Author Topic: Taking the BFR Beyond Mars  (Read 17226 times)

Offline Norm38

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Re: Taking the BFR Beyond Mars
« Reply #20 on: 10/29/2017 03:46 AM »
Power for the ship is a real problem. Solar power is impossible for a crewed ship past Mars. RTG isn't going to cut it either. This isn't a probe that can run on 200W by trickle charging a battery for one daily pulse of activity.
Actual fission reactors are needed (discounting a fusion breakthrough). And those aren't coming anytime soon.

Offline Robotbeat

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Re: Taking the BFR Beyond Mars
« Reply #21 on: 10/29/2017 04:28 AM »
Power for the ship is a real problem. Solar power is impossible for a crewed ship past Mars. RTG isn't going to cut it either. This isn't a probe that can run on 200W by trickle charging a battery for one daily pulse of activity.
Actual fission reactors are needed (discounting a fusion breakthrough). And those aren't coming anytime soon.
Man, I disagree with almost EVERY sentence here.

Solar power isn't "impossible" beyond Mars. Even at Jupiter, you're only off the power produced at Mars by a factor of 11. And for Ceres, it's just a factor of 3 different from Mars. So a ship designed for, say, 200 people at Mars (in a pinch) could handle like 18 at Jupiter and still a healthy 65 or so people at Ceres. Those are huge crew sizes for typical NASA deep space plans, so power for crew is not going to be the ultimate limiting factor.

RTG would work, too. You'd need a lot of Pu238, but you can also use the cheaper Americium. A high performance 10 ton RTG/ASRG could produce like tens of kilowatts, plenty for a small crew. That much Americium would be hundreds of millions of dollars, but that's not too bad for a truly deep space mission.

Actual fission reactors are also not far away, either. NASA's Kilopower reactor tech is progressing fairly quickly. They did a small proof of concept test generating actual electricity using actual small-scale fission with Department of Energy help a couple years ago, and the prototype Kilopower reactor KRUSTY is pretty far along. Could easily be done within a decade if desired and fully funded.
« Last Edit: 10/29/2017 04:29 AM by Robotbeat »
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Offline Norm38

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Re: Taking the BFR Beyond Mars
« Reply #22 on: 10/29/2017 10:26 AM »
Has solar advanced that far?  Mars is about 1/2 Earth solar incidence. So it's factors of 6 and 22 for Ceres / Jupiter. Look at the ISS array size to keep 6 people alive.  A ship is not hauling 22 of those to Jupiter. What is it down to now? How is even a small integer practical?

1 ton RTG per 1-3kW sounds like a lot to me. Does that include the radiators? I'll go read up on Kilopower.

Offline tea monster

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Re: Taking the BFR Beyond Mars
« Reply #23 on: 10/29/2017 11:46 AM »
Once you add a decent sized reactor and cooling system, you pretty much have a deep-space tug. Chuck some 3X nested hall thrusters or a VASIMIR on that thing and you have a deep space vessel that can be used for all sorts of things like ferrying suites of probes to the outer planets - or pushing an ITS spacecraft out to the Jovian system. The ITS gets its power from the tug and the high-power ion engines can reduce trip time. The ITS can be used as a lander and a hab.

Offline DrRobin

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Re: Taking the BFR Beyond Mars
« Reply #24 on: 10/29/2017 12:06 PM »
I've analysed using this technique to fly to Titan (all on a single tank!) with a reasonable aerobrake entry speed of ~7.7 km/s, to land on the surface. It's the easiest Outer System target to reach, but takes ~3 years, so it's an endurance mission. Launching there in a Parabolic solar orbit takes 2.5 years, but means a re-entry speed of ~10.5 km/s. That *might* be doable with a Magnetoshell braking system.

If Titan is the easiest, let's talk about that! Certainly, it would worth to establish a research base there, if it possible. I guess, the return trip would be possible also on a single tank, after refueling on Titan surface. Could you calculate?

Thanks to qraal for starting this thread! I played around a good bit with these numbers after the first ITS reveal last year. My take home: Titan is "easy" to get to and -for multiple reasons- a good place for a sustainable colony, but is is harder to get back from, so maybe overall a good target for one-way settlement voyages. Ceres is hard to get to but easy to get back from so -also for multiple reasons- an attractive target for building as much a possible on site with local materials and selling tankers full of volatiles back to Earth. Callisto is in principle reasonable for two-way traffic _if_ you can aerobrake at Jupiter, given scary-looking reentry speeds and radiation. And as been mentioned before, all of them are a lot easier from Mars orbit that from LEO, making Mars a promising gateway to parts further out.

