Author Topic: Taking the BFR Beyond Mars  (Read 15878 times)

Offline qraal

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Taking the BFR Beyond Mars
« on: 10/28/2017 07:17 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.

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.

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. The BFS needs to aerocapture into Jupiter, then do a bi-elliptical transfer to Callisto. Shaving off ~1.5 km/s in the aerocapture from an entry speed of 60.7 km/s (it's a 500 day solar orbit, not a 1,000 day Hohmann) is the *hard* part. I feel it'll require some tricky tweaking of the TPS. Or a working Magnetoshell braking system. The BFS ends up in low Callisto orbit, so a pre-emplaced Tanker will be needed to land.

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.

Offline qraal

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Re: Taking the BFR Beyond Mars
« Reply #1 on: 10/28/2017 07:25 AM »
Note: I am assuming a *small* crew of 10 for these missions. If 100 people are provisioned for 180 days on a standard Mars payload, then 10 can be provisioned for 1,800 days. ISRU will be needed for all the missions to get back to Earth or survival, if it's a one-way "settlement" mission.

Callisto has water ice, dry ice and some sort of organics available in its surface material.

Titan has methane, ethane, acetylene (!!) and water ice. Lots of organicky crud with the texture of instant coffee in those dunes (or something like that.) Plus nitrogen. It also has super-rotation in its upper atmosphere, which means we could use stratospheric tethered floating wind-turbines for power. Fill them with either methane or the 0.1% hydrogen in the atmosphere.

Offline KelvinZero

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Re: Taking the BFR Beyond Mars
« Reply #2 on: 10/28/2017 10:39 AM »
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)

Offline MATTBLAK

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Re: Taking the BFR Beyond Mars
« Reply #3 on: 10/28/2017 11:01 AM »
Apparently; Ceres requires a fair plane-change maneuver to reach it that is fuel-intensive, depending on the method of propulsion. Dawn had no worries because of it's high-Isp ion drive. I'm not sure what it's difference to the Earth's solar system ecliptic position is, but I think it's more than 7 degrees, but less than 10.
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Offline KelvinZero

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Re: Taking the BFR Beyond Mars
« Reply #4 on: 10/28/2017 11:33 AM »
Apparently; Ceres requires a fair plane-change maneuver to reach it that is fuel-intensive, depending on the method of propulsion. Dawn had no worries because of it's high-Isp ion drive. I'm not sure what it's difference to the Earth's solar system ecliptic position is, but I think it's more than 7 degrees, but less than 10.
Yeah I have heard that Ceres is well off the plane of the ecliptic an this makes it very awkward.. just wondering why it was easier from Mars.

According to this there are about 200 asteroids greater than 100km in size.
https://en.wikipedia.org/wiki/Asteroid#Size_distribution

I think people assign far too much importance to Ceres. One of these 200 would surely be in a far more convenient orbit, depending what our goal is, and have a more convenient mix of ice and ore on the surface. >100km is plenty of material.

Offline Hauerg

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Re: Taking the BFR Beyond Mars
« Reply #5 on: 10/28/2017 11:41 AM »
I do not think that we assign too much importance to Ceres.
Looks like it had an OCEAN!

Offline MATTBLAK

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Re: Taking the BFR Beyond Mars
« Reply #6 on: 10/28/2017 12:17 PM »
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.
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Offline qraal

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Re: Taking the BFR Beyond Mars
« Reply #7 on: 10/28/2017 12:23 PM »
That's minor compared to the fact you need to propulsively brake into orbit around it. No aerobraking, no handy highly elliptical orbit momentum bank. From Mars, the delta-vee is lower overall. But the trip is a bit longer.

Apparently; Ceres requires a fair plane-change maneuver to reach it that is fuel-intensive, depending on the method of propulsion. Dawn had no worries because of it's high-Isp ion drive. I'm not sure what it's difference to the Earth's solar system ecliptic position is, but I think it's more than 7 degrees, but less than 10.

Offline Hauerg

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Re: Taking the BFR Beyond Mars
« Reply #8 on: 10/28/2017 12:30 PM »
10.6

Offline qraal

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Re: Taking the BFR Beyond Mars
« Reply #9 on: 10/28/2017 12:42 PM »
Can you unpack that a bit for me?

10.6

Offline geza

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Re: Taking the BFR Beyond Mars
« Reply #10 on: 10/28/2017 01:00 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?

