Calculating dv for new planets

[Yukon0009]

Is there any way to calculate the dv used for landing/ascent/getting to on a custom planet?

[SanderB]

you can in the tracking center select the planet and view the escape velocity under attributes. If the planet has no atmosphere the dV to orbit and back is escape velocity & 70.7% plus maybe 5-10% depending on the TWR of the vessel used. If there is an atmosphere, the calculation gets more complicated.

[Superfluous J]

Also, the Transfer Window Calculator mod works with custom planets, so that would get you the dV to transfer burn from Kerbin (or any other planet) to there.

- - - Updated - - -

I believe all charts that give dV for atmo ascents use actual testing. And IMO actual testing for the other values is a good idea too. Doesn't matter if the math says you can do it in 1300m/s if it always takes your 1400. You'd better bring 1400. Plus some spare.

[GoSlash27]

yukon,

It can be calculated precisely for airless bodies, but you'll need to make a very rough estimate if it has an atmosphere.

For an airless body you will need it's radius and mass.

First you find the velocity of an orbit at it's surface

Vo= sqrt(6.67*10^-11*M/r)

where M is in kg and r is in meters.

You have to achieve this velocity before you can Hohmann transfer to your desired orbital altitude. This is straight DV plus or minus whatever sidereal rotation it gives you.

Next you calculate the velocity at your desired orbital altitude using the same formula.

The velocity is lower at the orbital altitude, so you take the difference between the two. This is the DV required to move your orbit from the surface to your desired altitude

Add the 2 together and you have the theoretical minimum DV required to achieve orbit, but this assumes 2 instant impulses. Since your thrust isn't infinite you'll need to pad it a bit for safety.

ex. the Mun.

Mun has a mass of 9.76x10^20 kg and a surface radius of 2x10^5 m.

orbital velocity at surface= sqrt(6.67x10^-11 x 9.76x10^20 / 2.00x10^5)= 571 m/sec.

orbital velocity at 14 km altitude= sqrt(6.67x10^-11 x 9.76x10^20 / 2.14x10^5)= 552 m/sec.

Hohmann transfer DV= 571-552= 19 m/sec

Total DV = 571+19= 590 m/sec.

Best,

-Slashy

Edited September 3, 2015 by GoSlash27

[tbarcello]

https://play.google.com/store/apps/details?id=com.amagi82.kerbalspaceapp

I got you. Don't worry, I got you. Upper right corner in that app, seriously.

Or did I completely misunderstood the question?

Edited September 3, 2015 by tbarcello

[Gaarst]

Use this equation (Vis-viva) for Hohmann transfers. Just use the Hohmann transfer orbit for semi-major axis.

[Renegrade]

Mun has a mass of 9.76x10^20 kg and a surface radius of 2x10^5 m.

orbital velocity at surface= sqrt(6.67x10^-11 x 9.76x10^20 / 2.00x10^5)= 571 m/sec.

orbital velocity at 14 km altitude= sqrt(6.67x10^-11 x 9.76x10^20 / 2.14x10^5)= 552 m/sec.

Hohmann transfer DV= 571-552= 19 m/sec

Total DV = 571+19= 590 m/sec.

I use a similar method, but I actually calculate the intermediate orbit. Ex - assume A is 0km altitude, and B is 14km altitude, I'd calculate the PE=A, AP=A orbit (A/A), then the necessary delta-v to change that to an A/B orbit (at A), and then the delta-v to round it out (using the orbital velocity of the A/B orbit at B compared to the orbital velocity of a B/B orbit (at any point)).

It's a bit longer, but the CPU doesn't mind.

(there's a slight discrepancy, so I have to assume that the A/A straight to B/B is missing out on something.. Oberth effect perhaps?)

[GoSlash27]

I use a similar method, but I actually calculate the intermediate orbit. Ex - assume A is 0km altitude, and B is 14km altitude, I'd calculate the PE=A, AP=A orbit (A/A), then the necessary delta-v to change that to an A/B orbit (at A), and then the delta-v to round it out (using the orbital velocity of the A/B orbit at B compared to the orbital velocity of a B/B orbit (at any point)).