Offline RonM

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Re: Taking the BFR Beyond Mars
« Reply #25 on: 10/29/2017 01:07 PM »
Has solar advanced that far?  Mars is about 1/2 Earth solar incidence. So it's factors of 6 and 22 for Ceres / Jupiter. Look at the ISS array size to keep 6 people alive.  A ship is not hauling 22 of those to Jupiter. What is it down to now? How is even a small integer practical?

1 ton RTG per 1-3kW sounds like a lot to me. Does that include the radiators? I'll go read up on Kilopower.

Solar panel tech has greatly improved since the ISS panels were made and research continues. Look at the Juno mission at Jupiter. It's solar powered and the panels were built about a decade ago.

By the time anyone is ready to fly BFR to Jupiter or Ceres, solar panels will not be a concern.

Online guckyfan

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Re: Taking the BFR Beyond Mars
« Reply #26 on: 10/29/2017 01:13 PM »
By the time anyone is ready to fly BFR to Jupiter or Ceres, solar panels will not be a concern.

Maybe for the ECLSS for a crew of 10-20. But for the MW needed to produce propellant?

Offline RonM

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Re: Taking the BFR Beyond Mars
« Reply #27 on: 10/29/2017 01:32 PM »
By the time anyone is ready to fly BFR to Jupiter or Ceres, solar panels will not be a concern.

Maybe for the ECLSS for a crew of 10-20. But for the MW needed to produce propellant?

Just like Mars, it's going to take multiple ships to create an infrastructure before the first crewed mission. It will probably take a fission reactor for the propellent plant.

This isn't going to work with a single BFR flight.

Offline Ludus

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Re: Taking the BFR Beyond Mars
« Reply #28 on: 10/29/2017 01:42 PM »
To get to Callisto requires using two BFS Tankers which then transfer propellant to the BFS after all three boost into a Trans-Jupiter Insertion orbit. The Tankers can then return to Earth after a long loop.
...
Ceres, from Earth orbit, is a real pain. But it'd be pretty simple from Mars. Thus Mars will be the Gateway to Ceres and the Asteroids.
A pet idea of mine is to use a full BFS Tanker as a departure stage to push the ship on it's way. The trick is that the trajectory could skim past earth, boosting just before closest approach for maximum Oberth effect, and also to allow the now empty tanker to be immediately recaptured at earth.

Why is Ceres a real pain from Earth but pretty simple from Mars? (honest layman's question)
That is one hell of an idea...

If BFS can hold propellant for long durations (Musk mentioned cryo coolers) then you can have all the inherent dV for braking and return.

That's very powerful.

If you were using a filled Tanker as a departure stage, how would you do it? What would be the minimum mod to the standard BFS? Lock them heat shield to heat shield for the boost?

Offline philw1776

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Re: Taking the BFR Beyond Mars
« Reply #29 on: 10/29/2017 03:35 PM »
I believe that aerobraking a crewed ship at Jupiter leaves you with a soon to be dead crew after transiting Jupiter's intense radiation belts.  Some form of non-existant at this time magnetic field shield would be needed.
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Offline Robotbeat

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Re: Taking the BFR Beyond Mars
« Reply #30 on: 10/29/2017 03:42 PM »
I believe that aerobraking a crewed ship at Jupiter leaves you with a soon to be dead crew after transiting Jupiter's intense radiation belts.  Some form of non-existant at this time magnetic field shield would be needed.
Wrong.

You can use supplies, polyethylene, and water to block the radiation, which is lower energy than cosmic rays and closer to the easier-to-block solar particles.
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Offline DrRobin

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Re: Taking the BFR Beyond Mars
« Reply #31 on: 10/29/2017 03:46 PM »
By the time anyone is ready to fly BFR to Jupiter or Ceres, solar panels will not be a concern.

Maybe for the ECLSS for a crew of 10-20. But for the MW needed to produce propellant?

Just like Mars, it's going to take multiple ships to create an infrastructure before the first crewed mission. It will probably take a fission reactor for the propellent plant.

This isn't going to work with a single BFR flight.
It may well take a fission reactor to get propellant production started at a useful rate in a reasonable amount of time, but you'd also start by bringing solar panels from Earth, then bringing equipment to make solar panels locally, then equipment to make that equipment, etc., all with as much local material as feasible, so the rate of power (and propellant) production can ramp up (more than!) exponentially. For Ceres in particular, once you can fill up tankers there and send them out to refill spaceships "part way" to other destinations, a lot of beyond-Mars trips get a lot easier.

