How do I know what my rocket is capable of?

[Kalzzz]

I'm planing a interplanetary journey bud I don't know how do calculate my delta v. Don't say you're a delta v map and calculator because those make no sense to me. Plz help me

[GoSlash27]

 You say you don't know how to calculate your DV, but don't want to know how to calculate your DV either. So... what are you asking?

 

[cubinator]

Delta-V is a measure of how much your rocket can accelerate before running out of fuel. The equation for it goes like this:

DeltaV = 9.81 x I_sp x ln(full mass / dry mass)

I_sp is a measure of how quickly an engine consumes fuel (higher is better). You can find this in the part description in the ship editor, or in the part's right-click menu.

Full mass is how much the rocket weighs when it's full of fuel, and dry mass is how much the rocket weighs when it's all out of fuel. The rocket gets lighter as the fuel drains from the tanks, so the dry mass is less than the full mass.

You can calculate the Delta-V of a rocket stage in the ship editor by looking at the mass in the engineer's report, the mass after draining all the fuel tanks, and the I_sp of the rocket engine it uses. (You probably only have one type of engine on your ship. For engines with different I_sps it gets more complicated.) Repeat this process for each stage in your rocket to find the total Delta-V.

Once you know know your Delta-V, you can use a Delta-V map to find out how far your rocket can go on an optimal route. Here is a nice map:

So, a ship with a Delta-V value of about 3500 can accelerate 3500 m/s in zero gravity, and will be able to reach Kerbin orbit. A ship with 5300 m/s will be able to land on the Mun, but it needs an additional 900 m/s to be able to return to Kerbin. I calculated all that simply by adding up all the numbers between Kerbin and Mun. A round trip to Duna would require about 8200 m/s, eyeballing values for Kerbin and Duna because the atmosphere can help or hinder you depending on the design of your ship and how you fly it.

The other thing to consider when going interplanetary is the phase angle. You need to have both the origin and destination planets aligned precisely to get to where you're going. Here's a diagram showing what angles you should use:

And here's a diagram I made showing how you should burn when leaving a planet:

Now that's a lot of information, and it's no surprise it doesn't all make sense at first. It is rocket science, after all! My best piece of advice is the simplest one: Just try it. See what works and what doesn't. The only real way to find out how to do interplanetary missions well is to try a few, find the issues, and try to fix them in the next runs.

[GoSlash27]

This is like saying you want to build a house but we shouldn't talk about carpentry because it makes no sense to you. You simply cannot plan an interplanetary mission without knowing what your DV is. That is the basic unit of measure in this realm.

 You don't necessarily need to know how to compute what DV your rocket has or how much DV you need to get to your destination (there are mods for that), but you absolutely must know what "DV" means if you're going to plan an interplanetary trip. You need to know how much DV your ship has and how much DV it takes to complete your trip.

 

Edited December 25, 2016 by GoSlash27

[mikegarrison]

Surely this is more appropriate for the gameplay forum.

[KSK]

6 hours ago, mikegarrison said:

Surely this is more appropriate for the gameplay forum.

The joy of KSP - sometimes serious physics and gameplay overlap.  This thread could go in either forum in my opinion.

One thing to add to @cubinator's excellent post, 'ln' means 'take the natural logarithm of the stuff in the brackets' Any scientific calculator (or scientific calculator program on your computer), will have an ln button, so don't worry about that part.

 

[Jonfliesgoats]

Fill out the blanks in this webpage, select ISP instead of VEX and you will get your Delta V.

http://www.strout.net/info/science/delta-v/

DV to get to a particular body changes with relative position.  Fill in the blanks on this webpage to determine transfer requirements.

https://alexmoon.github.io/ksp/#/Kerbin/80/Jool/100/false/ballistic/false/2/161

Edited December 25, 2016 by Jonfliesgoats

[Wjolcz]

Just download KER and instal the dV map for KSPedia (by @5thHorseman iirc). It's the easiest way.

[Vanamonde]

Thread moved to Gameplay Questions. 

And yes, there is only one way to know how far your rocket can go, and that's to compare its dV to the values needed by the game. If you do not want to do the math yourself, then some mods that have been recommended can do it, but it will need to be done one way or another. 

[bewing]

Have to disagree a tiny bit with Vanamonde. There is another way, called "Sandbox mode".

Copy your rocket over to sandbox mode, launch it, fly it to your interplanetary destination, and see how much fuel you have left.

If you want a good estimation of total deltaV without a calculator: use SetOrbit to put your rocket in a high orbit around Eeloo, note the orbital speed, and hit Z. When the fuel runs out, note your new orbital speed and subtract.

 

[DD_bwest]

11 hours ago, Jonfliesgoats said:

Fill out the blanks in this webpage, select ISP instead of VEX and you will get your Delta V.

http://www.strout.net/info/science/delta-v/

DV to get to a particular body changes with relative position.  Fill in the blanks on this webpage to determine transfer requirements.

https://alexmoon.github.io/ksp/#/Kerbin/80/Jool/100/false/ballistic/false/2/161

this right here if you arent strong at math.  ive knowm more than a few who are hopeless at proper calculations but have had alot of succeas with that site.

add 15 to 20 percent from any delta v map, as getting the most efficient burns can be a challenge.   the extra reserve fuel can really save you sometimes

[Spricigo]

7 hours ago, bewing said:

Have to disagree a tiny bit with Vanamonde. There is another way, called "Sandbox mode".

