Author Topic: BFS Design Requirements  (Read 18871 times)

Offline mlindner

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BFS Design Requirements
« on: 02/06/2018 01:56 AM »
A recording of the FH press conference of today has been posted:

https://soundcloud.com/geekwire/elon-musk-discusses-the-launch-and-flight-of-the-falcon-heavy-rocket

https://www.geekwire.com/2018/elon-musk-explains-spacexs-falcon-heavy-rocket-risky-revolutionary/

The recent recording on Falcon Heavy they discussed the design requirements for the BFS and that they'll be starting ground hops next year.

He also went over design requirements of the BFS.

1. Reuse boost stage, upper stage and fairing.
2. Launch every few hours.
3. Booster will land 10 minutes after lift off.
4. Heat shield capable of re-entering from interplanetary velocities.
5. Control itself through vacuum, rarified gas, thin atmosphere, thick atmosphere, hypersonic, supersonic, transsonic, subsonic in different types of atmosphere.
6. Land on unimproved terrain.
7. Take off on unimproved terrain.


Emphasis mine.
« Last Edit: 02/06/2018 01:56 AM by mlindner »
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Offline DigitalMan

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Re: BFS Design Requirements
« Reply #1 on: 02/06/2018 02:06 AM »
A recording of the FH press conference of today has been posted:

https://soundcloud.com/geekwire/elon-musk-discusses-the-launch-and-flight-of-the-falcon-heavy-rocket

https://www.geekwire.com/2018/elon-musk-explains-spacexs-falcon-heavy-rocket-risky-revolutionary/

The recent recording on Falcon Heavy they discussed the design requirements for the BFS and that they'll be starting ground hops next year.

He also went over design requirements of the BFS.

1. Reuse boost stage, upper stage and fairing.
2. Launch every few hours.
3. Booster will land 10 minutes after lift off.
4. Heat shield capable of re-entering from interplanetary velocities.
5. Control itself through vacuum, rarified gas, thin atmosphere, thick atmosphere, hypersonic, supersonic, transsonic, subsonic in different types of atmosphere.
6. Land on unimproved terrain.
7. Take off on unimproved terrain.


Emphasis mine.

Number 6 has been the most difficult for me to visualize a solution for.  There may always be a non zero chance the surface will destabilize either through heat or weight

Online meekGee

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Re: BFS Design Requirements
« Reply #2 on: 02/06/2018 03:30 AM »
A recording of the FH press conference of today has been posted:

https://soundcloud.com/geekwire/elon-musk-discusses-the-launch-and-flight-of-the-falcon-heavy-rocket

https://www.geekwire.com/2018/elon-musk-explains-spacexs-falcon-heavy-rocket-risky-revolutionary/

The recent recording on Falcon Heavy they discussed the design requirements for the BFS and that they'll be starting ground hops next year.

He also went over design requirements of the BFS.

1. Reuse boost stage, upper stage and fairing.
2. Launch every few hours.
3. Booster will land 10 minutes after lift off.
4. Heat shield capable of re-entering from interplanetary velocities.
5. Control itself through vacuum, rarified gas, thin atmosphere, thick atmosphere, hypersonic, supersonic, transsonic, subsonic in different types of atmosphere.
6. Land on unimproved terrain.
7. Take off on unimproved terrain.


Emphasis mine.

Number 6 has been the most difficult for me to visualize a solution for.  There may always be a non zero chance the surface will destabilize either through heat or weight

I think the surface is allowed to be carefully vetted and selected beforehand.

So a scout will find appropriate terrain, but won't have to pour a concrete pad.

Of course the first scout has to land without a scout, but an unmanned mission can do a fair bit of scouting too.

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Offline mlindner

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Re: BFS Design Requirements
« Reply #3 on: 02/06/2018 04:40 AM »
A recording of the FH press conference of today has been posted:

https://soundcloud.com/geekwire/elon-musk-discusses-the-launch-and-flight-of-the-falcon-heavy-rocket

https://www.geekwire.com/2018/elon-musk-explains-spacexs-falcon-heavy-rocket-risky-revolutionary/

The recent recording on Falcon Heavy they discussed the design requirements for the BFS and that they'll be starting ground hops next year.

He also went over design requirements of the BFS.