Offline DrRobin

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Re: Taking the BFR Beyond Mars
« Reply #32 on: 10/29/2017 04:08 PM »
Not beyond Mars but another interesting non-Lunar destination might be Earth's quasi-satellite 2016 HO3 (https://www.jpl.nasa.gov/news/news.php?feature=6537), the closest deep space (i.e. beyond Earth's Hill Radius) object in a (quasi) stable orbit. At what looks like a bit under 100 meters across, it's too small for much propellant production for BFR-scale vehicles but if it's on the order of a million kilograms of mass, it could provide raw materials enough to make a small deep space hotel, just a few million kilometers from home. I can't think of any compelling orbital mechanics reason to use it, but it would be the easiest past-the-Moon tourist destination.

Offline meekGee

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Re: Taking the BFR Beyond Mars
« Reply #33 on: 10/29/2017 04:23 PM »
To get to Callisto requires using two BFS Tankers which then transfer propellant to the BFS after all three boost into a Trans-Jupiter Insertion orbit. The Tankers can then return to Earth after a long loop.
...
Ceres, from Earth orbit, is a real pain. But it'd be pretty simple from Mars. Thus Mars will be the Gateway to Ceres and the Asteroids.
A pet idea of mine is to use a full BFS Tanker as a departure stage to push the ship on it's way. The trick is that the trajectory could skim past earth, boosting just before closest approach for maximum Oberth effect, and also to allow the now empty tanker to be immediately recaptured at earth.

Why is Ceres a real pain from Earth but pretty simple from Mars? (honest layman's question)
That is one hell of an idea...

If BFS can hold propellant for long durations (Musk mentioned cryo coolers) then you can have all the inherent dV for braking and return.

That's very powerful.

If you were using a filled Tanker as a departure stage, how would you do it? What would be the minimum mod to the standard BFS? Lock them heat shield to heat shield for the boost?
IMO I'd modify the tanker to be able to push.  Much lower acceleration than launch, everything still axial, and the BFS itself doesn't require any mods  so any mass penalty stays on Earth.
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Offline acsawdey

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Re: Taking the BFR Beyond Mars
« Reply #34 on: 10/29/2017 04:39 PM »
To get to Callisto requires using two BFS Tankers which then transfer propellant to the BFS after all three boost into a Trans-Jupiter Insertion orbit. The Tankers can then return to Earth after a long loop.
...
Ceres, from Earth orbit, is a real pain. But it'd be pretty simple from Mars. Thus Mars will be the Gateway to Ceres and the Asteroids.
A pet idea of mine is to use a full BFS Tanker as a departure stage to push the ship on it's way. The trick is that the trajectory could skim past earth, boosting just before closest approach for maximum Oberth effect, and also to allow the now empty tanker to be immediately recaptured at earth.

Why is Ceres a real pain from Earth but pretty simple from Mars? (honest layman's question)
That is one hell of an idea...

If BFS can hold propellant for long durations (Musk mentioned cryo coolers) then you can have all the inherent dV for braking and return.

That's very powerful.

If you were using a filled Tanker as a departure stage, how would you do it? What would be the minimum mod to the standard BFS? Lock them heat shield to heat shield for the boost?
IMO I'd modify the tanker to be able to push.  Much lower acceleration than launch, everything still axial, and the BFS itself doesn't require any mods  so any mass penalty stays on Earth.

One could imagine a structure that unfolds out of the cargo section of the tanker (assuming tanker is just BFS that doesn't carry cargo) and provides the same structural connections used to mate BFR to BFS, but in front of the ship. The structure has to carry the load down to the aft bulkhead of the cargo compartment which normally carries the cargo load. Might need to use only two Raptor vac engines at part throttle when boosting to keep the loads manageable. Normal load of 150 MT at 3g would mean you could push 1200 MT fuelled BFS at something like 0.4g. Or, maybe they build a tanker for boosting with heavier structure.

Offline meekGee

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Re: Taking the BFR Beyond Mars
« Reply #35 on: 10/29/2017 04:44 PM »
To get to Callisto requires using two BFS Tankers which then transfer propellant to the BFS after all three boost into a Trans-Jupiter Insertion orbit. The Tankers can then return to Earth after a long loop.
...
Ceres, from Earth orbit, is a real pain. But it'd be pretty simple from Mars. Thus Mars will be the Gateway to Ceres and the Asteroids.
A pet idea of mine is to use a full BFS Tanker as a departure stage to push the ship on it's way. The trick is that the trajectory could skim past earth, boosting just before closest approach for maximum Oberth effect, and also to allow the now empty tanker to be immediately recaptured at earth.

Why is Ceres a real pain from Earth but pretty simple from Mars? (honest layman's question)
That is one hell of an idea...

If BFS can hold propellant for long durations (Musk mentioned cryo coolers) then you can have all the inherent dV for braking and return.

That's very powerful.

