Author Topic: BFS Design Requirements  (Read 18761 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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Online 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 ;)

--- Tony

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?

I am regularly cheered when I remember that we live at the bottom of a huge gravity well. Once we achieve the prodigious feat of climbing out of it and plopping down elsewhere “up top”, the rest of the Solar System is ours for the taking.

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.

Offline Norm38

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Re: BFS Design Requirements
« Reply #20 on: 02/07/2018 05:39 PM »
I posted this question in a thread that's now locked, so I'll ask again here.

They will need to test landing and takeoff from unimproved terrain.  Nothing beats a real test.  So couldn't they use Spaceport America for that?  Brownsville may work too, but the soil in the New Mexico desert is probably a better match to Mars.  I would expect that long before the BFS ever lands on Mars, it will have made many desert landings on many different soil types. Demonstrating resistance to debris, object ingestion and the ability of the legs to handle large rocks, etc.

Offline vaporcobra

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Re: BFS Design Requirements
« Reply #21 on: 02/07/2018 11:24 PM »
Something I haven't seen posted WRT to 6 and 7. Elon is CEO and Director of a certain company that almost certainly has the most mature and capable machine learning algorithms yet developed for navigation, terrain/object recognition, and minute trajectory management. Add in SpaceX's literally unprecedented experience with autonomously landing rockets, and it shouldn't be hugely difficult to implement AI, radar, and imaging that can prepare BFS' legs or safely choose a landing site during the late stages of reentry.

HiRISE imagery has basically blanketed Mars with 0.5m resolution coverage and will continue to do so until it dies, and that global imagery (stereoscopic in many cases) is almost without a doubt enough to roughly point to a landing spot small enough for onboard sensors to be able to autonomously choose a landing site in the last 10-30 seconds before touchdown.

The real issue with 6 and 7 is FOD avoidance and tolerance - that's gonna be hugely difficult to successfully integrate with goals of reliable and total reuse, as well as economically viable lifespans between serious refurb.

Offline the_other_Doug

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Re: BFS Design Requirements
« Reply #22 on: 02/08/2018 12:19 AM »
Something I haven't seen posted WRT to 6 and 7. Elon is CEO and Director of a certain company that almost certainly has the most mature and capable machine learning algorithms yet developed for navigation, terrain/object recognition, and minute trajectory management. Add in SpaceX's literally unprecedented experience with autonomously landing rockets, and it shouldn't be hugely difficult to implement AI, radar, and imaging that can prepare BFS' legs or safely choose a landing site during the late stages of reentry.

HiRISE imagery has basically blanketed Mars with 0.5m resolution coverage and will continue to do so until it dies, and that global imagery (stereoscopic in many cases) is almost without a doubt enough to roughly point to a landing spot small enough for onboard sensors to be able to autonomously choose a landing site in the last 10-30 seconds before touchdown.

The real issue with 6 and 7 is FOD avoidance and tolerance - that's gonna be hugely difficult to successfully integrate with goals of reliable and total reuse, as well as economically viable lifespans between serious refurb.

I will be honest -- I don't think nearly as much of Mars has been imaged at 50cm resolution as you seem to think.  HiRISE is a great instrument, but its highest resolution strips are fairly narrow, and Mars is still an entire planet.  With land area equal to Earth's -- just without the intervening oceans/seas.

There is likely 3- to 20-meter resolution photography covering 80% of the planet, and down to .5-meter resolution coverage on another 20%.  Enough to offer a lot of very high resolution looks at candidate landing sites, but not covering every portion of the planet.

Not even the Earth is mapped at .5-meter resolution to that large of an extent...
-Doug  (With my shield, not yet upon it)

Offline vaporcobra

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Re: BFS Design Requirements
« Reply #23 on: 02/08/2018 01:15 AM »
Something I haven't seen posted WRT to 6 and 7. Elon is CEO and Director of a certain company that almost certainly has the most mature and capable machine learning algorithms yet developed for navigation, terrain/object recognition, and minute trajectory management. Add in SpaceX's literally unprecedented experience with autonomously landing rockets, and it shouldn't be hugely difficult to implement AI, radar, and imaging that can prepare BFS' legs or safely choose a landing site during the late stages of reentry.

HiRISE imagery has basically blanketed Mars with 0.5m resolution coverage and will continue to do so until it dies, and that global imagery (stereoscopic in many cases) is almost without a doubt enough to roughly point to a landing spot small enough for onboard sensors to be able to autonomously choose a landing site in the last 10-30 seconds before touchdown.

The real issue with 6 and 7 is FOD avoidance and tolerance - that's gonna be hugely difficult to successfully integrate with goals of reliable and total reuse, as well as economically viable lifespans between serious refurb.

I will be honest -- I don't think nearly as much of Mars has been imaged at 50cm resolution as you seem to think.  HiRISE is a great instrument, but its highest resolution strips are fairly narrow, and Mars is still an entire planet.  With land area equal to Earth's -- just without the intervening oceans/seas.