1. Reuse boost stage, upper stage and fairing.
2. Launch every few hours.
3. Booster will land 10 minutes after lift off.
4. Heat shield capable of re-entering from interplanetary velocities.
5. Control itself through vacuum, rarified gas, thin atmosphere, thick atmosphere, hypersonic, supersonic, transsonic, subsonic in different types of atmosphere.
6. Land on unimproved terrain.
7. Take off on unimproved terrain.


Emphasis mine.

Number 6 has been the most difficult for me to visualize a solution for.  There may always be a non zero chance the surface will destabilize either through heat or weight

Number 7 I have issue with. Most anywhere you will be taking off on the same piece of terrain you landed on as nothing will be able to move it except on Earth. For Number 6 your thrust is greatly reduced as you have a mostly empty vehicle. For Number 7 you are using the full thrust of the vehicle on a dirt surface.
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Offline biosehnsucht

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Re: BFS Design Requirements
« Reply #4 on: 02/06/2018 06:09 AM »
Since it's not an air-breathing jet engine, number 7 seems easier assuming no damaged occurred during number 6 and you can ignite the engines close enough together that the force of the engines prevents anything from kicking up into them (and the rest of the vehicle base is sufficiently "armored" that dust/rocks bouncing off don't matter, nothing getting stuck in crevices that must move, etc). Number 6 and the seconds following completion of number six seem like a more difficult problem to solve ... as that seems like a more likely time to damage the engines themselves.

Offline jebbo

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Re: BFS Design Requirements
« Reply #5 on: 02/06/2018 06:34 AM »
1. Reuse boost stage, upper stage and fairing.

Fairing?  Doesn't this suggest the BFS design might have changed somewhat over the last year?

--- Tony

Offline Cheapchips

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Re: BFS Design Requirements
« Reply #6 on: 02/06/2018 06:59 AM »

More likely shorthand for having integrated the upper stage and fairing in the BFS design. 

Offline jebbo

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Re: BFS Design Requirements
« Reply #7 on: 02/06/2018 08:13 AM »
Probably.. I was thinking more about a tanker variant where there might be a bigger advantage if the tanker payload is really stripped down and always one way.

But time will tell, I guess ;)

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Offline M.E.T.

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Re: BFS Design Requirements
« Reply #8 on: 02/06/2018 09:46 AM »
Surely landing on "unprepared terrain" is most easily achieved by having the legs come out of the belly, and having it land like a Harrier jet. Thus, no chance of toppling over and exploding.

Of course, the engineering required  to make that possible probably makes the vehicle impractical in many other aspects.

Offline speedevil

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Re: BFS Design Requirements
« Reply #9 on: 02/06/2018 10:34 AM »
Surely landing on "unprepared terrain" is most easily achieved by having the legs come out of the belly, and having it land like a Harrier jet. Thus, no chance of toppling over and exploding.

The issue isn't solely toppling over and exploding, but excavating so much material that you can't take off again without foreign object damage to the engines or craft.

I have been unable to find any research into landing on unimproved surfaces other than the moon, which differs somewhat.


I note - this is a ~sonic jet not even impinging directly on tarmac tearing bits off.
(It's not very good tarmac).

Saying 'the engine thrust will keep the debris away' only sort-of-works for dust.
For large rocks - >30cm or so, if they attain even a modest velocity, the engine blast won't sufficiently deflect them to make them miss the engines at more than a few meters a second.
If they can actually make this work, there are great synergies with the boring company.


Online Robotbeat

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Re: BFS Design Requirements
« Reply #10 on: 02/06/2018 12:21 PM »
1. Reuse boost stage, upper stage and fairing.

Fairing?  Doesn't this suggest the BFS design might have changed somewhat over the last year?

--- Tony
I believe he was talking about F9 and FH at that point.

Do yourself a favor and listen to it yourself. I've seen a bunch of misunderstandings because people are going off of someone else's summary.
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Offline jebbo

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Re: BFS Design Requirements
« Reply #11 on: 02/06/2018 01:03 PM »
Do yourself a favor and listen to it yourself. I've seen a bunch of misunderstandings because people are going off of someone else's summary.

I plan to, just can't while I'm still at work  ;D

--- Tony

Offline mlindner

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Re: BFS Design Requirements
« Reply #12 on: 02/06/2018 01:07 PM »
1. Reuse boost stage, upper stage and fairing.