If you were using a filled Tanker as a departure stage, how would you do it? What would be the minimum mod to the standard BFS? Lock them heat shield to heat shield for the boost?
IMO I'd modify the tanker to be able to push.  Much lower acceleration than launch, everything still axial, and the BFS itself doesn't require any mods  so any mass penalty stays on Earth.

One could imagine a structure that unfolds out of the cargo section of the tanker (assuming tanker is just BFS that doesn't carry cargo) and provides the same structural connections used to mate BFR to BFS, but in front of the ship. The structure has to carry the load down to the aft bulkhead of the cargo compartment which normally carries the cargo load. Might need to use only two Raptor vac engines at part throttle when boosting to keep the loads manageable. Normal load of 150 MT at 3g would mean you could push 1200 MT fuelled BFS at something like 0.4g. Or, maybe they build a tanker for boosting with heavier structure.
Might not need to unfold.

The tanker is a BFS without a cabin, and optimised structurally.

If there's a longitudinal member -basically an axial through tube - it'll transfer load straight through, and so the BFS can have a nose mounted pusher ring.

With that arrangement, you can pull off some insane dV flights.
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Offline Kaputnik

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Re: Taking the BFR Beyond Mars
« Reply #36 on: 10/29/2017 07:11 PM »
RTGs would work better if optimised for convection cooling so a bit of redesign required.

What would a viable power source look like for a Titan mission- ASRG?
At least the methane ISRU would be easy- it's even subcooled for you already :D

Do you mean 'better than ASRG'?
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Online KelvinZero

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Re: Taking the BFR Beyond Mars
« Reply #37 on: 10/31/2017 10:49 AM »
Ceres is awesome - it should be the very next target for manned missions after Mars, no question - either by SpaceX or NASA. And more probes, too for that matter. At least one rover and another with a good drill - somewhat like the one on Mars Insight.
A bit off topic, but I looked up some other possibles. Here are a few.

NameCeres24_Themis65_Cybele19_Fortuna
Size960km198km302×290×232225×205×195
Density2.16 g/cm³2.79 g/cm³0.99 g/cm³2.70 g/cm³
Orbit2.56-2.98AU2.7-3.5AU3-3.8AU2.0-2.8AU
Inclination10.6°0.76°3.6°1.573°
IceLotsCompletely covered
in surface ice
Ice on surface,
and density~1
Dunno, but similar
spectra to Ceres

Ceres is really really big, and has lots of ice. But these others are big enough.. so it is ok to look at other characteristics. 24_Themis has a much lower inclination for example. Im not sure, but the wording implied the ice might be more exposed on the surface also, perhaps.

(Im a bit puzzled why the density of 65_Cybele is so close to one.. possibly a misprint? I didn't google anyone talking about this odd coincidence)
« Last Edit: 10/31/2017 11:17 AM by KelvinZero »

Offline mikelepage

Re: Taking the BFR Beyond Mars
« Reply #38 on: 11/04/2017 11:23 AM »
The slide for using the BFS to land on the Moon and return has the BFS 'parked' in a HEEO while it tanks up. With a top delta-vee of ~6.4 km/s for the BFS, storing launch momentum in such an orbit allows a very effective Oberth Maneuver for Outer Planet missions.
(Snip)
Ceres, from Earth orbit, is a real pain. But it'd be pretty simple from Mars. Thus Mars will be the Gateway to Ceres and the Asteroids.

From LEO, perhaps. Way easier to go to Ceres/asteroids from EML1 than Mars though.

Offline DrRobin

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Re: Taking the BFR Beyond Mars
« Reply #39 on: 11/04/2017 04:12 PM »
The slide for using the BFS to land on the Moon and return has the BFS 'parked' in a HEEO while it tanks up. With a top delta-vee of ~6.4 km/s for the BFS, storing launch momentum in such an orbit allows a very effective Oberth Maneuver for Outer Planet missions.
(Snip)
Ceres, from Earth orbit, is a real pain. But it'd be pretty simple from Mars. Thus Mars will be the Gateway to Ceres and the Asteroids.

From LEO, perhaps. Way easier to go to Ceres/asteroids from EML1 than Mars though.

Maybe this goes without saying but I am assuming you mean easier from EML1 than Mars surface. I forget who did a nice post with delta-V calculations a ways back showing how Mars orbit is a very attractive staging point for destinations further out.

Also, just to re-iterate, the largest factor making Ceres a difficult target is not the orbital inclination but rather the absence of aerocapture, requiring a large delta-V at the destination. That changes once there is enough infrastructure on Ceres to fill tankers locally and send them out to meet incoming ships part way to supply propellant for the arrival delta-V.

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