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Building for Interplanetary


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I'm a long-time player, but I've never been much good at efficiently building for interplanetary missions. Though I've gone interplanetary, it's pretty much just luck if I make it or not. No matter what, I can't seem to build a craft with more than 11km/s dV. I'm basically looking to have two questions answered:

 

1. How do I find out exactly how much dV a trip will take? I've seen dV maps, but don't have much of an idea how to read them.

2. How much dV should each stage have, starting from LKO?

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2 hours ago, Dizzle said:

I'm a long-time player, but I've never been much good at efficiently building for interplanetary missions. Though I've gone interplanetary, it's pretty much just luck if I make it or not. No matter what, I can't seem to build a craft with more than 11km/s dV. I'm basically looking to have two questions answered:

 

1. How do I find out exactly how much dV a trip will take? I've seen dV maps, but don't have much of an idea how to read them.

2. How much dV should each stage have, starting from LKO?

1. This or this site might help you

2. Doesn't really matter if you don't have an atmosphere

Edited by ToukieToucan
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2 hours ago, Dizzle said:

1. How do I find out exactly how much dV a trip will take? I've seen dV maps, but don't have much of an idea how to read them.

You read it basically like a subway map. It's no coincidence that the Community DV Map looks exactly like one :P

To figure out how much dV something costs, just trace out the route you want to take, and add up all the numbers you encounter. For example, using the above map, you can plan out a Mun landing like this:

Start at Kerbin surface
Go to low Kerbin orbit (+3400)
Go to Mun intercept (+860)
Go to low Mun orbit (+310)
Go to Mun surface (+580)

= Total cost of 5,150 m/s

Keep in mind that these are approximate numbers for a reasonably well flown but not completely perfect mission. It's a good idea to bring some margin for error 8for example, bringing 5,500 m/s for the above Mun mission).

And then, remember that you need to return. To do so, again trace out the route you want to take, and keep adding onto your total:

Start at Mun surface
Go to low Mun orbit (+580)
Got to Mun intercept (+310), which is equal to an escape burn
Go to low Kerbin orbit (+860)
Go to Kerbin surface (+3400)

You will notice that it's just the same numbers, the other way around. However, you are able to see on the dV map that there are those nice "aerobraking possible" arrows next to the last two legs of the journey. This means that if you intend to use the atmosphere to slow down, you don't have to pay this dV cost. And since you're performing a reentry at Kerbin, you are definitely using the atmosphere! So the 860 and 3,400 m/s positions are zeroed out, and you only add 580 and 310 to your previous total of 5,150. Your new total is 6,040 m/s.

Therefore, you should take about 6,250 m/s to 6,500 m/s worth of dV for a Mun land and return mission, if you want to have some margin.

The same thing applies to interplanetary trajectories. They are just longer than a trip to the Mun, so you have to count more stations, but it's still only just basic addition. Be on the lookout for opportunities to aerobrake, but keep in mind that your spacecraft needs to be able to survive the process (i.e. for your direct Kerbin reentry from the Mun you should be carrying a heatshield).

 

3 hours ago, Dizzle said:

2. How much dV should each stage have, starting from LKO?

Short answer: Depends on your engine choice. 3000 to 4000 is probably an acceptable number.

Long answer: learn about mass fractions and how they relate to dV.

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