delta-v to space, im getting crazy

[Th3F3aR]

First of all thanks for steppin in here

I know there might be maybe another thread open like this but after hours of googling and learning physics i basicly freak out now.

To the Question:

How do i calculate the amount of delta-v needed to get to "location X".

What do i know:

-I got the space ships specs

-I got tons of equations to calculate stuff

-I got no nerves left

Starting of:

My spaceship is a 3 Staged Asparagus SLS with a 7 T payload

SLS Weight:

1. Stage 40,61 T / 28,61 T -- Burntime 35s -- delta-v 1100,64m/s
   
2. Stage 24,33 T / 12,33 T -- Burntime 58s -- delta-v 2135,78m/s
   
3. Stage 8,05 T / 2,05 T -- Burntime 101s -- delta-v 4970,15m/s (vacuum)
   

TWR is about 2,7 in every Stage

delta-v complete (without payload) 8200,16 m/s

Stagewise calculated with Isp*LN(Mtotal/Mdry)

So far so good, i added the 7T payload and get following values

1. Stage delta-v 912,59m/s (Loss is 17%)
   
2. Stage delta-v 1517,5m/s (Loss is 39%)
   
3. Stage delta-v 1848,10m/s (Loss is 63%)
   

Seems legit as the payload is nearly equal to the 3rd Stages Weight --> output delta-v complete : 4278,19m/s

Now i calculated around a bit with different Stuff, Hohmann transfers with 0 x 200km ellipse and 2nd burn 200 x 200 orbit -> delta-v needed approx : 4200m/s ?!

delta-v for Kerbinorbit at 200km with Sqrt(GM/200km) -> 4200m/s

i calculated loss through atmosphere -> approx 1212m/s | The amount of delta-v to lift it to 200km approx 1215,23m/s (2Ekin+Epot)

and all these things but i never got the values i expected like 5900m/s - 6200m/s for kerbin orbit at 200km high (4500m/s lift of + escape atmosphere + loss through atmosphere + hohmann to circular orbit)

So i tried to find out how i can calculate the delta-v for my rocket from 0-15km burn -- gravity turn to 45° burn to 200km -- turn to circularize at 200km burn to circular orbit.

I couln't and the most sad thingy is i tried it in ksp with the 6 t payload and although my delta-v max was way below something like 4500m/s or 5000m/s i did the 400km circular orbit with approximately 40% of the last stages fuel left.

I am really desperate now, i recalculated the delta-v's of the stages 4 times and still don't get it where my problem is. Can anyone help please?

Many thanks

Th3F3aR

[Th3F3aR]

*Update*:

Okay, i found out another way, maybe im wrong on this too but it seems reasonable:

The delta-v needed for a Kerbin Orbit at 200km high is : Sqrt(GM/200km) -> 4201,19m/s (is this reliable?)

The delta-v caused by drag is : vo/Apg | vesc/Apg -> 1433m/s | 2027m/s

As i am Orbiting i assume the 1433m/s are the right choice ? This leads me to a total delta-v of 5634m/s (Sounds quite reasonable for me)

I finally found out (hope this is right) that stages in Asparagus are not calculated like in "normally" staged rockets.

With this i came to S1 2766m/s | S2 2766m/s | S3 1848m/s

That leads me to a total delta-v of 7380m/s

In this case i have approx 1746m/s left in my Stage 3 (94% of Fuel)

This complete calculation sounds more like the experience i had in KSP but i'm still not sure if its accurate (Tolerance of 5% is allowed )

Can someone explain me if i'm doing anything wrong and what??

BTW: Engines are LV T-30 (215Kn/320s(atm)/370s(vac)) | Fuel containers FL-T400(2,25T/0,25T)

Many Thanks

Th3F3aR

[Nibb31]

This should probably be moved to "Gameplay Questions and Tutorials".

5000m/s dV should be enough to get you to a 200km orbit in KSP. At 4200, you are probably running a bit short.