There is likely 3- to 20-meter resolution photography covering 80% of the planet, and down to .5-meter resolution coverage on another 20%.  Enough to offer a lot of very high resolution looks at candidate landing sites, but not covering every portion of the planet.

Not even the Earth is mapped at .5-meter resolution to that large of an extent...

OOOF, I was very over-optimistic ;D It's actually probably closer to 3% of the surface imaged by HiRISE itself, with 85%+ imaged by the context camera (about 20m/px). Still, the point is that HiRISE can almost without a doubt be tasked by SpaceX, so long as it is alive. That is probably the first cooperative component NASA would likely be able to offer ahead of actual funding, and that is only necessary if that ~3% somehow doesn't include most of the viable first-base locations SpaceX would consider.

Once we get into the 2020s, I have little doubt that SpaceX will have a fair amount of experience with designing, building, launching, and operating their own satellites, and orbital imaging isn't a huge stretch beyond that point. At worst, SpaceX contracts out to the several highly experienced satellite manufacturers that exist today, and options like Planet's Terra Bella branch are also a possibility as they gain their own experience.

Getting a bit too deep in the weeds here, my main point is more simply that imagery and safe, autonomous landing and last-mile touchdown decisions are almost certainly not the biggest hurdles ahead of BFS.

Offline speedevil

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Re: BFS Design Requirements
« Reply #24 on: 02/08/2018 11:25 AM »
Getting a bit too deep in the weeds here, my main point is more simply that imagery and safe, autonomous landing and last-mile touchdown decisions are almost certainly not the biggest hurdles ahead of BFS.

Some of the remaining issues might be greatly helped by a FH ~2020 launch of various probes.
Beagle 2 mostly landed safely (it was not debris at least, and it seems likely if you had many of them some would have worked) with 60kg aeroshell from Mars.

Impactor probes which shed much or all of their final velocity in impact at 150-50m/s or so have been planned or flown, with masses in the single digit kilos. ( https://forum.nasaspaceflight.com/index.php?topic=44853 )

A 'low risk' mission involving a simple bus which does a mars flyby and picks up entry data from a few dozen dispersed probes that may or may not work would provide a _lot_ of data on a wide area.

A day before encounter, the bus manoevers by a few meters a second and drops out impactors/very simple landers designed to have half of them fail.
Some small relay sats that aerobrake into orbit, for atmospheric studies.

Almost all of it can fail and work to some degree independently, even if you're left paying for time on the various mars relay sats.

Offline guckyfan

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Re: BFS Design Requirements
« Reply #25 on: 02/08/2018 12:18 PM »
The first 2 scout BFS can have rovers that search for the best landing sites and remove some lose rubble, if needed. Then mark the site with radio beacons and radar reflectors for final descent. They would have to sit on Mars for 4 years before return is possible. I don't think they will ever return, so some damage to them on landing can be accepted.

Offline DecoLV

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Re: BFS Design Requirements
« Reply #26 on: 02/17/2018 09:58 PM »
I am a little surprised we have not heard about plans for a concept vehicle, especially for Mars EDL  This is what I thought Red Dragon would do this year. It could be an itty-bitty balsa wood thing (well, not literally, just making a point), but just if your little spam-covered vehicle can actually get there and land. That's been a hard enough challenge for NASA spacecraft as it is.


Offline speedevil

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Re: BFS Design Requirements
« Reply #27 on: 02/17/2018 10:02 PM »
I am a little surprised we have not heard about plans for a concept vehicle, especially for Mars EDL  This is what I thought Red Dragon would do this year.

What does your concept vehicle do that BFS does not, and why?
Red Dragon was cancelled due to the realisation that BFS could in principle be developed in 5 years from internal revenues, and Red Dragon wasn't going to help at all with that.

Offline vaporcobra

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Re: BFS Design Requirements
« Reply #28 on: 02/17/2018 10:30 PM »
I am a little surprised we have not heard about plans for a concept vehicle, especially for Mars EDL  This is what I thought Red Dragon would do this year. It could be an itty-bitty balsa wood thing (well, not literally, just making a point), but just if your little spam-covered vehicle can actually get there and land. That's been a hard enough challenge for NASA spacecraft as it is.

As I understand it, Crew Dragon will give SpaceX the knowledge necessary to develop orbital, crew-rated spacecraft with long in-orbit or deep space longevity. By using Falcon 9 to test supersonic retropropulsion at points in Earth's atmosphere that are Mars atmosphere facsimiles, as well as countless experience recovering and reusing orbital Dragons, I think SpaceX believes they properly understand the flight regimes BFS will need to survive.

