Author Topic: Refueling BFB in Orbit  (Read 4497 times)

Offline 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.

Offline 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 »

Offline 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.

Offline 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.

Offline Ludus

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Re: Refueling BFB in Orbit
« Reply #7 on: 04/26/2018 05:07 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.

There has been discussion here on NSF (not AFAIK by SpaceX) about the idea of using a BFS (cargo/Tanker) as a sort of deep space booster for BFS. This is basically just adapting a Tanker so it can mate nose to tail with a BFS and using other Tankers to fill both it and the BFS with propellant. The Tanker Booster would end up looping back earth and the BFS would have significant new performance options. This seems in line with what you had in mind.

If this was going to be done repeatedly, the Tanker might be modified to optimize for the role as booster and left in orbit.
« Last Edit: 04/26/2018 05:11 AM by Ludus »

Offline Zed_Noir

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Re: Refueling BFB in Orbit
« Reply #8 on: 04/27/2018 09:10 AM »
Kind of pointless to fly a BFB to Mars.

However for outer system missions you could build a 12 meter diameter deep space stage with tankage for about 3500 tonnes of propellants with a single Raptor Vac. Boosted up to LEO partially empty with a BFB and the stage's header tanks to waiting tankers in orbit. Load the propellants from the tankers and attached the mission vehicle stack in orbit from a BFS.

The Deep Space stage is basically a simplified half size ITS booster core with some sort of fairing or nosecone and the tail section of a BFS.

Online niwax

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Re: Refueling BFB in Orbit
« Reply #9 on: 04/27/2018 10:32 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.

There has been discussion here on NSF (not AFAIK by SpaceX) about the idea of using a BFS (cargo/Tanker) as a sort of deep space booster for BFS. This is basically just adapting a Tanker so it can mate nose to tail with a BFS and using other Tankers to fill both it and the BFS with propellant. The Tanker Booster would end up looping back earth and the BFS would have significant new performance options. This seems in line with what you had in mind.

If this was going to be done repeatedly, the Tanker might be modified to optimize for the role as booster and left in orbit.

Do two BFS stages give that much more dv? I calculated a full BFB giving a 3000m/s boost to a 1335t BFS and having 3250m/s left to return. For a BFS as a push stage the margins are much tighter.

Offline Ludus

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Re: Refueling BFB in Orbit
« Reply #10 on: 04/28/2018 04:53 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.

There has been discussion here on NSF (not AFAIK by SpaceX) about the idea of using a BFS (cargo/Tanker) as a sort of deep space booster for BFS. This is basically just adapting a Tanker so it can mate nose to tail with a BFS and using other Tankers to fill both it and the BFS with propellant. The Tanker Booster would end up looping back earth and the BFS would have significant new performance options. This seems in line with what you had in mind.

If this was going to be done repeatedly, the Tanker might be modified to optimize for the role as booster and left in orbit.

Do two BFS stages give that much more dv? I calculated a full BFB giving a 3000m/s boost to a 1335t BFS and having 3250m/s left to return. For a BFS as a push stage the margins are much tighter.

I suppose that depends on where you want the booster BFS to end up. It would be in some very eccentric earth orbit returning on its own. Any Dv applied after the boost would be to adjust that orbit.

Online niwax

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Re: Refueling BFB in Orbit
« Reply #11 on: 04/28/2018 08:24 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.

There has been discussion here on NSF (not AFAIK by SpaceX) about the idea of using a BFS (cargo/Tanker) as a sort of deep space booster for BFS. This is basically just adapting a Tanker so it can mate nose to tail with a BFS and using other Tankers to fill both it and the BFS with propellant. The Tanker Booster would end up looping back earth and the BFS would have significant new performance options. This seems in line with what you had in mind.

If this was going to be done repeatedly, the Tanker might be modified to optimize for the role as booster and left in orbit.

Do two BFS stages give that much more dv? I calculated a full BFB giving a 3000m/s boost to a 1335t BFS and having 3250m/s left to return. For a BFS as a push stage the margins are much tighter.

