3833139*10^24(1÷3840-1÷3940) =9.1430963e+18

ignore the font change I have no idea what happend and I can't change it back

I think I'm just doing the maths wrong because even using your values I don't get the same number you have.

This is the only thing that I need help with, I've tried looking for ways to calculate gravitational drag but I couldn't find anything that didn't just talk about what drag is.

.

[Th3F3aR]

@Th3F3aR

Okay cool, there is just one more thing I'm having trouble with though, with this

dGravity is calculated by GM(1/o1-1/o2)

GM is a Gravitational Constant for Kerbin it is 3,53E+12

o1 is your starting Orbit (600km / 600000m) for Kerbin on Waterlevel

o2 is your destination Orbit (Kerbinradius+Destinationorbit)

When I do this a get a ridiculous number that I thing is wrong

.

Actually my bad :/

I forgott the Sqrt... sorry for the confusion, it's Sqrt(GM(1/o1-1/o2))

And another one too, the GM describes the "Gravitational Parameter" (i personally just say it is a Gravitational Constant as is doesn't change for the Planet ) resulting from (GravConst*PlanetMass)

Maybe thats why your equation is wrong

And in additation ! GM uses meter (somewhat like m³/s²) ! so if your using km you need to divide GM by 1000 !

Edited October 23, 2014 by Th3F3aR

[Bluelogic32]

@Th3F3aR I've got it now, thank you so much for your help. I can finally make my delta v map. I assumed you meant gravitational constant because of the value you gave. Where did you learn how to do this?

[Th3F3aR]

Glad you can get it done

Well sort of assembled all that together, trying around in my excel sheet where i have all that stuff located now with descriptions.

Mainly i got the info from a German page : http://www.bernd-leitenberger.de/grundlagen-der-raumfahrt.shtml

Then i also scanned through this english page : http://www.projectrho.com/public_html/rocket/multistage.php

and a lot of physics related pages too, as well as wiki (KSP wiki and Wikipedia) searches for those furmulas and finally

the thread i'd opened my self some days ago : http://forum.kerbalspaceprogram.com/threads/95693-delta-v-to-space-im-getting-crazy

All in all it was a lot of searching and scratching things together, and finally evaluating whats right and whats wrong by trial and compare.

Maybe i'll have the time to do a explanationsheet someday