Author Topic: BFR Space Tug  (Read 3130 times)

Offline ZachF

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BFR Space Tug
« on: 02/26/2018 05:58 PM »
So I've been playing around with the idea of a multi-purpose space tug that can fit inside the BFR payload bay. Interestingly enough, a space tug that can fit enough fuel from a full tanker trip fits inside the payload bay.

-The Space Tug would use a modified Raptor engine with a slightly larger expansion ratio for a few seconds of ISP gains. (and perhaps de-rated for thrust)
-It would be sized to fit roughly a full tanker load of fuel (150 tonnes), and have "prongs" on the rear capable of mating with the BFR refueling system.
-Space tug would have fold-out solar panels.
-It would wait in space, meet with BFR/Ss coming up to orbit, take their payloads and excess fuel as well.
-With spherical tanks it could be pretty mass efficient, if it had the same mass efficiency of a Falcon 9 US it would weigh about 6 tonnes empty. It would have an empty dV of >12,000m/s, more than a hydrogen ACES stage.

By my calculations a BFR could deliver a 20 tonne payload to orbit, rendezvous with a space tug in orbit, give the tug it's remaining fuel, and deliver the payload direct to geostationary, and come back. It could also be very useful for cleaning up all the space junk, with regular tanker flights refueling it, going back and forth pushing dead satellites/stages into the atmosphere. It would also be useful for space probes, and if you had to you could expend them for very long distance probes, or just fling smaller ones and retro-burn.

I think it would be extremely handy in a number of circumstances, when you don't want to use an expensive full BFS.

Sketch to follow
« Last Edit: 02/26/2018 06:19 PM by ZachF »

Offline ZachF

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Re: BFR Space Tug
« Reply #1 on: 02/26/2018 06:04 PM »
Sketch. When delivering a tug it could flip around, take the excess fuel (since it's so lightweight it would be nearly full afterwards), and be ready to go.
« Last Edit: 02/26/2018 06:29 PM by ZachF »

Offline spacenut

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Re: BFR Space Tug
« Reply #2 on: 02/26/2018 06:06 PM »
I think it is a great idea.  Have you considered a large SEP tug that can run on lox? 

Offline ZachF

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Re: BFR Space Tug
« Reply #3 on: 02/26/2018 06:21 PM »
I think it is a great idea.  Have you considered a large SEP tug that can run on lox?

How well does EP work with lox?

Offline Exastro

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Re: BFR Space Tug
« Reply #4 on: 02/26/2018 06:25 PM »
I think it is a great idea.  Have you considered a large SEP tug that can run on lox?

How well does EP work with lox?

Gridded ion thusters are probably a no-go, but an ELF thruster has demonstrated decent performance in the lab running on gaseous O2.

Offline Lar

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Re: BFR Space Tug
« Reply #5 on: 02/26/2018 07:34 PM »
The tug needs the clamshell variant (unless shipped up kitted and then assembled in orbit which seems implausible) as it won't fit out a standard cargo hatch but a lot of us assume the clamshell is likely to be developed quite early.

Neat idea and one that seems likely to be done sooner than other specialty vehicles. Especially the "take on excess propellant" part...
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Offline ZachF

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Re: BFR Space Tug
« Reply #6 on: 02/26/2018 07:56 PM »
The tug needs the clamshell variant (unless shipped up kitted and then assembled in orbit which seems implausible) as it won't fit out a standard cargo hatch but a lot of us assume the clamshell is likely to be developed quite early.

Neat idea and one that seems likely to be done sooner than other specialty vehicles. Especially the "take on excess propellant" part...

Yes, it would need the full cargo bay variant to deliver it to space, but I think that will fly early if not first.

Once they are delivered to space they would spend most of their time up there... They would behave a lot like an ACES stage. When they need maintenance they could be slotted to go down with a BFR, it does not weigh much so it should easily be within downmass capabilities.

« Last Edit: 02/26/2018 07:57 PM by ZachF »

Offline docmordrid

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Re: BFR Space Tug
« Reply #7 on: 02/26/2018 08:12 PM »
We may have already seen a BFR tug
DM

Offline Lar

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Re: BFR Space Tug
« Reply #8 on: 02/26/2018 10:22 PM »
We may have already seen a BFR tug

Perhaps indeed. Missing the propellant fill/drain connections but that's artistic license....
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Online kdhilliard

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Re: BFR Space Tug
« Reply #9 on: 02/26/2018 11:31 PM »
We may have already seen a BFR tug
Maaaybeee, ... but that slide appears at 29:55 in the 2017 Making Life Multiplanetary IAC presentation, as Mr. Musk says:
Quote
The size of this being a 9 meter diameter vehicle is a huge enabler for new satellites. We can actually send something that is almost nine meters in diameter to orbit. So for example, if you want to do a new Hubble, you could send a mirror that has ten times the surface area of the current Hubble, as a single unit. Doesn't have to unfold or anything.

Sounds like it was meant to be an artistic rendering of a large diameter space telescope.