How are you calculating your dV for each stage? I recommend using MechJeb. Even if you don't want to use the autopilot features, the stat readings in the VAB are useful for determining whether your rocket will make it or not.

[Th3F3aR]

Hi Nibb31,

Thanks for the intel, i thought as this has something to with "rocketscience" it might fit in here, i assume i can't move this thread by myself?

As my posts need to get passed thorugh by the moderators you missed my reply to the original post.

I calculated the delta-v = Isp*LN(mFull/mDry)

As i had learned now the asparagus Stages are calculated only with the weight of their parts. (and the payload?)

All in all id led me to a needed dV of 6348,12m/s for a 400km Orbit and a achieved dV of approx. 6890,61 m/s what is more than sufficient and quite close to the experience i had on trying.

[Guest]

Use Engineer Redux.

MechJeb is for cheaters.

[magnemoe]

This should probably be moved to "Gameplay Questions and Tutorials".

5000m/s dV should be enough to get you to a 200km orbit in KSP. At 4200, you are probably running a bit short.

How are you calculating your dV for each stage? I recommend using MechJeb. Even if you don't want to use the autopilot features, the stat readings in the VAB are useful for determining whether your rocket will make it or not.

Enginer do the same, one old version of mechjeb for 0.18-0.19 did a breakdown and showed losses because of atmosphere and gravity who was pretty cool.

[Rune]

Yeah, I think his question is more about the math behind the game. He is trying to derive delta-v to orbit before trying it!

And to the OP, of course you are doing it wrong. Orbital velocity at a 100km circular orbit is 2.2km/s, so no way you need more than that before applying drag and gravity losses (you seem to get 4.2km/s by this point, right?). In fact you need less than 2km/s since you are already moving about 200m/s eastwards due to Kerbin's rotation just by standing still. Really anything else you add to that is due to you fighting gravity (which depends mainly on how long do you spend fighting gravity to stay climbing, but take in mind "gravity" changes as you acquire "centrifugal force", and I quote those because I know they don't really exist but you get my meaning). Serious differential equations begin to apply here, of course, so I suggest a step through the wiki since I'm not explaining (or assuming you know) calculus, that doesn't really sound healthy.

Anyhow, once you have gravity losses, you can open the other big can of worms that is drag losses. Dependant on both your speed and trajectory, which itself is dependant on your T/W curve and your piloting. Good luck with that, even in real life we use statistical data mostly to figure that out. The end result, though, is around 4.5-4.6km/s to get to a low kerbin orbit of about 100km, if you fly an efficient ascent with >1.5 T/W at all times. That's the empirical experience talking, and nothing trumps that! For the rest, I really suggest the wiki entry for "gravity drag" at least.

Rune. Kudos for going all sciency on this!

[Th3F3aR]

Thank you for your honesty Rune,

i do know that i dont have the mathematical experience and everything i wrote is assembled by google and different Websites tranig to explain the math behind the rocket. That even in reallife statistic are used is interesting, i didnt want to get exact values but wanted to get an approximate direction. I tried it many times and also with different rockets. I have a Feeling of what i Need to achieve my goals, but i wanted to get behind it, starting to optimize and reducing waste and failure.

If you have sources that i could use to learn more id be very grateful, its also of interest for me to understand it better in the real world.

To my values, i tried to orientate on Ksp sources like the Delta v map, therefore i was quite happy with what i got. And yes i forgot the 174m/s kerbin /subtracts to my delta v.

Thank you very much

Th3F3aR

[magnemoe]

Yeah, I think his question is more about the math behind the game. He is trying to derive delta-v to orbit before trying it!