SpaceX likely judged Red Dragon to be too disimilar from BFS to be worth the high cost of R&D and nonexistent ROI. Instead, they'll begin serious testing of BFS prototypes as soon as early 2019, using Earth to flesh out its capabilities and fix any bugs. Red Dragon would be a distraction from the vehicle SpaceX actually needs and wants, although it would be an incredible symbolic achievement :)
« Last Edit: 02/17/2018 10:30 PM by vaporcobra »

Offline Dante2121

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Re: BFS Design Requirements
« Reply #29 on: 02/18/2018 12:26 PM »
Something I've been curious about what others think - I think some amount of the Merlin's iterative improvements and reliability can be traced to the fact they built so many of them. If the booster is successfully reused from the getgo - they just won't have as many chances to iteratively test changes.

With the Merlin every launch likely had a small incremental improvement to the engine. You won't have that opportunity with Raptor - which means changes will get grouped together in large sets - increasing risk. 

Offline livingjw

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Re: BFS Design Requirements
« Reply #30 on: 02/18/2018 12:37 PM »
Something I've been curious about what others think - I think some amount of the Merlin's iterative improvements and reliability can be traced to the fact they built so many of them. If the booster is successfully reused from the getgo - they just won't have as many chances to iteratively test changes.

With the Merlin every launch likely had a small incremental improvement to the engine. You won't have that opportunity with Raptor - which means changes will get grouped together in large sets - increasing risk.

But they have 38 engines per BFR/BFS, all of which will be tested. During development, testing will probably include individual engine FOD and RUD containment tests as well. I think it will be done similar to turbine engine development.

John

Offline speedevil

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Re: BFS Design Requirements
« Reply #31 on: 02/18/2018 01:53 PM »
But they have 38 engines per BFR/BFS, all of which will be tested. During development, testing will probably include individual engine FOD and RUD containment tests as well. I think it will be done similar to turbine engine development.

A heavy emphasis on 'mass' production from the start, rather than carefully hand-building each engine might also be valuable to them in terms of consistency, and something they probably have a better handle than pretty much anyone in the industry with.

They 'know' they are going to need quite a lot of them by 2024.
At least 6 BFS, a couple of tanker BFS, a couple for earthly operations, three BFRs say, one BFR worth in testing.

That's a couple of hundred engines. Even if you discount the P2P ambitions entirely.

The cheapness of LNG also doesn't hurt if you want to do 'ridiculous' lengths of testing, with lots of restarts per engine.

Throwing bolts into the turbopump and seeing how violently you can get them to explode would be an entertaining series of blooper reels.
Throwing rocks and gravel at the operating engine similarly - what happens if you fire up the bell 6" off sand, gravel, or very large rocks, if the ambitions of rough landing are to be achieved.

Offline Dante2121

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Re: BFS Design Requirements
« Reply #32 on: 02/18/2018 02:02 PM »
True it's 38 engines for one complete set - but that's going to be 38 engines at once (likely with exact same design) whereas 38 Merlin engines represented 4+ sets of engines and thus four opportunities to iterate.

I think building 15 cores a year over several years has allowed Spacex to iterate a lot more than building a couple BFS per year will allow.  This allowed pretty rapid iteration - in true Silicon Valley style.

Maybe they have learned a lot and won't need to iterate as much - but that remains to be seen. If they need to iterate - it's going to be a lot more expensive to iterate BFS than Falcon.

Offline speedevil

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Re: BFS Design Requirements
« Reply #33 on: 02/18/2018 02:25 PM »
Maybe they have learned a lot and won't need to iterate as much - but that remains to be seen. If they need to iterate - it's going to be a lot more expensive to iterate BFS than Falcon.

I'm not sure that follows, at least for many aspects.

It may be more expensive to iterate BFS/R in the sense that if you want to replace the whole fleet, yes, 10*$200M or whatever = ow.

But, other than re-usability or safety bugs, I'm not sure it largely is, for most things you may want to do with any BFS/R.

Given the rapid reusability and stackability, and more importantly, the vast overperformance of the stock BFS in the face of any plausible paying customer in the near term, there may be little incentive to replace first generation BFS that 'just' can launch 50 tons without refuelling in orbit to get down, because they're 50 tons overweight and the thermal protection doesn't work as well as later ones, and the legs are three times heavier.

Or, indeed, upgrading from ones that can 'just' launch 150 tons and are at baseline IAC performance.

About the only thing where you'd care lots about replacement is if you have a design that explodes every few launches, or if you are getting so many launches that the marginal costs of keeping a different inventory becomes important.

Otherwise, if they are safe, it just becomes a case of selecting a high performance pair if you've got a high performance launch requirement. (>100 tons to orbit, or Mars)

Offline acsawdey

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Re: BFS Design Requirements
« Reply #34 on: 02/18/2018 02:31 PM »
But they have 38 engines per BFR/BFS, all of which will be tested. During development, testing will probably include individual engine FOD and RUD containment tests as well. I think it will be done similar to turbine engine development.