Offline Exastro

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Re: BFR Space Tug
« Reply #10 on: 02/27/2018 12:28 AM »
The tug needs the clamshell variant (unless shipped up kitted and then assembled in orbit which seems implausible) as it won't fit out a standard cargo hatch but a lot of us assume the clamshell is likely to be developed quite early.

Neat idea and one that seems likely to be done sooner than other specialty vehicles. Especially the "take on excess propellant" part...

Would it make sense to consider balloon tanks, in the sense that when empty they can be folded up and stored in a small fraction of the volume they can hold?

Offline Lar

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Re: BFR Space Tug
« Reply #11 on: 02/27/2018 01:26 AM »
The tug needs the clamshell variant (unless shipped up kitted and then assembled in orbit which seems implausible) as it won't fit out a standard cargo hatch but a lot of us assume the clamshell is likely to be developed quite early.

Neat idea and one that seems likely to be done sooner than other specialty vehicles. Especially the "take on excess propellant" part...

Would it make sense to consider balloon tanks, in the sense that when empty they can be folded up and stored in a small fraction of the volume they can hold?

IANARS but I'd worry about sloshing about while partially full. hard sided tanks also take some blows from slosh but maybe can withstand it better.

If the balloon is in a frame to make it more rigid maybe that defeats the space savings? Still, maybe?
"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk
"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

Online speedevil

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Re: BFR Space Tug
« Reply #12 on: 02/27/2018 01:54 AM »
IANARS but I'd worry about sloshing about while partially full. hard sided tanks also take some blows from slosh but maybe can withstand it better.

If the balloon is in a frame to make it more rigid maybe that defeats the space savings? Still, maybe?

For at least some uses, launching empty, and never ever hitting atmosphere again would be interesting - with thrust very low compared to mass, which would drop peak loads a lot, and reduce slosh.

Offline rakaydos

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Re: BFR Space Tug
« Reply #13 on: 02/28/2018 07:06 AM »
Have you run the "LLO-Lunar Landing and back to LLO" numbers for this tug yet? I suspect TWR will be a bigger concern than DV, when it comes to max payload.

Offline Hominans Kosmos

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Re: BFR Space Tug
« Reply #14 on: 03/10/2018 07:48 AM »
The tug needs the clamshell variant (unless shipped up kitted and then assembled in orbit which seems implausible) as it won't fit out a standard cargo hatch but a lot of us assume the clamshell is likely to be developed quite early.

Neat idea and one that seems likely to be done sooner than other specialty vehicles. Especially the "take on excess propellant" part...

Would it make sense to consider balloon tanks, in the sense that when empty they can be folded up and stored in a small fraction of the volume they can hold?

I've never heard of materials that are compatible with liquid oxygen and are elastic at room temperature. Bladder tanks work for substances at near-standard temperature and pressures. Not likely to keep structural integrity near oxygen freezing temperatures. Perhaps not even near methane boiling point (warmer).

When reading about rockets, and you read "balloon tank", the writer does not mean party balloons or bulk fluid container bags. They mean thin metal that requires internal pressure to hold up against external forces. This is not collapsible.
« Last Edit: 03/10/2018 07:52 AM by Hominans Kosmos »

Online speedevil

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Re: BFR Space Tug
« Reply #15 on: 03/10/2018 10:56 AM »
I've never heard of materials that are compatible with liquid oxygen and are elastic at room temperature. Bladder tanks work for substances at near-standard temperature and pressures. Not likely to keep structural integrity near oxygen freezing temperatures. Perhaps not even near methane boiling point (warmer).

When reading about rockets, and you read "balloon tank", the writer does not mean party balloons or bulk fluid container bags. They mean thin metal that requires internal pressure to hold up against external forces. This is not collapsible.

These peopleare developing under a NASA contract.

Quote
NASA Kennedy Space Center
Teamed with Technology Applications Inc., Thin Red Lines inflatable UHPV technology is investigated as powerful alternative to cryogenic COPVs. UHPVs extreme simplicity permits incorporation of a much broader spectrum of corrosive content (LOX, LH2, LCH4, etc.) compatible liner materials. For propellant depots UHPV can be packaged for launch, filled in-situ, and repeatedly depleted to almost zero residual.

Offline Ludus

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Re: BFR Space Tug
« Reply #16 on: 03/11/2018 01:11 AM »
I think ACES was supposed to hold 70 tons of propellant so this about 2X. How would performance compare?

Offline Ronsmytheiii

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Re: BFR Space Tug
« Reply #17 on: 03/11/2018 10:53 AM »
Yes, it would need the full cargo bay variant to deliver it to space, but I think that will fly early if not first.
Alternatively, you could launch an engine/avionics section with limited propellant on a flight with another payload (aka piggyback), then lift larger propellant tanks on another flight with excess space/mass lift. This was how NASA was going to do it with their shuttle tug, the Orbital Maneuvering Vehicle:

http://www.pmview.com/spaceodysseytwo/station/sld013.htm
And this is a good reminder that just because one of your fellow space enthusiasts occasionally voices doubts about the SpaceX schedule announcements or is cautious about believing SpaceX has licked a problem before actually seeing proof that's true, it doesn't mean they hate SpaceX.

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