And to the OP, of course you are doing it wrong. Orbital velocity at a 100km circular orbit is 2.2km/s, so no way you need more than that before applying drag and gravity losses (you seem to get 4.2km/s by this point, right?). In fact you need less than 2km/s since you are already moving about 200m/s eastwards due to Kerbin's rotation just by standing still. Really anything else you add to that is due to you fighting gravity (which depends mainly on how long do you spend fighting gravity to stay climbing, but take in mind "gravity" changes as you acquire "centrifugal force", and I quote those because I know they don't really exist but you get my meaning). Serious differential equations begin to apply here, of course, so I suggest a step through the wiki since I'm not explaining (or assuming you know) calculus, that doesn't really sound healthy.

Anyhow, once you have gravity losses, you can open the other big can of worms that is drag losses. Dependant on both your speed and trajectory, which itself is dependant on your T/W curve and your piloting. Good luck with that, even in real life we use statistical data mostly to figure that out. The end result, though, is around 4.5-4.6km/s to get to a low kerbin orbit of about 100km, if you fly an efficient ascent with >1.5 T/W at all times. That's the empirical experience talking, and nothing trumps that! For the rest, I really suggest the wiki entry for "gravity drag" at least.

Rune. Kudos for going all sciency on this!

Yes, you don't want a too high TWR or your engine weight is killing you, not to slow as gravity drag do the same.

Now add budgets, you see why they order truckloads of ethanol.

[mhoram]

everything i wrote is assembled by google and different Websites tranig to explain the math behind the rocket.

If you have sources that i could use to learn more id be very grateful, its also of interest for me to understand it better in the real world.

Recently I gathered a lot of info about the Physics used in KSP. There you can find some physical and mathematical background infos and also references to more detailled descriptions.

[tomf]

Stupid question but are you using 0 and 200 as the radii in your hohmann calculations? If so you need to add The radius of Kerbin to that.

Trying to calculate DeltaV through the atmosphere seems to be really hard, people with much beter maths than me have shown that there is no anaytic solution. I plan my missions pretty meticulously but I normally take the 4500 figure quoted on the maps and then calculate every other delta v from there.

[tomf]

You can see some sample maths for airless bodiesin a thread I created ages back http://forum.kerbalspaceprogram.com/threads/27171-Calculating-interplanetary-delta-v

[Bobnova]

If you start with ~4300-4400m/s for a 70km orbit, you can ignore atmosphere entirely and just focus on gravity losses and old/new velocity I'd imagine.

[Red Iron Crown]

I calculated the delta-v = Isp*LN(mFull/mDry)

Not sure if this was a typo, but using that forumla will only calculate about 1/10th of the delta-V. The equation should be:

delta-V = Isp *9.82*ln(mFull/mDry) (in real life it's 9.80665, in KSP it's 9.82 because reasons)

For a three stage rocket with payload P, you have to run the equation once for each stage and add them together, with payload and stages above counted as dry mass for each stage:

dV = Stage1dV + Stage2dV + Stage3dV

= Isp1*9.82*((P+m1full+m2full+m3full)/(P+m1dry+m2full+m3full)) + Isp2*9.82*((P+m2full+m3full)/(P+m2dry+m3full)) + Isp3*9.82*((P+m3full)/(P+m3dry))

It doesn't matter what kind of staging used, the calculation is the same.

[Th3F3aR]

Not sure if this was a typo, but using that forumla will only calculate about 1/10th of the delta-V. The equation should be:

delta-V = Isp *9.82*ln(mFull/mDry) (in real life it's 9.80665, in KSP it's 9.82 because reasons)

No it was no Typo as i'm using the 3142m/s as Isp not the US 320s Therefore my g0 was already in the Equation

For a three stage rocket with payload P, you have to run the equation once for each stage and add them together, with payload and stages above counted as dry mass for each stage:

dV = Stage1dV + Stage2dV + Stage3dV

= Isp1*9.82*((P+m1full+m2full+m3full)/(P+m1dry+m2full+m3full)) + Isp2*9.82*((P+m2full+m3full)/(P+m2dry+m3full)) + Isp3*9.82*((P+m3full)/(P+m3dry))

It doesn't matter what kind of staging used, the calculation is the same.

