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Delta V AND TWR Calculator


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I hope someone can help. I'm at my wits end here.

I'm trying to get to Duna, but I keep on running out of fuel. All the maps, wiki's, etc. only specify how much Delta V is needed. Well, that's pretty useless.

I have a 13-stage rocket to get me there - each of the stages has twice the required Delta V for that stage (so I can do a return mission). None of the calculators specify how much TWR/Thrust is needed.

To explain: I have a Delta V of 1123 to break away from Kerbin orbit. This should be enough to reach a Minmus transfer. But how much TWR/Thrust do I need to break orbit? I have a TWR of 1.44, so that should be enough. Oh no. I run out of fuel after 1m 15s. And please don't tell me "you need more fuel". I could stack every fuel tank in the game together, get a kazillion Delta V, but if my TWR/Thrust isn't enough to break away from the gravitational pull, all that Delta V means squat!

So, is there a calculator somewhere that tells me for my vessel mass, who much Delta V AND TWR is needed for each stage. Any help please, because this is turning the game into all frustration and no fun/entertainment. ;.;

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TWR is only an important consideration when you are taking off or landing. For those events, you need to ensure that your TWR is greater than 1 (and that you have enough delta-v to get to orbit). However, once you are in orbit, TWR is no longer important and the only thing you need to worry about is delta-v.

For low TWR craft, it will sometimes be necessary to split an ejection burn into multiple parts across multiple orbits, slowly raising your apoapsis each pass then going around until you get back to periapsis for the next burn. However, if you have the patience to do this, then even ion engines are sufficient once you get to orbit.

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Thanks Stochasty. Then I need a calculator that tells me how much fuel I need to complete the burn because I keep on running out of fuel before the burn is complete, even though I have plenty of Delta V. Any suggestions?

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When you're lifting off, you need above 1:1 TWR on the body in question, and ideally more like 1.5:1 or greater otherwise you waste fuel just holding your rocket up against gravity.

In space, it doesn't matter much. Really low TWR means you need more delta-v, but that'll rarely be important for anything besides an ion drive and they have tons of dV anyway.

EDIT: Regarding running out of fuel, a few things may be the case. If it's in the launch, you may be going too slowly and thus fighting gravity for longer, too quickly and thus fighting too much drag, or using a poor flight profile and thus wasting energy.

If it's in orbital manouvres, bear in mind delta-V maps give the best reasonable case. For planetary transfers this is in suitable transfer windows when the planets line up right, at other times you'll need much more dV. (Conversely, judicious use of gravity assists can get you places with much less dV).

Edited by cantab
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I need a calculator that tells me how much fuel I need to complete the burn because I keep on running out of fuel before the burn is complete, even though I have plenty of Delta V.

If you have enough delta-V, then you have enough fuel. In rocketry, the two are (for all practical purposes) synonymous. Delta-V refers to change in velocity - it is a measure of how much you can change the velocity of your craft. Note that this does not mean one burn (although it can); it means the total amount of change in velocity that your ship can manage throughout the entire voyage - in essence, it is your budget for your flight.

If (according to the maps) you have enough delta-v for your voyage, and yet you are still running out of fuel, then the problem is not that you need more fuel. It's that you need a better (more efficient) flight plan. Getting to and from other planets is hard, but it has far more to do with when you leave and how you travel than it does with how much fuel you have. Almost any craft that is capable of a Mun landing and return can return from Duna - the delta-V requirements are almost identical - but you have to have the right flight plan to do it. The delta-v numbers given by the maps represent best-case scenarios for transfers made during the optimal transfer windows. If you aren't flying during one of those windows, then it can take much, much more fuel (prohibitively much, if you are flying at exactly the wrong time).

Nine times out of ten, careful flying trumps exhaustive building in this game. Make sure that the phase angle between Kerbin and Duna is correct before you leave, and make sure you have the right ejection angle when you go - if you do that, then you'll most likely find that the ship you have is already capable of doing the job. (If the terms phase angle and ejection angle don't mean anything to you, then I recommend that you look up a tutorial on interplanetary flight; it's not all that hard once you understand what to do, but it's not intuitive.)

