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Old 02-23-2017, 01:07 PM   #1
DaltonS
 
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Default [Space, Spaceships] Total ΔV for Interplanetary Travel

When calculating the total ΔV for an interplanetary trip, do we have to add the ΔV for breaking planetary orbit (both from the origin and braking to the destination) to that of the interplanetary transfer orbit?

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Old 02-23-2017, 01:22 PM   #2
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Default Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

You want the sum of the ΔV required to break orbit from the origin, the ΔV used in the transfer itself, and then, yes, the ΔV expended to attain orbit at the destination.

In theory. In practice, good piloting can reduce the ΔV needed on arrival, and if the destination has an appropriate atmosphere and you have a ship that can handle hypersonic atmospheric burns, you can use areobreaking to achieve orbit at the destination for a fraction of the ΔV cost.

All of this assumes a Hoffman transfer, which is generally both simple and low-ΔV-cost. The downsides are that it requires specific windows to work well, and it's fairly slow.

If one has very large reserves of ΔV, it's possible to fly from the origin to the destination directly, and I think there's notes on that in GURPS Space. Or Spaceships. Can't remember.

Best of luck!
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Old 02-23-2017, 01:24 PM   #3
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Default Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

On consideration, I think that's probably necessary.
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Old 02-23-2017, 01:34 PM   #4
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Default Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

How much difference is there if launching from Earth's surface as opposed to being launched from a facility already in orbit?
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Old 02-23-2017, 02:28 PM   #5
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Default Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

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How much difference is there if launching from Earth's surface as opposed to being launched from a facility already in orbit?
Per Spaceships p37, going surface to escape costs planetary escape velocity, while going from low orbit to escape costs 30% of escape velocity.
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Old 02-23-2017, 02:39 PM   #6
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Default Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

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Originally Posted by DaltonS View Post
When calculating the total ΔV for an interplanetary trip, do we have to add the ΔV for breaking planetary orbit (both from the origin and braking to the destination) to that of the interplanetary transfer orbit?
Realistically, the Oberth Effect means that the total ΔV is less than their sum; how much less depends on your thrust.

The limit case for infinite thrust is ΔV = sqrt( escape velocity ^ 2 + transfer velocity ^ 2 ) - orbital velocity, to either enter or leave orbit. For example, from low earth orbit (escape velocity = 11.2 km/sec, velocity = 7.92 km/s) to a Mars transfer orbit (2.9 km/s) requires sqrt( 11.2^2 + 2.9^2) - 7.92 = 3.65 km/s, which is barely more than the 3.28 km/s required to break orbit.

Launch direct from ground saves the fuel required to circularize your orbit.
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Old 02-23-2017, 04:11 PM   #7
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Default Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

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Originally Posted by PTTG View Post
You want the sum of the ΔV required to break orbit from the origin, the ΔV used in the transfer itself, and then, yes, the ΔV expended to attain orbit at the destination.
Thanks. This will help.
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Originally Posted by PTTG View Post
In theory. In practice, good piloting can reduce the ΔV needed on arrival, and if the destination has an appropriate atmosphere and you have a ship that can handle hypersonic atmospheric burns, you can use areobreaking to achieve orbit at the destination for a fraction of the ΔV cost.
I think areobraking would require a certain level of armor to withstand aerodynamic heating depending on the ΔV you want to shed.
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All of this assumes a Hoffman transfer, which is generally both simple and low-ΔV-cost. The downsides are that it requires specific windows to work well, and it's fairly slow.
That's where a One-Tangent Burn orbit calculator comes into play. Unlike a Hohmann transfer, you can adjust the launch windows by adjusting the ΔV used. The second burn has to be done at an angle to the flight path, requiring more ΔV.

Quote:
Originally Posted by PTTG View Post
If one has very large reserves of ΔV, it's possible to fly from the origin to the destination directly, and I think there's notes on that in GURPS Space. Or Spaceships. Can't remember.
Quote:
Brachistochrone (literally meaning “shortest time”) transfers are those that use constant thrust throughout the duration of the voyage, performing a 180° rotation halfway to decelerate until coming to rest at the target destination.
Pyramid 3/79: Space Atlas "Halfway to Anywhere"(p.27)
There is even a spreadsheet to go with the article. :) For some reason they missed One-Tangent Burns though.

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Old 02-23-2017, 04:26 PM   #8
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Default Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

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All of this assumes a Hoffman transfer, which is generally both simple and low-ΔV-cost. The downsides are that it requires specific windows to work well, and it's fairly slow.
So it won't work with Linux? :o)
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Old 02-23-2017, 05:39 PM   #9
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Default Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

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So it won't work with Linux? :o)
You can run it in linux, you just need to use grape-based ethanol fuel.
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Old 02-23-2017, 06:58 PM   #10
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Default Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

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You want the sum of the ΔV required to break orbit from the origin, the ΔV used in the transfer itself, and then, yes, the ΔV expended to attain orbit at the destination.
Just to be exhaustive there's the possibility of an change or orbital plane maneuver. It was part of the effect of launching from Cape Canaveral for the Moon that on the right days no change of plane was needed.

It's usually a glossed over effect and I know of no easy databases but the planets of our solar system mostly do have different orbital planes..
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