[Tutorial] A Guide to Basic Kerbal Rocket Design Through Rocket Science.

[MilkTheFrog]

MilkTheFrog, what you describe is known as a direct ascent, when the Mun (or Minmus) is in the correct position it is possible to make a single continuous burn to intercept the other body, then do a second burn after you have changed Sphere of Influence, this was the method that was going to be employed by the Soviet N1 rocket btw.

The more common method is to attain a parking orbit and then wait for the opportune time to begin a transfer burn.

Hmm, i may be getting confused with the terms here, but i thought direct ascent meant having one continuous burn from launch to get you to your destination. What i'm describing involves being in an orbit, then doing the burn to put you in a transfer orbit into the target body's SOI. I thought that was how most people get to the Mun? I've never seen anyone get to the apoapsis of their transfer orbit then burn to put themselves into the same orbit as the Mun... am i missing something here?

[sal_vager]

No you are right, I must have misread what was said before, a direct ascent is as you say, from launch to the destination.

You say you have not seen anyone round out their orbit to reach the Mun? I have a few times, it looks like more trouble than it's worth but I see new players attempt this.

I do as you describe, I get to orbit, then raise my Apoapsis to intercept the Mun, but I also do direct ascents occasionally, I guess they might be considered a Hohman Transfer from the ground.

[Chrischn89]

It's so complicated to calculate Delta Vs... I can't for example figure out how solid rocket booster come into play when calculating. Any advice on this?

[ForumHelper]

@up I'd like to know this as well.

[Black Haired Guy]

I've been browsing through here and I noticed many 9.81m/sec² in the formulas. I'm assuming this is the acceleration of gravity, but this would be earths standard surface gravity.

My first question is is Kerbin surface gravity the same as earth? Such as a less massive body but with a smaller radius would equal the same. I haven't done the math so is Kerbin g the same as earths g?

And with the thrust to weight formula the surface gravity would make sense to use. But for impule calcs why is 9.81 used especially in a munar or minmus transition?

[bsalis]

Finally did that spreadsheet. I attached it.

Anyone care to check the calculations?

The spreadsheet also has a tab with Inter-planetary transfer calculations as well.

(edit: small correction, LV-909 Small actually has a thrust of 20, not 90, change cell V17)

Edited August 9, 2012 by bsalis

[RaptorFuel]

I am trying to figure out Delta-V using your 3rd stage but I keep getting a different Delta-V then you got. I don't think I'm calculating ln right. I'm using a scientific calculator from http://web2.0calc.com/. I enter log(3.72/1.72) which gives me 0.335. Then I do the formula: 400*9.81*0.335 which gives me 1314. What am I doing wrong? Its been forever since I used one of these calculators.

Edited August 11, 2012 by RaptorFuel

[Kosmo-not]

I am trying to figure out Delta-V using your 3rd stage but I keep getting a different Delta-V then you got. I don't think I'm calculating ln right. I'm using a scientific calculator from http://web2.0calc.com/. I enter log(3.72/1.72) which gives me 0.335. Then I do the formula: 400*9.81*0.335 which gives me 1314. What am I doing wrong? Its been forever since I used one of these calculators.

This is because you need to use the natural logarithm.

ln(3.72/1.72) = 0.7714

[UmbralRaptor]

Sorry about the belated response.

My first question is is Kerbin surface gravity the same as earth?

Yes.

And with the thrust to weight formula the surface gravity would make sense to use. But for impule[sic] calcs why is 9.81 used especially in a munar or minmus transition?

The 9.81 in impulse calcs is because of the units used for specific impulse -- when they're listed in seconds, you have to multiply by 9.81 to get the effective exhaust velocity in m/s.