It's a bit longer, but the CPU doesn't mind.

(there's a slight discrepancy, so I have to assume that the A/A straight to B/B is missing out on something.. Oberth effect perhaps?)

Renegrade,

Just rounding errors, most likely. If you add together the DV on your kick and circularization burns, you should find that it equals the difference in orbital velocity between the first and second orbits.

Using the vis-visa for my example

initial orbit altitude 0, v= 570.5

tx orbit sma 207km with Vpe=580.1 and Vap=542.1

kick burn 580.1-570.5=9.6 m/sec

circ burn 551.5-542.1=9.4 m/sec

total= 19.0

final orbit altitude 14km, v=551.5

570.5-551.5 also equals 19.0.

Best,

-Slashy

Edited September 3, 2015 by GoSlash27

[John FX]

One thing which may be a factor in custom planets is if they have a very fast rotational speed then that will reduce/increase the Dv required to be stationary above the surface depending on your inclination.

If it is a whacky custom planet then the rotational speed on the surface could be higher than the orbit speed at that altitude although this would not be a planet that could exist in real life.

[tbarcello]

One thing which may be a factor in custom planets is if they have a very fast rotational speed then that will reduce/increase the Dv required to be stationary above the surface depending on your inclination.

If it is a whacky custom planet then the rotational speed on the surface could be higher than the orbit speed at that altitude although this would not be a planet that could exist in real life.

It will be a polar landing then

[Renegrade]

Renegrade,

Just rounding errors, most likely. If you add together the DV on your kick and circularization burns, you should find that it equals the difference in orbital velocity between the first and second orbits.

Using the vis-visa for my example

initial orbit altitude 0, v= 570.5

tx orbit sma 207km with Vpe=580.1 and Vap=542.1

kick burn 580.1-570.5=9.6 m/sec

circ burn 551.5-542.1=9.4 m/sec

total= 19.0

final orbit altitude 14km, v=551.5

570.5-551.5 also equals 19.0.

Best,

-Slashy

I'm getting A/A=570.701, A/B@A=580.271, A/B@B=542.309, and B/B=551.718. The total is coming out as 18.978, but a difference between A/A and B/B velocities is 18.984 (an actual difference of about 0.005429097345996).

All internal calculations are carried out with fifteen digit (aka double) precision, and the final is rounded as displayed. For example, A/A is internally something around 570.701322935211010.

Anyhow that seems a bit large for a rounding error :S

[GoSlash27]

I'm getting A/A=570.701, A/B@A=580.271, A/B@B=542.309, and B/B=551.718. The total is coming out as 18.978, but a difference between A/A and B/B velocities is 18.984 (an actual difference of about 0.005429097345996).

All internal calculations are carried out with fifteen digit (aka double) precision, and the final is rounded as displayed. For example, A/A is internally something around 570.701322935211010.

Anyhow that seems a bit large for a rounding error :S

Renegrade,

The discrepancy is from somewhere, but it's not a result of shortcutting the formula.

Also keep in mind you're talking an error of 5mm/sec, which is effectively nothing. KSP only shows velocity to the nearest .1m.

If you try the same exercise with a very eccentric transfer in a steep gravity well, you'll see it still works out the same.

*edit* Actually, having worked this in some extreme examples, I actually did find a discrepancy. I think you're right and the Oberth effect plays a part. In most cases though, the error will be low enough to safely ignore.

Best,

-Slashy

Edited September 3, 2015 by GoSlash27

[cantab]

For a vacuum body there's a simple in-game way to estimate it. Get a ship in low orbit, set up a deorbit manoeuvre, then set a second manoeuvre on the planet's surface and drag the retrograde handles back until you see it flip round. Add the two burns together and that's your delta-V for landing. (Hover over any manoeuvre in map view when the handles are closed to see the delta-V). Then add safety margin to taste.