John

Do you expect that they will do tests analogous to the blade-out test that is done on commercial turbofans, i.e. intentionally cause a turbopump failure to test the engine controller shutdown response and the containment system? I seem to recall hearing that spacex has already done a "nut ingestion" test.

Offline QuantumG

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Re: BFS Design Requirements
« Reply #35 on: 02/18/2018 10:07 PM »
Not a viable solution. SpaceX has already stated complete unwillingness to use FH for any Moon or Mars missions.

huh? I'm pretty sure they said "we can do it, just send money" like they do for everything.


Jeff Bezos has billions to spend on rockets and can go at whatever pace he likes! Wow! What pace is he going at? Well... have you heard of Zeno's paradox?

Offline speedevil

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Re: BFS Design Requirements
« Reply #36 on: 02/18/2018 10:18 PM »
Not a viable solution. SpaceX has already stated complete unwillingness to use FH for any Moon or Mars missions.

They really haven't.
They have said that they believe BFS is the future, and they recommend others wait for it.

If, in mid 2019, it looks like 2022 might actually happen for first BFS landing on Mars, things get serious.

If BFS is not able to launch anything to Mars for the 2020 window, and they need more information, they either have to launch in 2022 and risk it, or they can try and get that information by using a relatively cheap launch (for them) and repurposed Starlink sats and similar hardware.

Might this happen?
I would be shocked if they've decided in a concrete manner either way yet.

Offline MaxTeranous

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Re: BFS Design Requirements
« Reply #37 on: 02/19/2018 12:13 PM »
Not a viable solution. SpaceX has already stated complete unwillingness to use FH for any Moon or Mars missions.

huh? I'm pretty sure they said "we can do it, just send money" like they do for everything.

Indeed, they'll not pay for it off their own back but pretty sure if NASA waves a $5 billion commercial contract out there to go play on the Moon in 2020 then they'd submit FH as a proposal.

Offline livingjw

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Re: BFS Design Requirements
« Reply #38 on: 02/19/2018 03:08 PM »
But they have 38 engines per BFR/BFS, all of which will be tested. During development, testing will probably include individual engine FOD and RUD containment tests as well. I think it will be done similar to turbine engine development.

John

Do you expect that they will do tests analogous to the blade-out test that is done on commercial turbofans, i.e. intentionally cause a turbopump failure to test the engine controller shutdown response and the containment system? I seem to recall hearing that spacex has already done a "nut ingestion" test.

Yes. They already have shown a containment cover for the engine. Most likely Kevlar or similar. Safe containment and shutdown will probably be part of development. Probability of a malfunctioning engine goes up with the number of engines, and they have a lot of them.

John

Online envy887

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Re: BFS Design Requirements
« Reply #39 on: 02/20/2018 01:35 PM »
But they have 38 engines per BFR/BFS, all of which will be tested. During development, testing will probably include individual engine FOD and RUD containment tests as well. I think it will be done similar to turbine engine development.

John

Do you expect that they will do tests analogous to the blade-out test that is done on commercial turbofans, i.e. intentionally cause a turbopump failure to test the engine controller shutdown response and the containment system? I seem to recall hearing that spacex has already done a "nut ingestion" test.

They did a FOD ingestion test during Merlin dev that consisted of dropping a #10 nut into the turbopump inlet, supposedly it ate the nut and kept running fine. Haven't hear of a similar test during Raptor dev, but it would probably be good to have some FOD tolerance.

Offline speedevil

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Re: BFS Design Requirements
« Reply #40 on: 02/20/2018 02:10 PM »
They did a FOD ingestion test during Merlin dev that consisted of dropping a #10 nut into the turbopump inlet, supposedly it ate the nut and kept running fine. Haven't hear of a similar test during Raptor dev, but it would probably be good to have some FOD tolerance.

In order to be properly comparable to airliner engines, it should be able to ingest a canada goose.

Offline acsawdey

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Re: BFS Design Requirements
« Reply #41 on: 02/20/2018 03:31 PM »
But they have 38 engines per BFR/BFS, all of which will be tested. During development, testing will probably include individual engine FOD and RUD containment tests as well. I think it will be done similar to turbine engine development.

John

Do you expect that they will do tests analogous to the blade-out test that is done on commercial turbofans, i.e. intentionally cause a turbopump failure to test the engine controller shutdown response and the containment system? I seem to recall hearing that spacex has already done a "nut ingestion" test.

They did a FOD ingestion test during Merlin dev that consisted of dropping a #10 nut into the turbopump inlet, supposedly it ate the nut and kept running fine. Haven't hear of a similar test during Raptor dev, but it would probably be good to have some FOD tolerance.

Yes, the point would be to make sure that the combination of the control system and the containment system can handle events like the loss of a pump vane or a chunk of the turbine disk and get things shut down without damaging adjacent components.