@ForumHelper and Chrischn87: for a separate stage, it's the same as a liquid fueled stage, though you'll want to use the Ve figures directly from the wiki, though I don't think that was. For SRBs assisting a stage, You'll need to find the ÃŽâ€V for the duration of the burn, and then continue on with subsequent stages (bearing in mind the partially drained liquid fuel tanks) To find the overall Ve during the burn, sum up all various engine thrusts. Then divide by the sum of all the mass flows (.005*fuel unit consumption tonne/s for any stock LFE/tankage, 0.576 tonne/s for the RT-10, and 0.2 tonne/s for the RT-B20.). Initial mass - (Burn time * mass flow) == final mass for this stage.

Hopefully that was clear?

[RaptorFuel]

This is because you need to use the natural logarithm.

ln(3.72/1.72) = 0.7714

So how do i use a natural logarithm?

EDIT: Found a calc that had ln. http://www.eeweb.com/toolbox/calculator

Thanks for all the info

Edited August 11, 2012 by RaptorFuel

[MrPwner]

Is there a way to calculate delta v needed for an ascent, from bodies with or without an atmosphere, to a certain altitude other than 100km?

[Cykyrios]

Most probably, but you would have to take a lot of parameters into account: gravity loss, turning loss, drag loss when there's an atmosphere... Then you need a way to compute all this. MechJeb has a feature that shows you deltaV expenditure, but I'm not sure it can estimate how much deltaV it's going to use to get into orbit.

Once you are in orbit, though, things get much simpler.

[MrPwner]

Yeah I´ve been launching some rockets with MechJeb to check how much delta v it needed.

Checked with 100km, got indeed around 4700.

250 - 5200

500 - 5500

Maybe we can extrapolate and graph it or something?

[Cykyrios]

The problem comes mostly from the atmosphere, and also from the way you perform your gravity turn. Depending on the altitude you want to reach and your ascent profile, you may spend more or less time in the atmosphere, and turn at a different time/altitude. I'm no pro in this field though, but I'm sure you can extrapolate if you add a little margin of error.

[Vanamonde]

It seems that since we're going to be getting planets soon, I'm going to have give up my "Five tanks seemed to work last time" design method and bite the bullet and do some math. Do you gents have similar figures for Minmus flights?

Edited August 16, 2012 by Vanamonde

[AncientAstronaut]

Do you gents have similar figures for Minmus flights?

I was about to do a whole Delta-V map for Minmus but my game crashed again and I was just encountering too many problems with which to work. So I came up with an estimate for you.

To be safe, Trans-Minmus Injection should have about 1,500 m/s Delta V. Landing should be about 600 m/s. (This is really an estimate. I think it's probably less, but I'm giving you a number I know will be safe.) And for return, probably around 400. Kosmo-not is making a Minmus Delta-V map for you. His game crashes less than mine.

[Ziff]

I never really bothered to do a delta-v map for Minmus as it really isn't too far off from a Mun mission, but I would be interested in seeing what it is anyway. You need a little more delta-v depending on what method you use to transfer to minmus, but you save a lot on the landing because of the low gravity so it sort of evens out. Pretty much any craft I have that is Mun capable is also Minmus capable, but that's because I typically have an injection stage that has a bit of fuel left in it when my landing procedures start.

[Edit] Hey AncientAstronaut, what's the best way to go about creating a delta-v map anyway?

[Vanamonde]

Thanks for the numbers, AA. You're a mensch.

any craft I have that is Mun capable is also Minmus capable

Yes, when I'm feeling lazy, I just fly a Mun-design to Minmus. But I always arrive with enough extra fuel to float a boat, which is a habit I want to break before economics are added to the game.

So. Turns out I've forgotten how to do math. I keep getting different figures every time I try to add up the same rocket. This is embarrassing. But that just takes practice. More importantly, I'm trying to work backwards from the payload the mission requires to figure out how many tanks a stage needs. Can someone remind me how to solve for X number of tanks when:

Desired delta V =9.81*Isp*ln[(payload weight + X full tanks)/(payload weight + X empty tanks)] ?

I get as far as: (payload + X full tanks)/(payload + X empty tanks) =ln^-1(dV/(9.81*Isp)) and then I have no idea what to do with a fraction where the variable is in both the numerator and denominator. I mean, yeah, multiply both sides by the denominator, but then things get scary and I want my mommy.