Offline meekGee

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Re: Taking the BFR Beyond Mars
« Reply #11 on: 10/28/2017 02:20 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.

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Offline Kaputnik

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Re: Taking the BFR Beyond Mars
« Reply #12 on: 10/28/2017 10:12 PM »
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
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Offline qraal

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Re: Taking the BFR Beyond Mars
« Reply #13 on: 10/29/2017 12:12 AM »
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

Offline KelvinZero

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Re: Taking the BFR Beyond Mars
« Reply #14 on: 10/29/2017 12:28 AM »
I don't think you need to keep any landing fuel around for these tankers, right? They could aerobrake into earth orbit and be refueled before landing. It saves dragging that dead weight all around the solar system.

Offline Ludus

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Re: Taking the BFR Beyond Mars
« Reply #15 on: 10/29/2017 01:13 AM »
This also connects with BFR and science instruments thread. How much payload can a standard BFS cargo land on Titan?

How well does it work for Europa? No aerobraking so I suppose not well.

A cheap 50 ton robot Sub for the seas of Titan sounds pretty cool. Perhaps a variety of flying, swimming, crawling robots in the same payload.

Offline qraal

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Re: Taking the BFR Beyond Mars
« Reply #16 on: 10/29/2017 01:29 AM »
There's insufficient tank capacity for return in all the above cases. ISRU is needed.

In the case of Titan, the main need is oxidiser. Methane can be pumped straight out of the air, but there's nothing to burn it with. We're going to have to extract oxygen from either water or carbon dioxide. But how to minimise the needed oxygen?

Pondering the options led to a Google search and I found an old patent for a rocket engine using a mix of acetylene and ethane, with a smidge of oxygen, that might simplify things. That'd take development work on Titan, since acetylene isn't a forgiving compound to work with on Earth in pure form.

Acetylene + O2 gets an Isp of 415 seconds, so that's an encouraging start, but acetylene is touchy. Mixing it with N2 or CO would help stabilise it, but the N2 option would bring down the Isp quite a bit to a bit under 300 seconds. Acetylene + Carbon Monoxide as fuel gets an Isp of 350 seconds, which is quite exciting, so mining dry ice to crack into CO + O2 might mean local rocket propellant is straightforward to make, without the need for cracking water ice. The advantage over straight CH4+O2 is that the CO is used, rather than dumped, if CO2 cracking is how we're making O2.

Another option is acetylene/ammonia mix plus O2. Which the Russians have done some work on and got a decent discussion here back in 2013: Energomash Develops Revolutionary (?) Ammonia/Acetylene Rocket Engine ...but then the advantage against straight methalox isn't clear.

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.
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)

Offline qraal

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Re: Taking the BFR Beyond Mars
« Reply #17 on: 10/29/2017 01:36 AM »
I don't think robot subs mass 50 tons even on Earth. Remotely operated submersibles aren't over large and don't have to be as weighted down in 0.125 gee in a liquid half as dense as water. Going 200 metres down in methane/ethane probably means a pressure of less than 2 bar. To descend to 1,000 bar would need to penetrate the icy crust and go about ~80 km down. We're not likely to find access into the ice that deep, though we won't know until we go look ;-)

This also connects with BFR and science instruments thread. How much payload can a standard BFS cargo land on Titan?

How well does it work for Europa? No aerobraking so I suppose not well.

A cheap 50 ton robot Sub for the seas of Titan sounds pretty cool. Perhaps a variety of flying, swimming, crawling robots in the same payload.

Offline qraal

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Re: Taking the BFR Beyond Mars
« Reply #18 on: 10/29/2017 02:33 AM »
For the Trans Jupiter Insertion, quite right.

I don't think you need to keep any landing fuel around for these tankers, right? They could aerobrake into earth orbit and be refueled before landing. It saves dragging that dead weight all around the solar system.

Offline qraal

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Re: Taking the BFR Beyond Mars
« Reply #19 on: 10/29/2017 02:39 AM »
I assumed 150 tons, though most will be the life-support etc for the 10 crew. A cargo vehicle would need to be sent with it for setting up the base and ISRU. Personally I think the power should be sourced from in situ energy flows,
 rather than a reactor we've hauled across the solar system, though that will probably be needed initially. Wind power from high altitude seems more feasible on Titan than on Earth. Superconducting cables to floating wind-turbines seem perfectly suited to the job.

This also connects with BFR and science instruments thread. How much payload can a standard BFS cargo land on Titan?

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