Fairing?  Doesn't this suggest the BFS design might have changed somewhat over the last year?

--- Tony
I believe he was talking about F9 and FH at that point.

Do yourself a favor and listen to it yourself. I've seen a bunch of misunderstandings because people are going off of someone else's summary.

No this is about the BFS. This is info from the recording.
Internal combustion engine in space. It's just a Bad Idea.TM - Robotbeat

Offline tdperk

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Re: BFS Design Requirements
« Reply #13 on: 02/06/2018 01:20 PM »
I note - this is a ~sonic jet not even impinging directly on tarmac tearing bits off.
(It's not very good tarmac).

Good grief.  Is he taking off or plowing it?

Offline AC in NC

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Re: BFS Design Requirements
« Reply #14 on: 02/06/2018 01:47 PM »
Since it's not an air-breathing jet engine, number 7 seems easier assuming no damaged occurred during number 6 and you can ignite the engines close enough together that the force of the engines prevents anything from kicking up into them (and the rest of the vehicle base is sufficiently "armored" that dust/rocks bouncing off don't matter, nothing getting stuck in crevices that must move, etc). Number 6 and the seconds following completion of number six seem like a more difficult problem to solve ... as that seems like a more likely time to damage the engines themselves.

That's kind of where my head was at.  I wasn't worried as landing should have generally "scrubbed" the landing zone, and much of that scrub can have occurred before the legs come into play.  Except those legs at takeoff are going to be going through hell with a each Raptor 6-7x the thrust of the highest thrust 747 engines.


Offline speedevil

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Re: BFS Design Requirements
« Reply #15 on: 02/06/2018 01:51 PM »
That's kind of where my head was at.  I wasn't worried as landing should have generally "scrubbed" the landing zone,

This assumes the landing zone is a nice hard surface, with a scattering of rocks on it.
This works just fine then.
If you get even pockets of deeper excavation, you can end up with very large chunks becoming airborne.

Put some coffee granules or something in a cup - blow hard in a jet into the cup, and pretty much all of them come out.

Offline AC in NC

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Re: BFS Design Requirements
« Reply #16 on: 02/06/2018 02:22 PM »
That's kind of where my head was at.  I wasn't worried as landing should have generally "scrubbed" the landing zone,

This assumes the landing zone is a nice hard surface, with a scattering of rocks on it.
This works just fine then.
If you get even pockets of deeper excavation, you can end up with very large chunks becoming airborne.

Put some coffee granules or something in a cup - blow hard in a jet into the cup, and pretty much all of them come out.

Fair point.  Continuing the analogy just for discussion ...

this however seems more like a shot glass and a leaf blower rather than a cup and a jet (ie: the thrust "shield" is likely to be much larger than the excavation).  I'm just a layman thinking this through so don't have any special insight but I do see the liklihood of such a scenario getting big chunks moving.  But I'm not sure I can see those able to fight through the "thrust shield" (for lack of a better term) to damage the main body of the BFS or engines.  Legs though seem clearly vulnerable.

Offline BeamRider

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Re: BFS Design Requirements
« Reply #17 on: 02/06/2018 02:46 PM »
With the lower gravity of Mars, and the demonstrated (by then) ability to do “short hops”, perhaps with partial fuel load, does BFS have the option of landing on rough/loose dirt with low fuel load, and then hopping to a semi-prepared surface for takeoff? Might the reduced thrust required for a “light” landing and a hop mitigate the FOD problem quite a bit?

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Offline Brad_C

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Re: BFS Design Requirements
« Reply #18 on: 02/07/2018 05:19 PM »
8.  Add some landing engine ignition redundancy so that what happened to the Falcon Heavy core in the inaugural flight test doesn't happen to BFS upon landing. 

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Offline speedevil

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Re: BFS Design Requirements
« Reply #19 on: 02/07/2018 05:22 PM »
8.  Add some landing engine ignition redundancy so that what happened to the Falcon Heavy core in the inaugural flight test doesn't happen to BFS upon landing.
F9/H uses TEA/TEB hypergolic ignitors that have a finite amount of fuel.
Raptor, from memory uses spark ignitors, which do not have a meaningfully limited life.

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