Delta v map reading

[guitounet]

Hi guys,

 

i would like to know if someone can help me reading deltaV map (ie : http://wiki.kerbalspaceprogram.com/images/7/73/KerbinDeltaVMap.png).

I am not sure how to understand it.

Exemple of a round trip from kerbin to mun:

Should I add 3400+860+310+580+580+310+860+3400 to build  the right rocket ? Sure I will add some more dv because i'm  a not so good pilot and i like wasting fuel

Thanks for your help

[FullMetalMachinist]

21 minutes ago, guitounet said:

Should I add 3400+860+310+580+580+310+860+3400 to build  the right rocket ?

You've almost got it. You just don't need the second 860 and 3400. The reason is that you can use Kerbin's atmosphere to do that part of the work for you (called aerocapture or aerobraking). 

And yes, adding some extra is usually a good idea. I usually add 20-40% more. 

[diomedea]

DV map is pretty easy moving from Kerbin to other places. Is a bit less intuitive for other trips.

You'll spend around 3400 m/s to get in orbit (80 Km altitude) (Note, value depends on TWR, drag, trajectory and how accurately it is followed). Then, 860 m/s for the hohmann transfer to Mun, that brings apoapsis up to ~ 11400 Km, while periapsis still is at 80 Km. Then, 310 m/s at Mun periapsis to close the hyperbolic injection orbit to a circular low orbit. Then 580 m/s to land (you'll probably spend more unless you're able to perform a perfect suicide burn).

Now, to bring the craft back up in a low circular orbit around Mun, takes another 580 m/s. To eject from low orbit you need to accelerate to a hyperbolic orbit fast enough to bring you back to the same kind of hohmann transfer orbit you used to get to Mun: that requires 310 m/s. Burning just a few m/s more, you'll be able to lower periapsis around kerbin from 80 Km to a value low enough to have an aerocapture (35-40 Km, depends on drag). And generally you don't need to burn during aerocapture.

So the total amount is (theoretically) close to 3400+860+310+580, +580+310+a bit. Add some to deal with plane changes, piloting errors, non-instantaneous burns, imperfect timing, mid-course corrections.

[guitounet]

OK, thank you, think I understand now !

[WanderingKid]

What's throwing you off (slightly) is the atmosphere of Kerbin.

Let's take, for example, a trip from Dres to Moho and back again to land on Dres.  Insane or not, your calculation makes sense.  The reason for that is you would have to burn to slow down for orbit around Dres on your return, and then burn again to land safely.

However, because of the atmosphere on Kerbin you can Aerobrake off massive amounts of dV from the totals.  This counts for any planet with an atmosphere that you can safely aerobrake in.  Those planets are Eve (bring heatshields), Kerbin, Duna, Jool, and Laythe.

[Benjamin Kerman]

On 1/22/2017 at 2:02 PM, WanderingKid said:

What's throwing you off (slightly) is the atmosphere of Kerbin.

Let's take, for example, a trip from Dres to Moho and back again to land on Dres.  Insane or not, your calculation makes sense.  The reason for that is you would have to burn to slow down for orbit around Dres on your return, and then burn again to land safely.

However, because of the atmosphere on Kerbin you can Aerobrake off massive amounts of dV from the totals.  This counts for any planet with an atmosphere that you can safely aerobrake in.  Those planets are Eve (bring heatshields), Kerbin, Duna, Jool, and Laythe.

And even for Jool you don't need a heatshield because of its moons. More likely than not, you will be able to do a gravity capture using Laythe or Tylo. 

[PayadorPerseguido]

I have a few questions myself. The ammount of dV akin to reaching the low kerbin orbit is 3400m/s but when you get there, the actual speed of the craft is different, why is that? Is it because the dV difference is actually spent on atrition due to the atmosphere? If so, is this an example of momentum conservation?
This is probably too dumb, but hey, i did law school - not physics.

Thanks!!

Edited January 27, 2017 by PayadorPerseguido

[Benjamin Kerman]

10 minutes ago, PayadorPerseguido said:

I have a few questions myself. The ammount of dV akin to reaching the low kerbin orbit is 3400m/s but when you get there, the actual speed of the craft is different, why is that? Is it because the dV difference is actually spent on atrition due to the atmosphere? If so, is this an example of momentum conservation?
This is probably too dumb, but hey, i did law school - not physics.

Thanks!!