[Ziff]

It takes too much time for me to do it that way, I use the Kiocit plugin from .15 to figure out how much each stage weighs, then subtract the mass of the total fuel in that stage and plug it into the delta-v calculator here ->http://www.strout.net/info/science/delta-v/ with the engine Isp to give me the total delta-v of that stage. There is also a formula for figuring out what the Isp should be if you are using several different types of engines, I dunno where I put it though. Anyone have that?

Kiocoit plugin ->http://kerbalspaceprogram.com/forum/showthread.php/9063-Plugin-0-15-KICOIT-Kerbal-Engineer-0-8-Advanced-Brakes-v2-1?highlight=kicoit

[Major-Major]

Thanks for the numbers, AA. You're a mensch.

I get as far as: (payload + X full tanks)/(payload + X empty tanks) =ln^-1(dV/(9.81*Isp)) and then I have no idea what to do with a fraction where the variable is in both the numerator and denominator. I mean, yeah, multiply both sides by the denominator, but then things get scary and I want my mommy.

You're good, as long as you recognize that 'ln^-1 is actually the exponential function, e. So you have e^dV/(9.*81*Isp), which is just some number. Multiply the denominator, then solve for x. The equation stays linear in x, so it's not a big deal.

HOWEVER, I don't think it's going to give you integer values for x. I usually go the other way--pick a number of tanks, find the delta V for that number, and so on. All part of the fun.

[Vanamonde]

That looks like a useful plugin Ziff, but it doesn't seem to have been updated for .16 yet. Besides, I'd like to do it myself for a while until I understand what's going on, and then I wouldn't mind letting a calculator do it.

I forgot that inverse natural log is equivalent to e^x, thank you, but embarrassingly, it's the solving for X in that equation that I'm having trouble with. Substituting L for

e^{(dV/(9.81*Isp)}, I end up with X = payload * (L-1)/(full tank weight - L * empty tank weight). First of all, I'm not sure how to read that, and second of all, I suspect I went badly wrong somewhere and it's gibberish.

Edited August 17, 2012 by Vanamonde

[Ziff]

Sorry, I forgot to mention that it's not been updated. Even though it hasn't been updated it is still useful to get the total craft weight. It's a pain adding up all the fuel lines and struts, so I just use that mod to save me time.

[Vanamonde]

Does the plugin recognize the new parts and calculate their weights properly?

(Oh, and Major-Major, are you a Joseph Heller fan?)

Edited August 17, 2012 by Vanamonde

[Ziff]

Yes it does. It tracks all the weight/mass correctly, as well as amount of fuel, RCS fuel, and cost. It can also track weight by staging, although I don't really use that feature because my stages usually aren't set up in a normal fashion. It's the advanced features that no longer work. I think it used to track thrust, T:W ratios on Kerbin, Mun, and Minmus, total delta-v for each stage, and the Isp for that stage including all the engines. I really hope this get's updated because I build a lot of rockets, and the math becomes time consuming when you make as many design changes as I do.

[Major-Major]

Substituting L for

e^{(dV/(9.81*Isp)}, I end up with X = payload * (L-1)/(full tank weight - L * empty tank weight). First of all, I'm not sure how to read that, and second of all, I suspect I went badly wrong somewhere and it's gibberish.

Yeah, I kind of realized after the fact that I hadn't really answered your question. Anyway, that's the correct formula--the algebra works, and it's dimensionally correct, since you have (weight/weight), yielding a unit-less quantity. If you take that number and round up to the next fuel tank, it should work well.

Also, I think I'm going to undertake a Delta-V surface-to-orbit study, since empirical testing seems to be the only way to sort it out, what with all the complicating variables. Hopefully I can graph some meaningful results within a couple days. I'm particularly interested in how T:W ratio affects the achievable altitude...that might take some testing as well.

Oh yeah, and I love Catch-22. Sometimes I'll catch a Yossarian online somewhere, but I've only seen one other Major Major out there.