You mean "revert flight" ?  

[Vanamonde]

Quote

Copy your rocket over to sandbox mode, launch it, fly it to your interplanetary destination, and see how much fuel you have left.

This is actually my method because I am too lazy to do the math. But that doesn't mean I would recommend it to others. 

[cubinator]

3 hours ago, Vanamonde said:

This is actually my method because I am too lazy to do the math. But that doesn't mean I would recommend it to others. 

It's actually a lot faster to do the math than to fly all the way to another planet. You can even use this equation in flight:

For LF/OX:

dV = 9.81 * Isp * ln(current mass / (current mass - 0.005 * (# units LF + # units OX)))

For nuclear:

dV = 9.81 * 800 * ln(current mass / (current mass - 0.005 * # units LF)

It'll save you hours.

Edited December 26, 2016 by cubinator

[Vanamonde]

Quote

It'll save you hours. 

Yeah, but it sounds all hard and stuff. 

[bewing]

2 hours ago, cubinator said:

dV = 9.81 * Isp * ln(current mass / (current mass - 0.005 * (# units LF + # units OX)))

dV = 9.81 * 800 * ln(current mass / (current mass - 0.05 * # units LF)

It'll save you hours.

Just so long as you get the right number of zeros in your fuel units calculation ... otherwise it'll lose you hours.

 

 

[cubinator]

35 minutes ago, bewing said:

Just so long as you get the right number of zeros in your fuel units calculation ... otherwise it'll lose you hours.

 

 

Fixed, thanks!

[cubinator]

1 hour ago, Vanamonde said:

Yeah, but it sounds all hard and stuff. 

It may sound hard, but all you have to do is put in the right numbers and hit enter, and you'll know your exact delta-V. It's actually quite straightforward.

Let me give you an example. We'll use a simple ship: a Mk1 command pod (0.84 t), an FL-T800 fuel tank (4.5 t, 360 LF, 440 OX), and an LV-909 engine (0.5 t, 345 Isp). We'll assume it's in a circular 72 km orbit around Kerbin. Now to calculate delta-V:

1. The mass of the ship is 5.84 t (can be found from map view).
2. The Isp of the ship is 345 (can be found from engine's right-click menu).
3. The ship has 360 units of Liquidfuel and 440 units of Oxidizer (can be found from fuel tank's right click menu).
4. Now use this equation:

2 hours ago, cubinator said:

dV = 9.81 * Isp * ln(current mass / (current mass - 0.005 * (# units LF + # units OX)))

2 hours ago, cubinator said:

dV = 9.81 * 345 * ln(5.84 / (5.84 - 0.005 * (360 + 440)))

A little bit of button punching later, and:

dV = 3908.922 m/s

5. Use a delta-V map to find where you can go!

Spoiler

So, that ship could get to Ike and back with plenty of fuel to spare provided you manage to conserve 50 EC for two years and land upright with no legs.

[Vanamonde]

I know how and have done it before, Cubinator. But having done so, I have a rough idea what missions require and just eyeball them, adjusting as testing reveals necessary. 

[bewing]

5 hours ago, Vanamonde said:

This is actually my method because I am too lazy to do the math. But that doesn't mean I would recommend it to others. 

I would totally recommend it to others. DeltaV is all fine and good and stuff, but there is a lot more to learn about a new rocket than just its deltaV. There's the atmospheric stability, and if you're trying to gravity turn the thing, you want to know how much of a nudge it needs off the platform, and what and where your typical Ap is after getting the new rocket to orbit. And since that deltaV map is an absolute theoretical minimum, it much nicer to know how much fuel you actually need to get to your destination and landed. And, of course, you need a list of all the parts you forgot to put on the thing when you built it. And the best practical way that I know of to find all these wonderful things out ASAP is to just do a test launch. Once you've gotten the thing to LKO it doesn't take much more timewarping to get it to Jool or whereever to finish the testing.

[foamyesque]

I believe the deltaV maps actually do tend to build in a little bit of fudge, especially for things like ascent on atmospheric bodies. It's still a good idea to carry a bit more than they suggest, though, and the question of "how much" is best answered by test flights.

It's really dang helpful to have a target to aim for in the first place, though.

[cubinator]

2 hours ago, Vanamonde said:

I know how and have done it before, Cubinator. But having done so, I have a rough idea what missions require and just eyeball them, adjusting as testing reveals necessary. 

Ah, well in that case...test away!

I still think two minutes of calculator time is a lot easier and almost as good as twenty minutes of flight testing, though.

Edited December 27, 2016 by cubinator

[Spricigo]

Personally I never feel the need for very accurate TWR and delta-v calculations.

 I just design a ship for the mission,  if it fail or I feel it's overengineered I adjust accordingly. 

Doing the math is not necessary but knowing how it works is handy. 

[Loren Pechtel]

On 12/24/2016 at 6:24 PM, cubinator said:

Once you know know your Delta-V, you can use a Delta-V map to find out how far your rocket can go on an optimal route. Here is a nice map:

Huh??  This chart seems to imply that you can aerobrake on Kerbol.  My understanding of the situation:

1)  Kerbol doesn't have an atmosphere.

2)  There's a zone of certain destruction before you reach it anyway.

3)  You can't even reach the zone of destruction these days due to heat.

[foamyesque]

If I recall correctly, Kerbol does have an atmosphere, but it's only reachable if you turn off max temperatures.