Author Topic: Refueling BFB in Orbit  (Read 828 times)

Online sleepy-martian

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Refueling BFB in Orbit
« on: 04/24/2018 09:24 AM »
Hi there,

Has there been talks about the possibility to refuel BFB in Orbit? (from the videos it seems that in detaches only after reaching orbit).

Now this would obviously required a lot of tanker trips, but given the amount of thrust it generates I imagine you could do the mars trip way quicker, or even outside of the Transfer Windows, which would allow faster response times in case of an emergency (unexpected equipment breakdown and the likes).

Being able to bring stuff to, or evacuate people from Mars within a few months instead of in the worst case 2 years would definitely make the planet more accessible and alleviate some of the concerns of long isolation times.

BFB is has sea level engines though, which AFAIK can fire in vacuum at full thrust, but really shouldn't. So might the only possibility to pull this off be a 3 Stage BFR with the second stage having vacuum engines?

I could also image the BFR launching to LEO in the way we currently know it. And than another BFB bringing up the and mating the DS version of the booster.

Offline darkenfast

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Re: Refueling BFB in Orbit
« Reply #1 on: 04/24/2018 09:28 AM »
I'm pretty sure that the booster will get nowhere near orbital velocity.  The animations may have misled you.

Online speedevil

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Re: Refueling BFB in Orbit
« Reply #2 on: 04/24/2018 09:31 AM »
Hi there,

Has there been talks about the possibility to refuel BFB in Orbit? (from the videos it seems that in detaches only after reaching orbit).

It does not.
It detaches at somewhere around 3km/s, not the 8km/s or so to orbit.
Unfortunately, orbital mechanics isn't as simple as '3 times the thrust, 1/3 the delay'.
At optimal launch periods, it takes about 4.5km/s to get a hundred day transit.

At non-optimal launch periods, it needs about 300km/s. (cancel earths 30km/s velocity, accellerate to 5 times that velocity on a direct course, decellerate, and match velocity with Mars)
200km/s is well outside the possibilities with chemical.

I thought for a while about this a bit back, and couldn't come up with a reason to refuel the booster in orbit.
The booster in principle can get to orbit with no payload, it's just there is no point - once you're refuelling anyway.
(and of course, it can't get back from orbit, so you've thrown away the booster)
« Last Edit: 04/24/2018 09:34 AM by speedevil »

Online sleepy-martian

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Re: Refueling BFB in Orbit
« Reply #3 on: 04/24/2018 11:26 AM »
Okay thanks, that's  dead end then :)
« Last Edit: 04/24/2018 11:27 AM by sleepy-martian »

Offline rakaydos

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Re: Refueling BFB in Orbit
« Reply #4 on: 04/24/2018 03:00 PM »
Hi there,

Has there been talks about the possibility to refuel BFB in Orbit? (from the videos it seems that in detaches only after reaching orbit).

It does not.
It detaches at somewhere around 3km/s, not the 8km/s or so to orbit.
Unfortunately, orbital mechanics isn't as simple as '3 times the thrust, 1/3 the delay'.
At optimal launch periods, it takes about 4.5km/s to get a hundred day transit.

At non-optimal launch periods, it needs about 300km/s. (cancel earths 30km/s velocity, accellerate to 5 times that velocity on a direct course, decellerate, and match velocity with Mars)
200km/s is well outside the possibilities with chemical.

I thought for a while about this a bit back, and couldn't come up with a reason to refuel the booster in orbit.
The booster in principle can get to orbit with no payload, it's just there is no point - once you're refuelling anyway.
(and of course, it can't get back from orbit, so you've thrown away the booster)
In theory, a modified BFB may be useful for Titan ascent, or Venus Stratospher-to-orbit. Not something on the current mission docket, but there is a use for it.

Online niwax

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Re: Refueling BFB in Orbit
« Reply #5 on: 04/24/2018 05:43 PM »
I'm pretty sure that the booster will get nowhere near orbital velocity.  The animations may have misled you.

Sure the booster alone couldn't do SSTO? With an assumed 150t dry and 640t + 2300t of propellant it should be able to just like BFS alone. Getting a BFB to Mars could be interesting for a number of things too.

Online speedevil

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Re: Refueling BFB in Orbit
« Reply #6 on: 04/24/2018 05:51 PM »
I'm pretty sure that the booster will get nowhere near orbital velocity.  The animations may have misled you.

Sure the booster alone couldn't do SSTO? With an assumed 150t dry and 640t + 2300t of propellant it should be able to just like BFS alone. Getting a BFB to Mars could be interesting for a number of things too.

It will not normally do so, it could.
BFB to Mars is going to need a preprepared pad, but that's doable in principle?

Can you explain why it's interesting? I can't think of much.
The lower martian gravity well of 3.5km/s to LEO means BFS has around 500 tons of payload to LMO. This means you can top off a BFS in orbit with a couple of loads, getting 400 tons or so back to earth.

But, propellant is likely to remain cheaper on Earth than Mars, even though you lose much more of it getting up the gravity well, meaning there is little point.

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