[Space, Spaceships] Total ΔV for Interplanetary Travel

[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?

Dalton “who is working on a One-Tangent Burn Orbit spreadsheet” Spence

[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.

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!

[DaltonS]

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.

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.

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.

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.

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

[Humabout]

Originally Posted by DaltonS

Thanks. This will help.
I think areobraking would require a certain level of armor to withstand aerodynamic heating depending on the ΔV you want to shed.
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.


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

One-Tangent Burns weren't overlooked so much as intentionally omitted. The primary focus of the article was to give GMs reasonable bounds on realistic transfer times. That's why it covers the longest, most fuel-efficient transfers and the fastest, least fuel-efficient transfer. Enough variables exist in between these two senarios that it becomes unrealistic to provide a simple explaination of planning such voyages. I drew a pretty hard line at the GM having to track the motion of celestial bodies on a calendar, and you start to need that sort of thing to determine just how fast of a one-tangent burn is possible at a given time. If you gloss over that and arrive at the speed of plot, the article gives that speed upper and lower bounds, as it does for dV requirements.

After next spring, I may reprise the article with something more detailed, but I won't attempt this before I do more in depth study of the material. I probably won't present it through Pyramid unless Steven asks for it or there appears to be enough demand for something more math-heavy than the original.

[Anders]

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)

[PTTG]

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.

[DaltonS]

Originally Posted by PTTG

You can run it in linux, you just need to use grape-based ethanol fuel.

Perhaps if it's made from Grand Fenwick grapes. ;)

Dalton “what sort of ΔV can we get from Pinot Grand Fenwick?” Spence

[Fred Brackin]

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..

[RyanW]

Originally Posted by Fred Brackin

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.

Russia's got it even worse, with their even further north launch site, and the most sensible trajectory passing through unfriendly Chinese airspace during assent. The need to be accessible from Baikonur is a big part of why the ISS is placed in such a highly inclined orbit.

[Ulzgoroth]

On consideration, I think that's probably necessary.