If there are Canada Geese in the LOX tank I think SpaceX has bigger quality control issues. That said, maximum reported flight altitude for a Canada Goose is 9 km, at which point either Falcon or BFR would be moving at a good clip -- better make sure fairings and windows are bird-strike proof.

Offline IRobot

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Re: BFS Design Requirements
« Reply #42 on: 02/20/2018 03:33 PM »
They did a FOD ingestion test during Merlin dev that consisted of dropping a #10 nut into the turbopump inlet, supposedly it ate the nut and kept running fine. Haven't hear of a similar test during Raptor dev, but it would probably be good to have some FOD tolerance.

In order to be properly comparable to airliner engines, it should be able to ingest a canada goose.
If you can think of a way that a canada goose can avoid hitting the top of the rocket, somehow do a 180º turn and enter the turbopump at high speeds... they test for the faults they can think of. There is no air intake in front, so the airline test makes no sense.

Offline speedevil

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Re: BFS Design Requirements
« Reply #43 on: 02/20/2018 03:38 PM »
In order to be properly comparable to airliner engines, it should be able to ingest a canada goose.
If you can think of a way that a canada goose can avoid hitting the top of the rocket, somehow do a 180º turn and enter the turbopump at high speeds... they test for the faults they can think of. There is no air intake in front, so the airline test makes no sense.

It seems I need to add more smilies to my posts.
It was an attempted joke at inappropriate safety standards.
Though airframe birdstrikes are a moderate issue.

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Re: BFS Design Requirements
« Reply #44 on: 02/20/2018 08:32 PM »
They did a FOD ingestion test during Merlin dev that consisted of dropping a #10 nut into the turbopump inlet, supposedly it ate the nut and kept running fine. Haven't hear of a similar test during Raptor dev, but it would probably be good to have some FOD tolerance.

In order to be properly comparable to airliner engines, it should be able to ingest a canada goose.
If you can think of a way that a canada goose can avoid hitting the top of the rocket, somehow do a 180º turn and enter the turbopump at high speeds... they test for the faults they can think of. There is no air intake in front, so the airline test makes no sense.

Maybe if it nests inside the tank

Offline biosehnsucht

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Re: BFS Design Requirements
« Reply #45 on: 02/20/2018 11:58 PM »
Perhaps try ingesting a COPV? Surely an albatross is as good as a goose?

Offline Lar

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Re: BFS Design Requirements
« Reply #46 on: 02/21/2018 12:46 AM »
Can we do a 180 on the thread humor level before the great post ingester in the sky throws an impeller from too much deletion?

Oh never mind. Carry on.
"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 Patchouli

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Re: BFS Design Requirements
« Reply #47 on: 02/21/2018 03:33 AM »
I think they should purposefully test a Raptor for hard start next to another running engine to see if they can prevent a domino effect failure like what happened on the N1.
« Last Edit: 02/21/2018 03:35 AM by Patchouli »

Offline Lars-J

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Re: BFS Design Requirements
« Reply #48 on: 02/21/2018 03:58 AM »
I think they should purposefully test a Raptor for hard start next to another running engine to see if they can prevent a domino effect failure like what happened on the N1.

With FH flying, can't the N1 ghost be laid to rest? C'mon.

Offline Patchouli

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Re: BFS Design Requirements
« Reply #49 on: 02/21/2018 04:08 AM »
FH is a different beast it has three cores with much simpler plumbing and GG cycle engines .
The FAA would never allow BFS for P2P unless they can demonstrate it can land after a catastrophic engine failure plus there is no launch escape system which means fault tolerance is your only safety net.
« Last Edit: 02/21/2018 04:51 AM by Patchouli »

Offline docmordrid

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Re: BFS Design Requirements
« Reply #50 on: 02/21/2018 11:07 PM »
There's no LAS on an airliner either, and AIUI Raptor engines have integrated frag shields which mimic the effect of Octaweb cells - so it should  have engine-out.
DM

Offline tchernik

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Re: BFS Design Requirements
« Reply #51 on: 03/05/2018 09:12 PM »
I have a question that may fit within this thread: what are the design requirements for automated cargo delivery to the Moon?, both with BFS return and expendable missions.

I understand an orbit-refueled BFS is designed to be capable of doing a landing and return mission to the Moon without human intervention, given it can do landing and return with its own reserves of fuel. The cargo is less, but it can do a full Apollo-like mission profile.

The same mission profile on Mars would probably require people in situ, to ensure the ISRU fuel production and transfer works OK.

If BFS can deliver cargo to the lunar surface on automated mode, how such delivery of cargo to the surface would be performed?

BFS is a very tall vehicle, with the Raptors and the fuel tanks between the payload and the lunar surface. Elon Musk presentations and some media show BFS lowering stuff with cranes on the Moon, but they don't seem very automated.

If they actually aren't, that requires people in place to do the placement of the cargo. Is that the way this is supposed to happen?

Or if they are automatic, is anyone developing such cranes?

