Author Topic: Taking the BFR Beyond Mars  (Read 16668 times)

Online oldAtlas_Eguy

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Re: Taking the BFR Beyond Mars
« Reply #60 on: 11/17/2017 11:15 PM »
Most miss the point that with an on orbit (in flight) refueling with a SC and tanker set architecture there is not actually a limit on the delta V possible with this system. It is only a matter of costs since for each 6.3km/s of delta V is an equation of 6.3*N where number of launches is = 5^N + 5^(N-1) +...+5(N-N)

N   DV (km/s)  Launches
1      6.3             6
2    12.6            31
3    18.9          156

The problem comes in how to get the tankers back in some reasonable amount of time or if there is enough funds to expend them.
« Last Edit: 11/17/2017 11:20 PM by oldAtlas_Eguy »

Offline qraal

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Re: Taking the BFR Beyond Mars
« Reply #61 on: 11/17/2017 11:34 PM »
Nicely summarised. Thanks for the equation too! Quite right. I'm only looking at minimal architectures at this point, for initial feasibility. But faster flights will help!

Titan is very attractive as a target thanks to the aerocapture angle, but the long trip time is a problem. I don't think it's insuperable. Artificial gravity by spin should be perfectly feasible, if we're talking *Titan-gee* levels. It's a *pioneering* effort after all, not a programmatic mission, that I'm envisaging. A fast elliptical transfer or even a parabolic orbit can be done for less than a full-tank. Starting with a full tank and using the remaining fuel for initial power production will help expand the ultimate ISRU options, without presuming nuclear will be readily available. I'm hoping the Kilopower effort by NASA pays off and mini-reactors become commercially available for Outer Solar System ventures, but the more In Situ power supplies that can be tapped, the better. Thus the appeal of high altitude windpower that Titan seems to have plenty of.


Most miss the point that with an on orbit (in flight) refueling with a SC and tanker set architecture there is not actually a limit on the delta V possible with this system. It is only a matter of costs since for each 6.3km/s of delta V is an equation of 6.3*N where number of launches is = 5^N + 5^(N-1) +...+5(N-N)

N   DV (km/s)  Launches
1      6.3             6
2    12.6            31
3    18.9          156

The problem comes in how to get the tankers back in some reasonable amount of time or if there is enough funds to expend them.

Offline speedevil

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Re: Taking the BFR Beyond Mars
« Reply #62 on: 11/18/2017 02:42 PM »
The problem comes in how to get the tankers back in some reasonable amount of time or if there is enough funds to expend them.

If you're expending them, you can also in principle remove most engines, for some tons of weight saving.
Tankers that don't need to launch full, and never need to land can get by with many fewer.

At that point you might also consider how much an absolutely minimal tanker that had one raptor, and was carried to orbit mostly empty on cargo could cost.

Fuel is cheap, but at some point the exponentials of trying to recover tankers going quite fast away from you means that the logistics get expensive.

I do wonder on the cost of a simple aluminium one-raptor tanker, with very little else. (it would of course have a compatible base-frame for propellant transfer)
Expendable looks somewhat different if the margins required to launch to orbit don't need to be there, and you have a rapidly reusable (though expensive in terms of capital) propellant transfer for those expendables.

Online rakaydos

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Re: Taking the BFR Beyond Mars
« Reply #63 on: 11/18/2017 05:57 PM »
The problem comes in how to get the tankers back in some reasonable amount of time or if there is enough funds to expend them.

If you're expending them, you can also in principle remove most engines, for some tons of weight saving.
Tankers that don't need to launch full, and never need to land can get by with many fewer.

At that point you might also consider how much an absolutely minimal tanker that had one raptor, and was carried to orbit mostly empty on cargo could cost.

Fuel is cheap, but at some point the exponentials of trying to recover tankers going quite fast away from you means that the logistics get expensive.

I do wonder on the cost of a simple aluminium one-raptor tanker, with very little else. (it would of course have a compatible base-frame for propellant transfer)
Expendable looks somewhat different if the margins required to launch to orbit don't need to be there, and you have a rapidly reusable (though expensive in terms of capital) propellant transfer for those expendables.
At that point, it's no longer a tanker. "Flyback Drop Tank" is what you're looking for.

