Author Topic: BFS - one and a half stage silliness.  (Read 1972 times)

Offline speedevil

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BFS - one and a half stage silliness.
« on: 09/16/2018 12:34 PM »
In short: BFS can, even if not capable of SSTO with a payload get 40 tons or so to orbit with suborbital refuelling.

While we are waiting for the big reveal - I will flesh out an idea I've mentioned before.

I had intended for this post to have results of simulations and optimisations - this has not happened.

In order to simplify the numbers, the following assumptions are made:

1) BFS is capable of SSTO to LEO with 0 tons of payload and safely returning to the surface of earth.
This implies a number of things about the BFS that were explored more fully in that thread. Some changes in these directions seem to have been recently announced.
2) BFS is capable of landing safely and accurately +-1m.
3) BFS can transfer propellant at 100t/min to another similar vehicle, as is reasonable given the likely pressures in the tanks,
and flow in the pipework even without pumps. this is around the normal groundside fill rate of F9, proportionately. (see above
thread).
4) BFS can reenter fine at considerably lower velocity than nominal.
5) Enough ground support to launch two BFS is available, and one droneship.

In short, two BFS take off, on a slightly more lofted than normal trajectory, and at around LEO-2700m/s, with 100 tons of
propellant left each, match courses closely, and then cut engines.
They then proceed to transfer propellant so that one has a hundred tons extra, and the other only enough to land.
This naively gives the first BFS around 50 tons of extra propellant at LEO, or around 40 tons payload to LEO.

With effective gravity being around 3.5m/s^2 (inc RCS), although a somewhat more lofted trajectory allows you time - that time is not
 infinite, and is directly traded with making the impact of the tanker on the atmosphere harder.

At the nominal rate of 100 tons/minute, it will take around a minute to transfer the propellant. Add another minute for slow
approach and 10s for separation, and you’re at 450m/s delta-v required on the tanker, if you need to reduce its velocity all the
way to the nominal vertical interface velocity.

The orbital vehicle is not affected by this, as it can miss the atmosphere with minimal cosine losses with a slight change in
thrust vector.

If you need to reduce the tankers interface vertical velocity all the way to normal, that takes the effective payload of the orbital vehicle down to around 30 tons.

‘Impossible’ ?
Routine testing must have already developed the ability to hit a nonmoving surface accurately and on target, at a terminal deceleration of several G, and a final acceleration of 1G,  in the face of winds. It cannot have an error of more than several m/s.
F9S2 ignites the engine for boostback burns within 10 seconds.
The BFS RCS can do landing in 60mph winds, and RCS clusters have been mentioned as ‘10 ton’, ‘milligee’ several times,
which is consistent with a 0.5m/s^2 capability at 180 tons mass.

In more detail:
* ‘Park’ the BFS as you would for landing, 20m offset, pointing the opposite direction, at 1m/s together, and 5m apart in Z.
* At 0.5m/s^2 bring the vehicle axis into line.
* Mate two vehicles, slowing as you close.
* Apply ullage thrust.
* Dump pressurant partially out of orbital BFS, and use pressure to transfer 100 tons in a minute, around the same rate as it
   is done on the ground. (the pipe diameter and length is consistent with this being easily possible with no pumps and 2
   bar head).
* Undock and translate to clear the engines.
* One BFS ascends to orbit.
One lands on a drone ship.

The performance numbers for using this are similar to the SSTO BFS thread.

If we assume launches cost $5M for two(**), then the numbers break down something like:


Orbit    Delta-v  Capacity  Launch Cost to SpaceX
LEO      0            40        1    5
ISS    400            30        1    5
gto(*)1390            27        2    10
GTO   2500            17        3    15
TLI   3200            15        4    20
Mars(*)  ?            150       30   150


*)Typical SpaceX reusable ‘GTO’ launches with reuse, nominal BFS mission to Mars.
**) Less fuel, no need for stacking, is designed to operate from much closer to a flat pad than BFR, simplified operations.
The count goes up lots for higher orbits, and it might even be cheaper to use a disposable BFR than to do this for Mars.
Is two years delay waiting on BFR worth $150M?
You’re spending $150M anyway on leaving the BFS on Mars, and more on the payload.

Numbers in this post are approximate, and if the arbitrary LEO-2700m/s is not optimal, there would be some margin.
I have not detailed the landing fuel required, as that is included in the 'can get to orbit and return' and affects only the 'dry' mass.
I have been assuming the total mass is of the order of 85 tons - 85 vs 100 tons does not change the numbers much.

If you believe it won't work, please supply specific reasons that are ideally backed by numbers, and not simply assuming BFS can't work anyway, or that because this scheme is silly it can't work. I am quite aware that this is a silly idea, and unlikely to occur in practice.
« Last Edit: 09/16/2018 12:37 PM by speedevil »

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Re: BFS - one and a half stage silliness.
« Reply #1 on: 09/16/2018 01:58 PM »
This seems very high risk. Maybe a slight risk reduction if they launch bimese already coupled?
"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 Elmar Moelzer

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Re: BFS - one and a half stage silliness.
« Reply #2 on: 09/16/2018 02:22 PM »
This seems very high risk. Maybe a slight risk reduction if they launch bimese already coupled?
But don't they dock end to end for propellant transfer? And then it would effectively be two stages ;)
I don't think that any of this makes much sense since it only complicates things compared to just using the booster.
I think a better effort for them would be to just focus on improving the engines and the vehicle mass.