I suppose that depends on where you want the booster BFS to end up. It would be in some very eccentric earth orbit returning on its own. Any Dv applied after the boost would be to adjust that orbit.

I guess you could do a LEO -> HEO transfer and return using aerobraking. But is that any different from transferring to HEO using onboard power and then refueling? The thing that needs a really big booster is breaking earth orbit and returning.

Offline CorvusCorax

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Re: Refueling BFB in Orbit
« Reply #12 on: 06/13/2018 05:08 AM »
To simplify the entire discussion, look at the following:

To get from surface to orbit you need a big, freakish rocket. Not only do you need to accelerate to 9 km/s ish, but you need to have a thrust to weight ratio larger than one right from the get go, or you get nowhere.

Thus, thrust to weight ratio is everything in a booster. That's what its optimized for. ISP matters... not so much.


For in orbit manouvers thrust to weight is negligible. Extreme case: electrical propulsion. All that matters is Specific impulse.

The BFS already has vacuum optimized high ISP engines, the BFBs sea level raptors would be nothing but dead weight, firing them would get u less far than treating them as ballast and using the BFB as just a large external tank for BFS.

But if all you need is a large tank in orbit, make a dedicated one. It would likely be based on tanker-BFS.

Unlike the regular tanker, which brings fuel from the surface to orbit, this orbital fuel depot never needs to reenter. So you can strip the sea level raptors, and maybe only gice it one single vacuum raptor. That shoild be enough to get it to orbit initially - and empty.

In orbit it would be fuelled by regular tankers until.full.


Then u would attach it to a BFS in tandem combo, like the shuttles external tank, or if thats too complicated just accelerate in formation to the crewed BFS, then after the initial  burn, transfer the fuel to the main BFS to replenish its tanks to maximum. Maybe in a very high earth orbit or slow escape trajectory.

Then the BFS does the rest.of its burn.

The almost empty fuel tank would then decelerate just enough to remain in a HEO. It could lower its Periapsis to aerobrake its Apoapsis back to LEO then be refueled again.

And if you need even more fuel use multiple of them, like a tanker fleet, just like the airforce has been doing.

Example: 3 Tankers and.one BFS accelerate toA highly elyptic almost escape earth orbit. One tanker refuels the other tanker, the third refuels the BFS. The two empty tankers return to LEO with aerobraking. The BFS and the tanker burn on to a transjovian orbit. The tanker once more refuels the ship, then aerobrakes into orbit around a jobian moon. The BFS uses the fuel to do whatever. Like land on a Jupiter moon. Mybe the tanker has enough fuel left to refill the BFS for its journey back home :)

At the end you'd have one deep space tanker stranded in orbit around a jupiter moon. I thinks thats an acceptable expense for that kind of mission, maybe send it to hibernate until the in situ propellant plant on Europa gets on-line a few years later :)


Online hkultala

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Re: Refueling BFB in Orbit
« Reply #13 on: 06/13/2018 05:36 AM »

But if all you need is a large tank in orbit, make a dedicated one. It would likely be based on tanker-BFS.

Unlike the regular tanker, which brings fuel from the surface to orbit, this orbital fuel depot never needs to reenter. So you can strip the sea level raptors, and maybe only gice it one single vacuum raptor.

That shoild be enough to get it to orbit initially - and empty.

In orbit it would be fuelled by regular tankers until.full.

Makes absolutely no sense to launch empty fuel tank to orbit. It's always better to launch as full.

And there is no much point in modified BFS which cannot re-enter.

Quote
Then u would attach it to a BFS in tandem combo, like the shuttles external tank, or if thats too complicated just accelerate in formation to the crewed BFS, then after the initial  burn, transfer the fuel to the main BFS to replenish its tanks to maximum. Maybe in a very high earth orbit or slow escape trajectory.

Then the BFS does the rest.of its burn.

Better to use ordinary BFS-tanker for this.