Ah. This is a common misunderstanding. Your orbital speed will be a lot different from the dV it takes to get to orbit. This is because you have to include the energy spent accelerating upwards as well as the energy spent moving sideways. IRL, you can reach space with a rocket the size of a telephone pole, but to get into orbit you have to use an Atlas. This was the difference between the Mercury Redstone and the Mercury Atlas. 

[FullMetalMachinist]

10 hours ago, PayadorPerseguido said:

The ammount of dV akin to reaching the low kerbin orbit is 3400m/s but when you get there, the actual speed of the craft is different, why is that?

To expand on @Benjamin Kerman's answer, it also has to do with the rockets having lower efficiency in atmosphere. When you look at the engine stats in the VAB, there will be two numbers for Isp, one at sea level the other in vacuum. The oft-quoted 3400 m/s number is if you were to calculate the dV as if the whole flight took place in vacuum. This is just to make the math easier. 

In reality, when the first stage fires at sea level they are less effective, and it basically takes more than 1 m/s vac dV to actually give the craft 1 m/s atmo dV. That difference depends on the engine, but they are all worse in atmo. 

Combine that with gravity losses (if your rocket can accelerate at 20 m/s/s, gravity gobbles up 9.8 of that) and drag losses, and you get the roughly 1000 m/s difference that you see between the orbital speed and how much dV it took to get there. 

Also note that the magic 3400 number is an approximation that has been experimentally arrived at. There is no easy way to mathematically determine that number. Some rockets can get to orbit using less than that, some take more. 

[Spricigo]

13 hours ago, PayadorPerseguido said:

I have a few questions myself. The ammount of dV akin to reaching the low kerbin orbit is 3400m/s but when you get there, the actual speed of the craft is different, why is that? Is it because the dV difference is actually spent on atrition due to the atmosphere? If so, is this an example of momentum conservation?
This is probably too dumb, but hey, i did law school - not physics.

Thanks!!

Actually the cause of the discrepancy its because this is an example of variation of momentum.

DeltaV is given by the Tsiolkovsky Rocket Equation, it is based on the principle of momentum conservation: the propellant moves down, the rocket moves up. So far, so good.

But we don't have only this, we also have Gravity and Drag, external forces that change the momentum.  Newton's 2nd Law.

 

 

[Noah Foloni]

On 1/22/2017 at 4:02 PM, WanderingKid said:

What's throwing you off (slightly) is the atmosphere of Kerbin.

Let's take, for example, a trip from Dres to Moho and back again to land on Dres.  Insane or not, your calculation makes sense.  The reason for that is you would have to burn to slow down for orbit around Dres on your return, and then burn again to land safely.

However, because of the atmosphere on Kerbin you can Aerobrake off massive amounts of dV from the totals.  This counts for any planet with an atmosphere that you can safely aerobrake in.  Those planets are Eve (bring heatshields), Kerbin, Duna, Jool, and Laythe.

So to get to Minmus and Back i'd need 5940 m/s of dV, but i got to minmus and back with only 4000

[InfernoSD]

29 minutes ago, DaBakonAder said:

So to get to Minmus and Back i'd need 5940 m/s of dV, but i got to minmus and back with only 4000

From what did you arrive at the number 4000?

Bear in mind that in-game dV readings take atmosphere into consideration. A rocket on the launchpad or in the VAB will appear to have a low dV because of engine efficiency being low while in atmosphere. And that number will visibly go up as you exit the atmosphere. You can also click the dV button to switch these calculations between vaccum and atmosphere.

[paul_c]

43 minutes ago, DaBakonAder said:

So to get to Minmus and Back i'd need 5940 m/s of dV, but i got to minmus and back with only 4000

Some engines are very inefficient in atmosphere, but are good in a vacuum. For example, the Spark is 16.543kN in atmosphere, but 60kN in vacuum.

Your rocket probably took 3000m/s to get near/to LKO, then that remaining reported 1000m/s was actually 4000m/s in vacuum, thus is was enough to do a Minmus trip (numbers are approximate to illustrate the effect).

[king of nowhere]

56 minutes ago, DaBakonAder said:

So to get to Minmus and Back i'd need 5940 m/s of dV, but i got to minmus and back with only 4000

and besides what the other guys said, you only need around 5000 m/s for a minmus round trip. the 340 for plane change are a maximum value, you more often end up spending 50-100 for it. and then, once you are at minmus orbit, to come back to kerbin you only need to pay the 150 for intercept, and then aerobrake can take care of the rest.

[paul_c]

1 hour ago, king of nowhere said:

and besides what the other guys said, you only need around 5000 m/s for a minmus round trip. the 340 for plane change are a maximum value, you more often end up spending 50-100 for it. and then, once you are at minmus orbit, to come back to kerbin you only need to pay the 150 for intercept, and then aerobrake can take care of the rest.