I see, but the thing is, that i get a very lousy delta-v with this calculation for my Rocket (about than 4300m/s) but get into 400km Orbit with the last stage about 50% fuel left (approx 2000m/s)

To tomf : Yes i forgott the Kerbin radius in this case

Thats why i tried everything again:

Basicly i started off with vo (200km) Orbit 2100m/s , then calculate the drag by Apg = a/g -> 1,692, the vdrag -> vo/Apg = 1241m/s and finally the Gravity loss Sqrt(GM*(1/r1-1/r2)) whic is at r1 = 600km, r2 =800km about 1213m/s, summing it up i get to the 4554m/s to leave Kerbin atmosphere and get an Orbit at 200km with this rocket.

Back to the staging, if i calculate it as you mentioned Red Iron Crown i'll get for a ~7T Payload about 4354 m/s (and drag would get higher). So the 4554 to the 4354 -> No go, but i proved it wrong as i tested it 4 times and had the same result, about 50% left fuel on 400km orbit (instead of 200).

With the other Staging calc where the mfull is just the stage itself 6 tanks+2engines+payload+struts etc i get that value i expierenced, quite precisely

With this (even if very very inaccurate) calculations i have i am pleased, cause it workes.

So im glad you all helped me, i have a long way to go but you all made it a bit shorter

I don't wanna bother you guys but if you have intention to give me more explanation im grateful.

Greetings

Th3F3aR

[Red Iron Crown]

No it was no Typo as i'm using the 3142m/s as Isp not the US 320s Therefore my g0 was already in the Equation

Pardon my confusion. It's usually called exhaust velocity (Ve) when multiplied by g0 and given in meters per second, Isp is reserved for the non-multiplied figure given in seconds. Good to know that you know the difference.

I see, but the thing is, that i get a very lousy delta-v with this calculation for my Rocket (about than 4300m/s) but get into 400km Orbit with the last stage about 50% fuel left (approx 2000m/s)

The must be an arithmetic error somewhere. If you list the parts in each stage I could try running the rocket equation on it and showing my workings.

Also, are you using an aerodynamic mod like FAR or NEAR? Either reduces the dV requirements to orbit by about 1000m/s due to reduced drag losses.

[Th3F3aR]

Pardon my confusion. It's usually called exhaust velocity (Ve) when multiplied by g0 and given in meters per second, Isp is reserved for the non-multiplied figure given in seconds. Good to know that you know the difference.

Ah Okay then i my expression was wrong:confused:

The must be an arithmetic error somewhere. If you list the parts in each stage I could try running the rocket equation on it and showing my workings.

Also, are you using an aerodynamic mod like FAR or NEAR? Either reduces the dV requirements to orbit by about 1000m/s due to reduced drag losses.

Edit: No Mods used, all Stock (No Far or Near)

As you wish:

Parts:

Complete:

LV-T30 -- 5x -- 215kN -- 1,25T

FL-T400 -- 15x -- 2,25T Full -- 0,25T Dry

FTX 2 -- 4x -- 50kg

TT-70 -- 4x -- 50kg

TR-18A -- 1x -- 50kg

EAX 4 -- 8x -- 20kg

Weight is given per each part, summing it up 40,61T SLS weight, add 7T Payload --> 47,61T

Dry Mass of first Stage is 28,61T(6x FL-T400 Dry) SLS weight, add 7T Payload --> 35,61T

Full Mass of Stage 2 would be 24,33T + 7T Payload -> 31,33T (Dropped 2 LV-T30 + 2 FTX 2 + 2TT-70 + 6 FL-T400 + 4 EAX 4 = 4280kg)

Dry Mass of Stage 2 is 12,33T + 7T Payload -> 19,33T (6x FL-T400 Dry)