Quote

The almost empty fuel tank would then decelerate just enough to remain in a HEO. It could lower its Periapsis to aerobrake its Apoapsis back to LEO then be refueled again.


.. and it could never reach earth surface for maintainance.

Better to have atmospheric-capable BFS-tanker which can be brought back to earth for maintainance.

Offline speedevil

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Re: Refueling BFB in Orbit
« Reply #14 on: 06/13/2018 09:07 AM »
.. and it could never reach earth surface for maintainance.

Better to have atmospheric-capable BFS-tanker which can be brought back to earth for maintainance.
To be annoying, place full BFS on top of full BFR, and at 150km altitude, do a 1km/s burn, transfer fuel from BFS, BFS seperates and lands as normal, though at slightly lower entry speed.

BFR does a 5km/s or so retroburn, followed by an aerodynamic phase, then another retroburn and later landing.

This requires no greater delta-v than launch.

(But is still silly for the reasons mentioned upthread).

Offline CorvusCorax

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Re: Refueling BFB in Orbit
« Reply #15 on: 06/13/2018 02:13 PM »

Better to use ordinary BFS-tanker for this.


From a performance point of view, incorrect. It makes sense only because using an"ordinary" BFST (Big falcon spacetanker) avoids the complexity of needing yet another version, which for SpaceX is a biggy.

The fact that you can surface land it is a nice bonus, but its also possible to do maintenance in orbit, so that is by no means a neccesity Once the ship needs "dry dock" to repair u might as well scrap it and build a new one. ( remove the engines and utilities/avionics, store them.on a spacestation. Reenter the crippled empty hull)

The downside is that you will always drag along dead weight such as sealevel engines. For the suggested mission where the tanker stays in Jupiter orbit, this really is a waste.

But hey, if you have orbital maintenance facilities, you could also launch a BFS with sea level engines, then remove them for a deep space mission ;)

Offline biosehnsucht

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Re: Refueling BFB in Orbit
« Reply #16 on: 06/13/2018 07:08 PM »
Seems more likely we'll see a combination tug / depot model that launches much like BFTanker but is shaped differently (straight section longer, more sharp curve at top) which jettisons the (now smaller) nose section and reveals a second set of fuel exchange / docking hardware, so that you can both modularly create fuel depots and also reusable (but can never leave space) tug / booster modules.

You could basically insert a 2nd (or more) in orbit, making the BFS itself a 3rd (or higher) stage after the fact.

Online hkultala

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Re: Refueling BFB in Orbit
« Reply #17 on: 06/15/2018 08:48 PM »

Better to use ordinary BFS-tanker for this.


From a performance point of view, incorrect. It makes sense only because using an"ordinary" BFST (Big falcon spacetanker) avoids the complexity of needing yet another version, which for SpaceX is a biggy.

No, not only because of that. It makes sense because it's the only way to get that BFS maintained with a reasonable cost and without huge and expensive infrastructure that does not exist.

Quote
The fact that you can surface land it is a nice bonus, but its also possible to do maintenance in orbit, so that is by no means a neccesity Once the ship needs "dry dock" to repair u might as well scrap it and build a new one. ( remove the engines and utilities/avionics, store them.on a spacestation. Reenter the crippled empty hull)

In-orbit maintainance is VERY EXPENSIVE and requires huge and expensive facilities that simply do not exist.

The reaction engines guys made a study how to use skylon for mars mission. They ended up needing a huge station where they can build their mars craft from pieces which skylon can carry to orbit.

Quote
The downside is that you will always drag along dead weight such as sealevel engines. For the suggested mission where the tanker stays in Jupiter orbit, this really is a waste.

The weight of these engines is just a couple of tonnes. Not a big waste.

Quote

But hey, if you have orbital maintenance facilities, you could also launch a BFS with sea level engines, then remove them for a deep space mission ;)

Also, if you have magic unicorn rocket engine with isp of 10000s and T/W of 1000 you could also do many interesting things.
« Last Edit: 06/15/2018 08:48 PM by hkultala »

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