Offline speedevil

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Re: BFS Design Requirements
« Reply #52 on: 03/05/2018 09:21 PM »
Or if they are automatic, is anyone developing such cranes?

For the case of the moon, common industrial cranes are typically slow enough that a couple of seconds teleoperation delay is quite managable by a skilled operator.

Especially as there is no pressing need for speed, and having the crane go at a maximum speed that is safe enough that the several people watching it do its job can stop it in an emergency is a complete solution to the problem.

Mars teleoperation is slow enough that you actually need to solve this problem in an automated manner.
The very most basic 'put that load over there' operations are quite simple to automate, especially at slow speed, more will require some thought.

Online envy887

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Re: BFS Design Requirements
« Reply #53 on: 03/11/2018 01:38 AM »
I think they should purposefully test a Raptor for hard start next to another running engine to see if they can prevent a domino effect failure like what happened on the N1.

If they want to fly passengers P2P they will likely have to show that the engine casing contains a RUD, just like airliner engines are tested for turbine/compressor blade containment.

Offline docmordrid

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Re: BFS Design Requirements
« Reply #54 on: 03/11/2018 02:06 AM »
I think they should purposefully test a Raptor for hard start next to another running engine to see if they can prevent a domino effect failure like what happened on the N1.

If they want to fly passengers P2P they will likely have to show that the engine casing contains a RUD, just like airliner engines are tested for turbine/compressor blade containment.

AIUI Raptor has a frag containment shield, but instead of being an external Octaweb cell it's integral.
« Last Edit: 03/11/2018 02:10 AM by docmordrid »
DM

Offline alang

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Re: BFS Design Requirements
« Reply #55 on: 03/11/2018 07:47 AM »
With respect to unimproved terrain:
As I recall the Harrier was quoted to be able to operate from forest clearings which suggests some terrain improvement but how much?
I suspect that there's been a lot of military engineering research in this area for lightweight surface improvement even if it's beyond my search skills.
I can see that 'refractory glass cloth' is used in welding applications but only up to 1000 degrees C. Also, the mechanical effect of the blast force would need to be managed and it would  need to be anchored. It wouldn't have to survive very long though.
I mention glass because an  in situ resource is likely to be silica and such a material could have other uses if it could be manufactured locally.

Offline Archibald

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Re: BFS Design Requirements
« Reply #56 on: 03/11/2018 07:57 AM »
I think they should purposefully test a Raptor for hard start next to another running engine to see if they can prevent a domino effect failure like what happened on the N1.

With FH flying, can't the N1 ghost be laid to rest? C'mon.

A reasonable case could be make that Elon Musk is a little more mentally balanced and under far less pressure than the unfortunate Vasily Mishin... https://en.wikipedia.org/wiki/Vasily_Mishin
« Last Edit: 03/11/2018 02:07 PM by Archibald »
... that ackward moment when you realize that Jeff Bezos personal fortune is far above NASA annual budget... 115 billion to 18 billion...

Offline speedevil

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Re: BFS Design Requirements
« Reply #57 on: 03/11/2018 07:54 PM »
I can see that 'refractory glass cloth' is used in welding applications but only up to 1000 degrees C. Also, the mechanical effect of the blast force would need to be managed and it would  need to be anchored.

A related topic might be how rich can you operate a Raptor before it goes out.

Might jet impingement at 2km/s not 4km/s be easier on the surface?

If the vacuum raptors could be operated at 30% throttle very off-mixture, that may create a very different impingement pattern on the surface, at a much lower peak pressure than one sea-level raptor in the middle. (or the cluster of three)

Online envy887

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Re: BFS Design Requirements
« Reply #58 on: 03/11/2018 11:37 PM »
I can see that 'refractory glass cloth' is used in welding applications but only up to 1000 degrees C. Also, the mechanical effect of the blast force would need to be managed and it would  need to be anchored.

A related topic might be how rich can you operate a Raptor before it goes out.

Might jet impingement at 2km/s not 4km/s be easier on the surface?

If the vacuum raptors could be operated at 30% throttle very off-mixture, that may create a very different impingement pattern on the surface, at a much lower peak pressure than one sea-level raptor in the middle. (or the cluster of three)

I would be rather surprised if the normal exhaust of the landing engines was anywhere near 1270 K. The SSME exhaust was about half that. The nozzle expands and accelerates the exhaust gas, and the energy to do that comes from the heat in the gas itself. An ideal nozzle would expand it all the way to the condensation point.

Online Robotbeat

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Re: BFS Design Requirements
« Reply #59 on: 03/11/2018 11:56 PM »
There's no LAS on an airliner either
Airliners are 10,000x safer than rockets, so adding a "LAS" would only decrease safety.
Bingo.
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Offline woods170

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Re: BFS Design Requirements
« Reply #60 on: 03/12/2018 06:46 AM »
There's no LAS on an airliner either, and AIUI Raptor engines have integrated frag shields which mimic the effect of Octaweb cells - so it should  have engine-out.