Probably has some use in the future, but I would expect a clean sheet design for a cheap external tank. Possibly an inflatable around the outer rim that also protects the main tanks from heating the same way the main tanks protect the header tanks, and plugs into the refueling pipes withoutblocking the engines.

Offline qraal

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Re: Taking the BFR Beyond Mars
« Reply #64 on: 11/26/2017 08:06 PM »
There's a Russian propellant combination which I think is really interesting in a Titan context - acetam / atsetam.

A 30/70 mix of acetylene/ammonia, with O2 oxidiser. Titan has acetylene deposits on the surface and very likely has ammonia. Of course, there's lots of methane and it's possibly the easiest for initial ISRU - but acetam seems to get an Isp in the low 400s.

If the ice on Titan is a eutectic of water/ammonia, then there'll be ammonia produced by water-mining for oxygen. Of course we might need to use dry ice for oxygen, so really ground-truthing local resources will decide.

Offline KelvinZero

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Re: Taking the BFR Beyond Mars
« Reply #65 on: 11/30/2017 03:02 PM »
This mentions doing a "solar fry by" that could also be useful for a fast HSF mission within the solar system.
https://www.centauri-dreams.org/?p=38728

Im thinking perhaps you could use this just to accelerate some fuel depots. The crew BFS could catch the depots as they fly past the earth, leapfrogging from the slowest up to the fastest.

Offline qraal

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Re: Taking the BFR Beyond Mars
« Reply #66 on: 12/01/2017 09:15 AM »
Coming from so close to the Sun means the orbits are nearly radially outwards, thus significantly different to an orbit that launches cotangentially to Earth's orbit, which is needed to minimise delta-vee.

Plus the "Solar Fryby" (a phrase I coined BTW) needs a Jupiter Gravity Assist to get so close to the Sun.

This mentions doing a "solar fry by" that could also be useful for a fast HSF mission within the solar system.
https://www.centauri-dreams.org/?p=38728

Im thinking perhaps you could use this just to accelerate some fuel depots. The crew BFS could catch the depots as they fly past the earth, leapfrogging from the slowest up to the fastest.

Offline KelvinZero

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Re: Taking the BFR Beyond Mars
« Reply #67 on: 12/01/2017 09:50 AM »
Coming from so close to the Sun means the orbits are nearly radially outwards, thus significantly different to an orbit that launches cotangentially to Earth's orbit, which is needed to minimise delta-vee.

Plus the "Solar Fryby" (a phrase I coined BTW) needs a Jupiter Gravity Assist to get so close to the Sun.

This mentions doing a "solar fry by" that could also be useful for a fast HSF mission within the solar system.
https://www.centauri-dreams.org/?p=38728

Im thinking perhaps you could use this just to accelerate some fuel depots. The crew BFS could catch the depots as they fly past the earth, leapfrogging from the slowest up to the fastest.

Yes, as I said you would use this just for depots. It could take a decade to set up but the goal is to minimise the transit time for the crew. Once there is constant traffic there would be no waiting for setup of course.

The goal can't be just to minimise delta-v. Im hazy here but I think if you do that you will always have very long flight times on the order of the year of your target.. im guessing roughly a 1/3? eg eight months to mars. Think how long it would be for Saturn with an orbital period of almost 30 years. Im guessing around 10 years.

Of course each depot can only vary from the previous by about 10km/s, or whatever a single BFS can do, and the first can only vary from earth's current velocity by the same amount.

Offline Patchouli

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Re: Taking the BFR Beyond Mars
« Reply #68 on: 12/31/2017 11:14 PM »
There's a Russian propellant combination which I think is really interesting in a Titan context - acetam / atsetam.

A 30/70 mix of acetylene/ammonia, with O2 oxidiser. Titan has acetylene deposits on the surface and very likely has ammonia. Of course, there's lots of methane and it's possibly the easiest for initial ISRU - but acetam seems to get an Isp in the low 400s.

If the ice on Titan is a eutectic of water/ammonia, then there'll be ammonia produced by water-mining for oxygen. Of course we might need to use dry ice for oxygen, so really ground-truthing local resources will decide.
Nuclear thermal engines can be designed to use strait ammonia as propellant which would get an ISP of 470 to 550 seconds?

Water also could be used as a propellant but the ISP is terrible but it's very dense and can be mined from Enceladus as well.
« Last Edit: 12/31/2017 11:54 PM by Patchouli »

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