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 02-23-2017, 01:07 PM #1 DaltonS     Join Date: Aug 2004 Location: Hamilton, Ont. CANADA [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? Dalton “who is working on a One-Tangent Burn Orbit spreadsheet” Spence
 02-23-2017, 01:22 PM #2 PTTG     Join Date: Feb 2011 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!
 02-23-2017, 01:24 PM #3 Ulzgoroth   Join Date: Jul 2008 Re: [Space, Spaceships] Total ΔV for Interplanetary Travel On consideration, I think that's probably necessary. __________________ I don't know any 3e, so there is no chance that I am talking about 3e rules by accident.
 02-23-2017, 01:34 PM #4 Phantasm     Join Date: Jun 2006 Location: On the road again... 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? __________________ "Life ... is an Oreo cookie." - J'onn J'onzz, 1991 "But mom, I don't wanna go back in the dungeon!" The GURPS Marvel Universe Reboot Project A-G, H-R, and S-Z, and its not-a-wiki-really web adaptation. Ranoc, a Muskets-and-Magery Renaissance Fantasy Setting
02-23-2017, 02:28 PM   #5
Ulzgoroth

Join Date: Jul 2008
Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

Quote:
 Originally Posted by Phantasm 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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02-23-2017, 02:39 PM   #6
Anthony

Join Date: Feb 2005
Location: Berkeley, CA
Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

Quote:
 Originally Posted by DaltonS 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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Last edited by Anthony; 02-23-2017 at 03:36 PM.

02-23-2017, 04:11 PM   #7
DaltonS

Join Date: Aug 2004
Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

Quote:
 Originally Posted by PTTG 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.
Quote:
 Originally Posted by PTTG 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.
Quote:
 Originally Posted by PTTG 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 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.

Dalton “still working on his Mars Semi-Cycler Spence

02-23-2017, 04:26 PM   #8
Anders

Join Date: Jan 2005
Location: Gothenburg, Sweden
Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

Quote:
 Originally Posted by PTTG 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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02-23-2017, 05:39 PM   #9
PTTG

Join Date: Feb 2011
Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

Quote:
 Originally Posted by Anders So it won't work with Linux? :o)
You can run it in linux, you just need to use grape-based ethanol fuel.

02-23-2017, 06:58 PM   #10
Fred Brackin

Join Date: Aug 2007
Re: [Space, Spaceships] Total ΔV for Interplanetary Travel

Quote:
 Originally Posted by PTTG 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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Fred Brackin

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