Full Mass of Stage 3 would be 8,05T + 7T Payload -> 15,05T (Dropped 2 LV-T30 + 2 FTX 2 + 2TT-70 + 6 FL-T400 + 4 EAX 4 = 4280kg)

Dry Mass of Stage 3 is 2,05T + 7T Payload --> 9,05T (3x FL-T400 Dry)

Thats basically the staging, note that i calculated s3 with the Orbit Isp (370s) Ve ( 3633,60m/s)

Correct me if im wrong but with these stage parameters i'll get

s1 dV = Veatm*LN((40610+7000)/(28610+7000)) --> 919,43m/s

s2 dV = Veatm*LN((24330+7000)/(12330+7000)) --> 1536,74m/s

s3 dV = Vevac *LN((15050+7000)/(2050+7000)) -->1898,27m/s

total dV = s1+s2+s3 --> 4354,43m/s

As i said, it is Asparagus staged, dropping s1 after a Burntime of 35,07s, s2 after 58,46s and s3 after 101,4s.

Burntimes are calculated with (AmountOfFuel(kg)/(AmountFuelConsumed(kg)*Engines)) -> Leads to Isp if divided by AmountOfFuel(kg)

The calculation i used (as i saw in this Forum, which said it was for asparagus staging) just recalculates s1 + s2 like the following:

s1 weight (6x FL-T400 + 2x LV-T30 + 2x FTX 2 + 2x TT-70 + Payload) 22020kg (the struts with 20kg are egligible)

s2 weight = s1 weight

dV s1 = 1856,39m/s = s2 dV

Greetings

Th3F3aR

Edited October 10, 2014 by Th3F3aR
FAR or NEAR Question

[Red Iron Crown]

If I understand correctly, you have a center stack with a single LVT30 and 3 FL-T400s along with the 7t payload and decoupler, surrounded by 4 stacks each made of an LVT30 and 3 FL-T400s and a radial decoupler. The four stacks are split into two asparagus stages. Struts and fuel lines don't matter as their mass is ignored by the simulation.

So, my workings:

Stage 3

7t Payload

0.075t Decoupler

1.25t LVT-30

6.75/0.75t 3xFL-T400

Total Mass = 15.075/9.075 mWet/mDry

Delta-V = Isp*g0*ln(mWet/mDry) (I'll use vacuum Isp here as you did)

= 370*9.82*ln(15.075/9.075)

= 1844m/s

Stage 2

15.075t Payload

0.1t 2xDecoupler

2.5t 2xLVT-30

13.5t/1.5t 6xFL-T400

Total Mass = 31.175/19.175 mWet/mDry

Delta-V = Isp*g0*ln(mWet/mDry) (Switching to atmo Isp)

= 320*9.82*ln(31.175/19.175)

= 1527m/s

Stage 1

31.175t Payload

0.1t 2xDecoupler

2.5t 2xLVT-30

13.5t/1.5t 6xFL-T400

Total Mass = 47.275/35.275 mWet/mDry

Delta-V = Isp*g0*ln(mWet/mDry)

= 320*9.82*ln(47.275/35.275)

= 920m/s

Vessel Total

Delta-V = dVs1 + dVs2 + dVs3

= 1844 + 1527 + 920

= 4291m/s

If your third stage reaches orbit with half its fuel remaining:

12.075/9.075t mWet/mDry

Delta-V = Isp*g0*ln(mWet/mDry)

= 370*9.82*ln(12.075/9.075)

= 1037m/s remaining

That seems like a lot left over from an ascent that usually requires about 4,500m/s. Are you sure there is 50% fuel remaining in the stage and not a single tank 50% full with the other two empty?

Edit: Fixed the bad arithmetic. With the corrected numbers, it seems unlikely that the vessel can reach a circular orbit.