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Hmm - then I'm not going to get much fun out of KSP. I just want to put a rocket together, go to another planet, do some sight-seeing and come back. I don't want to strategically plan every single rocket, trip, manoeuvre I make. That means KSP sandbox isn't really sandbox after all. I'll go back to simpler games then, 'coz KSP is probably then not for me.

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For planetary transfers this is in suitable transfer windows when the planets line up right, at other times you'll need much more dV.

I'm suspecting that this is the heart of the issue. Making efficient interplanetary transfers is as much about making the transfer at the right time as it is about meeting the minimum delta-v requirements. You can make transfers outside the optimal windows, but it generally takes much more delta-v than it would otherwise.

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I just want to put a rocket together, go to another planet, do some sight-seeing and come back.

"Just."

It's called rocket science for a reason; if these things were easy to do we'd be doing them in real life rather than video games. :cool:

That said, if you think this game isn't for you then that's your prerogative. However, I recommend that you stick with it at least for a while; KSP has a steep learning curve, but the thrill of finally making it to another planet is well worth the struggle.

Edit:

Except I'm not having issues with transfer windows. I don't even get that far. I'm out of fuel before i've even broken free of Kerbin's SOI.

Really? In that case, something is going wrong. It takes less than 1,000 m/s delta-V to break free from Kerbin's SoI once you are in orbit. Describe to us what you are doing and we may be able to help.

Edited by Stochasty
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How low is your parking orbit? 70-100 km is good, and it makes life easier if it's pretty much circular. If you're in an orbit around Kerbin that's too high, it's going to take more fuel to escape.

Of course, if you just want to fly around and are fed up of running out of gas, hit Alt-F12 and enable infinite fuel.

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Really? In that case, something is going wrong. It takes less than 1,000 m/s delta-V to break free from Kerbin's SoI once you are in orbit. Describe to us what you are doing and we may be able to help.

I'm in 70km orbit, burning at Periapsis. By that time, I'm on stage 10 of my rocket - where I expect to be. I then have about 1000 Delta V left to break away from Kerbin's SOI, but that stage runs out of fuel before I get there.

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I'm in 70km orbit, burning at Periapsis. By that time, I'm on stage 10 of my rocket - where I expect to be. I then have about 1000 Delta V left to break away from Kerbin's SOI, but that stage runs out of fuel before I get there.

Well there's your problem... the estimates given in these plans are only estimates. You need to be able to have a bit of wiggle room. Still, looking at your craft, it looks like you should be more than fine reaching Duna. 13k delta v is usually okay for reaching and returning imo. Just go to the next stage and continue onwards.

edit 1: If I may be as bold to offer a suggestion, you should really look into a less-is-more mentality. Space flight is often about reducing mass before adding power. You might find a leaner machine to work better.

edit 2: Going over a little more closely, I see that you don't have enough delta v toleave Kerbin orbit in your tenth stage. You need to use some of the fuel in your eighth stage. If you were, however, to put a little more fuel in your final stage, say enough for 6k delta-v and just worry about getting that stage into LKO you'll be golden.

Edited by AmpsterMan
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Hmm... Looking at your design now, with decent piloting you should be able to get into LKO with stages 13 and 10, at least with stock aerodynamics. From there, stage 8 should be enough to get you out to Duna and still have a little fuel left over, even if you completely emptied stage 10 out getting into LKO.

In fact, I'd say that with decent piloting and taking into effect that you can aerocapture / aerobrake at both ends of the trip, you should be able to make the trip out to Duna and back with delta-v left over to spare.

EDIT: Ninja'd, as seems to be the running trend.

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I've decided to take a staged approach:

Stage 1: Eventual Dune Lander

Stage 2: Eventual Dune Orbiter

Stage 3: Kerbin Escape - Dune Capture

Stages 1 - 3 "thrown" separately into space at a Minmus orbital plane, assembled and then flown to Duna.

In theory, that should work. But I've learned theory doesn't work 90% of the time in KSP :-). Thanks for everybody's help.

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