Offline rakaydos

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Re: BFS - one and a half stage silliness.
« Reply #3 on: 09/16/2018 02:31 PM »
This seems very high risk. Maybe a slight risk reduction if they launch bimese already coupled?

I'm not so sure. Judging by the complications of Falcon heavy,  it may be better to have both ships launch outside each other's shock comes, with active 2-way relative positioning the whole way up and only approaching for docking when atmospheric effects are neligable.

Offline speedevil

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Re: BFS - one and a half stage silliness.
« Reply #4 on: 09/16/2018 03:32 PM »
The thought of BFS-heavy did arise - and I neglected it for the reasons mentioned above.

The rapid rendevous can be tested incrementally. For example - toss out a beachball, and see if you can touch it with the nose.

While the outlined scenario had the transfer taking two minutes or so, this only matters much if you are actually trying to use it operationally.

Transferring ten tons at a lower delta-v to LEO, at the right rate, and taking five minutes to approach and mate and five minutes to undock lets you test everything much more gently and still have plenty of propellant to safely slow each craft before entry.

Much of this is stuff that has to work anyway.
If you can't not come to rest against a fixed known point at non-1G acceleration, you're gonna have a real bad time landing on the Moon or Mars.
If BFR has any hope of actually landing the way it's supposed to, it has to land with several hundred tons of thrust on a launch mount at 1G in wind.

This puts the bits together in a different way, but they're not really different bits.

Offline IRobot

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Re: BFS - one and a half stage silliness.
« Reply #5 on: 09/16/2018 03:52 PM »
This saves no money and creates unnecessary risks. I bet that if you further "refine" it, you will end up with a BFR + BFS.

Offline speedevil

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Re: BFS - one and a half stage silliness.
« Reply #6 on: 09/16/2018 09:15 PM »
This saves no money and creates unnecessary risks. I bet that if you further "refine" it, you will end up with a BFR + BFS.
It may mean you have capability a year or two earlier than waiting for the development of BFR, and may enable you to move a Mars launch up a synod.

Similarly, it may enable BFS to take over payload launches somewhat earlier.

If you already have BFR, there is little to no point in doing this.

Offline biosehnsucht

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Re: BFS - one and a half stage silliness.
« Reply #7 on: 09/17/2018 11:30 PM »
Surely, even a stubby BFR + regular BFS would be easier to develop than such suborbital gymnastics as refueling while suborbital.

Offline speedevil

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Re: BFS - one and a half stage silliness.
« Reply #8 on: 09/18/2018 12:38 AM »
Surely, even a stubby BFR + regular BFS would be easier to develop than such suborbital gymnastics as refueling while suborbital.

Gymnastics are largely a matter of timing.
Nearly all of the obviously hard parts can be tested very early in low suborbital on flights you'd be doing anyway for negligible amounts of extra delta-v - simply fly the precise maneuvers and record for next time to see how well you can (feeding the control system the appropriate inputs) match your prior flight as the 'other side'.
There  is nearly no cost to this - you want to ensure that you can cut off at a precise vector and exercise the RCS anyway.
Versus the obvious cost and delay of even a stubby BFR.

Offline ppb

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Re: BFS - one and a half stage silliness.
« Reply #9 on: 09/22/2018 07:12 PM »
The thought of "bimese" or if you will conjoined BFS's intrigues me. You've dismissed it out of shock interaction concerns, but aren't the early shuttle designs with glideback first stages basically this? Now if the two BFS lox and ch4 tanks are connected, the orbital BFS simply draws down the suborbital BFS's tanks and ends up on orbit with as much as fuel as it would with the tricky suborbital docking/fuel transfer maneuver. The suborbital BFS could boost back to the launch site or land on a downrange ASDS.  Aside from the shock worries, what are the show stoppers for this idea? Maybe the T/W is too low with big gravity losses, so maybe add a couple more Raptors? Eliminating the whole other BFB design is a big plus. The whole on orbit tanker scheme works early on because of the crossfed tanks ( although designing the plumbing and external connectors is a challenge). Payload to orbit is probably reduced, but the whole architecture and fabrication gets much simpler and they'll have full capability much sooner.
Quam celerrime ad astra

Offline ChrisWilson68

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Re: BFS - one and a half stage silliness.
« Reply #10 on: 09/22/2018 07:19 PM »
It may mean you have capability a year or two earlier than waiting for the development of BFR

It all depends on whether the BFR booster is earlier on the schedule than refueling.  I could easily imagine that development of refueling could be pushed out to after initial operational capability of BFR and BFS to put satellites in orbit.

I really think Starlink is going to be driving initial BFS/BFR development, and refueling isn't needed for that.  Cheap reuse is needed for Starlink.

Offline speedevil

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Re: BFS - one and a half stage silliness.
« Reply #11 on: 09/22/2018 07:23 PM »
It may mean you have capability a year or two earlier than waiting for the development of BFR

It all depends on whether the BFR booster is earlier on the schedule than refueling.  I could easily imagine that development of refueling could be pushed out to after initial operational capability of BFR and BFS to put satellites in orbit.

I really think Starlink is going to be driving initial BFS/BFR development, and refueling isn't needed for that.  Cheap reuse is needed for Starlink.

This also implies a second version of BFS with refueling capability.
This would be quite ambitious for Mars in 2022, as seems to be still  on schedule.

Also, as noted above, in principle, the above idea gets you able to launch Starlink before BFR.
« Last Edit: 09/22/2018 07:24 PM by speedevil »

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