Correct. Remember that view of the BFS aft end from the 2017 IAC? I have it from SpaceX sources that the actual design is more "buttoned-up" with similar-function safeguards in place, for protection against engine RUD, as F9 has.
« Last Edit: 03/12/2018 06:49 AM by woods170 »

Offline Archibald

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Re: BFS Design Requirements
« Reply #61 on: 03/12/2018 11:59 AM »
There's no LAS on an airliner either
Airliners are 10,000x safer than rockets, so adding a "LAS" would only decrease safety.
Bingo.

a remarkable example of an aircraft LAS going the wrong way is the saga of the ejection pods on the B-58, XB-70, F-111, and B-1 (and later (never-happened) developments on Space Shuttle and Hermes, post STS-51L)

In most case classic ejection seats bet them any time, anywhere.

 The most extreme example being the Su-27 / Mig-29 / Buran K-36 seats.

http://www.proairshow.com/MiG%20Crash%20Seq.htm
« Last Edit: 03/12/2018 12:03 PM by Archibald »
... that ackward moment when you realize that Jeff Bezos personal fortune is far above NASA annual budget... 115 billion to 18 billion...

Online Robotbeat

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Re: BFS Design Requirements
« Reply #62 on: 03/12/2018 12:27 PM »
And ejection seats also aren’t very safe. They’d be a net reduction in safety if added to a typical airliner today.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline IainMcClatchie

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Re: BFS Design Requirements
« Reply #63 on: 03/12/2018 11:00 PM »
I would be rather surprised if the normal exhaust of the landing engines was anywhere near 1270 K. The SSME exhaust was about half that. The nozzle expands and accelerates the exhaust gas, and the energy to do that comes from the heat in the gas itself. An ideal nozzle would expand it all the way to the condensation point.

You're talking about static temperature.  That's not what is experienced by a surface on which the flow impinges.

The flow slows down as it gets very close to the surface, exchanging velocity for pressure and temperature.  For a rocket blasting at a flat surface, like on the ASDS, at the stagnation point where the exhaust has come to a stop, it's back up to the same total temperature that it was at in the combustion chamber.  I'm a little fuzzy on the details, but I think the total pressure (and thus density and heat transfer) is much lower because of expansion between nozzle exit and deck surface.

Offline Archibald

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Re: BFS Design Requirements
« Reply #64 on: 03/13/2018 06:10 AM »
And ejection seats also aren’t very safe. They’d be a net reduction in safety if added to a typical airliner today.

Imagine 800 seats popping out of a crashing A380... what a mess it would be...
« Last Edit: 03/13/2018 01:13 PM by Archibald »
... that ackward moment when you realize that Jeff Bezos personal fortune is far above NASA annual budget... 115 billion to 18 billion...

Online niwax

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Re: BFS Design Requirements
« Reply #65 on: 03/13/2018 06:47 AM »
And ejection seats also aren’t very safe. They’d be a net reduction in safety if added to a typical airliner today.

Imagine 800 sieges popping out of a crashing A380... what a mess it would be...

As the saying goes, please put on your seatbelt so we can identify the corpses by their seat number.

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Re: BFS Design Requirements
« Reply #66 on: 03/13/2018 01:11 PM »
There's no LAS on an airliner either
Airliners are 10,000x safer than rockets, so adding a "LAS" would only decrease safety.
Bingo.

a remarkable example of an aircraft LAS going the wrong way is the saga of the ejection pods on the B-58, XB-70, F-111, and B-1 (and later (never-happened) developments on Space Shuttle and Hermes, post STS-51L)

In most case classic ejection seats bet them any time, anywhere.

 The most extreme example being the Su-27 / Mig-29 / Buran K-36 seats.

http://www.proairshow.com/MiG%20Crash%20Seq.htm
bold mine
What "saga of the ejection pods" are you referring to?
Paul

Offline Archibald

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Re: BFS Design Requirements
« Reply #67 on: 03/13/2018 01:13 PM »
replace "wrong word" by "long and unfortunate story"
... that ackward moment when you realize that Jeff Bezos personal fortune is far above NASA annual budget... 115 billion to 18 billion...