Edited October 10, 2014 by Red Iron Crown

[Th3F3aR]

If I understand correctly, you have a center stack with a single LVT30 and 3 FL-T400s along with the 7t payload and decoupler, surrounded by 4 stacks each made of an LVT30 and 3 FL-T400s and a radial decoupler. The four stacks are split into two asparagus stages. Struts and fuel lines don't matter as their mass is ignored by the simulation.

Correct !

I updated the weight now and i am (as before) in common with you at s3 and s2

Stage 3

...

= 370*9.82*ln(15.075/9.075)

= 1844m/s

Yup

Stage 2

...

= 320*9.82*ln(31.175/19.175)

= 1527m/s

Yup!

Stage 1

31.175t Payload

0.1t 2xDecoupler

2.5t 2xLVT-30

13.5t/1.5t 6xFL-T400

Total Mass = 47.275/35.275 mWet/mDry

Delta-V = Isp*g0*ln(mWet/mDry)

= 320*9.82*ln(47.275/35.275)

= 1308m/s

I don't get it , my excel spits out 920,67m/s...

If your third stage reaches orbit with half its fuel remaining:

12.075/9.075t mWet/mDry

Delta-V = Isp*g0*ln(mWet/mDry)

= 370*9.82*ln(12.075/9.075)

= 1037m/s remaining

That seems like a lot left over from an ascent that usually requires about 4,500m/s. Are you sure there is 50% fuel remaining in the stage and not a single tank 50% full with the other two empty?

In fact the last Stage reaches Orbit with more than half fuel (sadly i can't remember the exact value) on a 200km apoapsis.

I am sure that it was the stage fuel, maybe it was 40% but it was enough to get me to a escape trajectory to the Mün.

But i'll try that again later and report the result.

[Red Iron Crown]

I don't get it , my excel spits out 920,67m/s...

Right you are! I must have misentered something in my spreadsheet. Thanks for catching that.

In fact the last Stage reaches Orbit with more than half fuel (sadly i can't remember the exact value) on a 200km apoapsis.

I am sure that it was the stage fuel, maybe it was 40% but it was enough to get me to a escape trajectory to the Mün.

But i'll try that again later and report the result.

When you say "orbit with a 200km apoapsis" what is the periapsis? If you haven't circularized your numbers will be way off as you're just on a much less costly sub-orbital trajectory.

[Th3F3aR]

When you say "orbit with a 200km apoapsis" what is the periapsis? If you haven't circularized your numbers will be way off as you're just on a much less costly sub-orbital trajectory.

Its an Off-perigree so No periapsis, After circularisation and a Hohmann transfer to 400 km i have the 40% left.

You See with the "normal" rocket equation there is not even a slight chance of success. But the testing shows that it works

Greetings

Th3F3aR

Edited October 10, 2014 by Th3F3aR
Typo

[Red Iron Crown]

Can you share the craft file? I'd like to take a closer look to see what's going on tonight.

[Padishar]

Towards the end of the first post you say the payload is 6 ton. Is this a typo? Can you list what the payload consists of? It may be that there are a significant number of "physics insignificant" parts in it so the effective mass is much lower...

[problemecium]

tl;dr for this whole thread: 4500 m/s, as a rule of thumb. A skilled pilot, or a (reasonably) modded ship, may be able to do it in less, but not by any significant amount.

[Th3F3aR]

@ Red Iron Crown

Excuse my late response i had quite a lot of stuff to do this weekend. As i had now time to get to my computer i tried it out again.

I must excuse, cause i reviewed the ship i used, the payload for that test was about 5,5 T so i made a mistake on that, i'm sorry cause i confused all of you

I did it now with a 7,155 T payload and got the result, that your calculations were correct, i just made it to a 100km x 20km Orbit which is according to the 4500m/s needed and the 4300m/s avail, quite precisely.

i wanted to post the screenshots i made and send you the craftfile, but actually i don't know how this works in this forum

But i want to thank you very very much for claryfying, investing that much effort in my question and again excuse me for my bad memory