Kerbin -> Minmus Orbit return, or Kerbin to Minmus landing and return?

[king of nowhere]

19 minutes ago, paul_c said:

Kerbin -> Minmus Orbit return, or Kerbin to Minmus landing and return?

landing and return.

3400 kerbin orbit + 930 minmus intercept + 150  minmus orbit +180 minmus landing + 180 minmus take off + 150 kerbin intercept. total 4990. add something for plane correction, as i said, it's usually 50-100 m/s. from there, you aerobrake the rest of the way.

[Noah Foloni]

On 1/21/2021 at 10:54 AM, InfernoSD said:

From what did you arrive at the number 4000?

Bear in mind that in-game dV readings take atmosphere into consideration. 

Ok. So the built in reader calculates for atmosphere, so it *LOOKS* like 4000, but its really more like 5000

22 hours ago, king of nowhere said:

landing and return.

3400 kerbin orbit + 930 minmus intercept + 150  minmus orbit +180 minmus landing + 180 minmus take off + 150 kerbin intercept. total 4990. add something for plane correction, as i said, it's usually 50-100 m/s. from there, you aerobrake the rest of the way.

 

On 1/21/2021 at 11:03 AM, paul_c said:

Some engines are very inefficient in atmosphere, but are good in a vacuum. For example, the Spark is 16.543kN in atmosphere, but 60kN in vacuum.

Your rocket probably took 3000m/s to get near/to LKO, then that remaining reported 1000m/s was actually 4000m/s in vacuum, thus is was enough to do a Minmus trip (numbers are approximate to illustrate the effect).

So a configuration of 2 reliant side boosters + swivel for kerbin -> LKO + a terrier to get to Minmus + a terrier to land & return to Kerbin will LOOK like 4000, but calculating Vacuums & what not, its more like 5000

[Superfluous J]

7 minutes ago, DaBakonAder said:

Ok. So the built in reader calculates for atmosphere, so it *LOOKS* like 4000, but its really more like 5000

You can change the reader to show vacuum, and should in general.

I wish there was an option to make it the default in fact.

[Noah Foloni]

Okay How do I Do that

please tell me

 

[king of nowhere]

1 hour ago, DaBakonAder said:

So a configuration of 2 reliant side boosters + swivel for kerbin -> LKO + a terrier to get to Minmus + a terrier to land & return to Kerbin will LOOK like 4000, but calculating Vacuums & what not, its more like 5000

I'd hazard more like 6000. the terrier is very inefficient in atmosphere, multiply by 4 all the deltaV of the last stage. and the swivel also is much better in space.

 

57 minutes ago, DaBakonAder said:

Okay How do I Do that

please tell me

 

click on deltaV to open the deltaV instruments. then select vacuum. You can also pick different altitudes or different planets, for example to simulate an eve lander.

[Noah Foloni]

Ok. the fact you can change for vacuums is cool within its self, but the fact you can also simulate other atmospheres? Wow.

26 minutes ago, king of nowhere said:

I'd hazard more like 6000. the terrier is very inefficient in atmosphere, multiply by 4 all the deltaV of the last stage. and the swivel also is much better in space.

 

 

The Terrier is my In Vacuum engine, and the Swivel/Reliant is my atmosphere engine

[Spricigo]

On 1/22/2021 at 4:20 PM, DaBakonAder said:

The Terrier is my In Vacuum engine, and the Swivel/Reliant is my atmosphere engine

Worth noticing how performance chances with altitude. To the point is often not only viable, but desirable to use a "vacuum engine" while still in the atmosphere.

[Superfluous J]

2 hours ago, Spricigo said:

Worth noticing how performance chances with altitude. To the point is often not only viable, but desirable to use a "vacuum engine" while still in the atmosphere.

Yeah above about 10km you're basically in space as far as vacuum engines are concerned.

[vv3k70r]

On 1/22/2017 at 8:17 PM, guitounet said:

Should I add 3400+860+310+580+580+310+860+3400 to build  the right rocket ?

It would be good. But over 4k dV can be spare for aerobraking. Keep around 25-50% of that for bad manouvers for first time.

When You heading home make PE directly low around >60km and do some orbits in real time controling the vessel for loosing speed and PE.

Of course there are more agresive aproach lower to atmosphere, but not for starter.

Aerobraking perform retrograde to expose engines - they are good with heat in game (pressure in bell is somehow not consider).