[xtoro]

Hmm, I seem to be one of the few that does trial and error? Math is one of my strong points but for some reason I prefer sending probes, even though it does cost more in my career, its more interesting for me.

I see it as this: KSC knows the orbit of Sarnus, and that's it. So I use MechJeb to find a launch window and to fine tune my approach because I hate fiddling with maneuver nodes. I send a probe to do a flyby and gather science, and I try to use my remaining fuel to circularize. Using the dV spent in trying to circularize, I'll guestimate what I need and then double it so that I might be able to also visit a moon or two.

I know it's not cost-efficient but its fun going into the unknown not knowing if the mission will succeed or not. And I think of the early days of NASA's Moon exploration when they just sent probes to gather data doing flyby's and impacts dozens of times using the early Pioneer and Ranger probes, while the USSR was using their Luna probes. It was very Kerbal times back then, with launch failures, flybys and crashes into the moon, and that's how I play

[Superfluous J]

Hmm, I seem to be one of the few that does trial and error? Math is one of my strong points but for some reason I prefer sending probes, even though it does cost more in my career, its more interesting for me.

I see it as this: KSC knows the orbit of Sarnus, and that's it. So I use MechJeb to find a launch window and to fine tune my approach because I hate fiddling with maneuver nodes. I send a probe to do a flyby and gather science, and I try to use my remaining fuel to circularize. Using the dV spent in trying to circularize, I'll guestimate what I need and then double it so that I might be able to also visit a moon or two.

I know it's not cost-efficient but its fun going into the unknown not knowing if the mission will succeed or not. And I think of the early days of NASA's Moon exploration when they just sent probes to gather data doing flyby's and impacts dozens of times using the early Pioneer and Ranger probes, while the USSR was using their Luna probes. It was very Kerbal times back then, with launch failures, flybys and crashes into the moon, and that's how I play

I'm similar to you except I plan the mission out roughly with maneuver nodes when the dV charts can't help. I send nothing anywhere without at least being moderately confident that - barring myself screwing something up - they can get home. For me the fun is in not screwing anything up. Which I almost invariably do anyway

[nightingale]

I'm similar to you except I plan the mission out roughly with maneuver nodes when the dV charts can't help. I send nothing anywhere without at least being moderately confident that - barring myself screwing something up - they can get home. For me the fun is in not screwing anything up. Which I almost invariably do anyway

And you screwing up is where the fun is for your viewers.

[magico13]

For me the fun is in not screwing anything up. Which I almost invariably do anyway

To be fair, it's not always directly your fault. Like when your ship decides to keep pumping fuel to a very massive and inefficient engine, thus trapping you in orbit of Jool

[Renegrade]

*edit* Actually, having worked this in some extreme examples, I actually did find a discrepancy. I think you're right and the Oberth effect plays a part. In most cases though, the error will be low enough to safely ignore.

Oh yeah, definitely low enough to ignore. In fact, my off the cuff method is simply to take the orbital velocity for a 0m altitude orbit and add twenty percent anyhow hehe

I was just curious as to the source of the (itty bitty) discrepancy.

[Superfluous J]

To be fair, it's not always directly your fault. Like when your ship decides to keep pumping fuel to a very massive and inefficient engine, thus trapping you in orbit of Jool

I'm pretty sure my ship was doing exactly what I (forgot I) told it to do.

And while that engine was too much for what was on the ship at the time, it was fine for the whole trip and throwing it away meant losing the fuel it was (supposed to be restricted to) using.

[GoSlash27]

Hmm, I seem to be one of the few that does trial and error? Math is one of my strong points but for some reason I prefer sending probes, even though it does cost more in my career, its more interesting for me.

xtoro,

Actually, I think you're in the large majority and folks like myself are in the minority.

I plan out the entire mission on paper before I design the hardware and then I design the hardware before I set foot in the VAB. There aren't very many people who do it that way.

But you know... lots of ways to play this game.

Best,

-Slashy