Offline Slarty1080

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Re: BFS Design Requirements
« Reply #68 on: 03/20/2018 08:32 PM »
The issue of FOD on Mars has been discussed in this thread:
https://forum.nasaspaceflight.com/index.php?topic=37466.20

Quote from reply #24
ADDED: Your question deserves a better answer so I looked up what NASA scientists have found in their studies. The following statement is a verbatim copy of a summary that addresses the problem. It was a section of the Mars Design Reference Architecture, Addendum A,  published in 2009:

"5.9.1 Summary and recommendations
The predictions and recommendations for a 40-t spacecraft on Mars are described in summary in this section. The next section of the report will then explain in detail how these predictions were obtained.
The engine exhaust plume from a 40-t lander on Mars will blow dust, sand, gravel, and even rocks up to about 7 cm in diameter at high velocity. These ejecta will cause significant damage to any hardware that is already placed on the martian surface within the blast radius. However, the blast radius is modest, extending out to approximately 1 km. The largest debris is accelerated by the plume to lower velocities and, thus, falls closer to the landing site; and the smallest particles are attenuated by the martian atmosphere, also falling closer to the landing. Thus, maintaining the distance of about 1 km between the landing site and any existing surface assets will completely solve this problem for all sizes of debris.
A second concern arises because the exhaust from the large engines will form deep, narrow craters that are directly beneath each of their nozzles, and these craters will redirect the supersonic jet of gas with sand and rocks up toward the landing spacecraft. This has been demonstrated in large-scale engine tests in sand and clay (Alexander, et al, 1966) 25, small-scale experiments (Metzger, 2007) 26, numerical simulations (Liever, et al, 2007) 27, and soil dynamics analysis (see section 5.10.2.3), so there is no question that this will occur. It did not occur in the Apollo and Viking missions because the thrust was lower and/or because the lunar regolith had higher shear strength and less permeability than martian soil. These variables have been taken into consideration in this report. An example of a small-scale test is provided in figure 5-55. The impact of debris striking the lander will be sufficient to cause damage to the lander, possibly resulting in LOM and LOC, and therefore must be prevented. Of special concern is damage to the engine nozzles, because with a multiple-engine lander the debris that is ejected by one engine will be aimed directly at the other engines. One mitigation approach is to add shielding to the spacecraft to block the debris. This will increase the mass of the lander and, therefore, reduce the mass of the payload by approximately 1 t."

So FOD is a significant issue. Other points to consider from this thread:

* Mars gravity only 38% of Earths so less thrust required on take off than on Earth
* Mars atmosphere only 1% of Earths so more gas dispersion than on Earth
* Longer landing legs move the engines further away from the surface
* Landing sites are likely to be covered with large quantities of very loose materials
* If a large enough angle can be achieved, engine gimbaling could be very useful in deflecting FOD
* Either berms will be needed or the BFS will need to land perhaps 1km from the habitat due to FOD
* Take off will require more thrust with full tanks and will be over a pit excavated by the decent engines
* It might be possible to place deflectors under the engines to mitigate problems at take off
* Difficult to test scenario due to Gravity / atmosphere differences between Mars and Earth
The first words spoken on Mars: "Humans have been wondering if there was any life on the planet Mars for many decades … well ... there is now!"

Offline tdperk

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Re: BFS Design Requirements
« Reply #69 on: 03/20/2018 08:48 PM »
The issue of FOD on Mars has been discussed in this thread:
https://forum.nasaspaceflight.com/index.php?topic=37466.20

Quote from reply #24
ADDED: Your question deserves a better answer so I looked up what NASA scientists have found in their studies. The following statement is a verbatim copy of a summary that addresses the problem. It was a section of the Mars Design Reference Architecture, Addendum A,  published in 2009:

"5.9.1 Summary and recommendations
The predictions and recommendations for a 40-t spacecraft on Mars are described in summary in this section. The next section of the report will then explain in detail how these predictions were obtained.
The engine exhaust plume from a 40-t lander on Mars will blow dust, sand, gravel, and even rocks up to about 7 cm in diameter at high velocity. These ejecta will cause significant damage to any hardware that is already placed on the martian surface within the blast radius. However, the blast radius is modest, extending out to approximately 1 km. The largest debris is accelerated by the plume to lower velocities and, thus, falls closer to the landing site; and the smallest particles are attenuated by the martian atmosphere, also falling closer to the landing. Thus, maintaining the distance of about 1 km between the landing site and any existing surface assets will completely solve this problem for all sizes of debris.
A second concern arises because the exhaust from the large engines will form deep, narrow craters that are directly beneath each of their nozzles, and these craters will redirect the supersonic jet of gas with sand and rocks up toward the landing spacecraft. This has been demonstrated in large-scale engine tests in sand and clay (Alexander, et al, 1966) 25, small-scale experiments (Metzger, 2007) 26, numerical simulations (Liever, et al, 2007) 27, and soil dynamics analysis (see section 5.10.2.3), so there is no question that this will occur. It did not occur in the Apollo and Viking missions because the thrust was lower and/or because the lunar regolith had higher shear strength and less permeability than martian soil. These variables have been taken into consideration in this report. An example of a small-scale test is provided in figure 5-55. The impact of debris striking the lander will be sufficient to cause damage to the lander, possibly resulting in LOM and LOC, and therefore must be prevented. Of special concern is damage to the engine nozzles, because with a multiple-engine lander the debris that is ejected by one engine will be aimed directly at the other engines. One mitigation approach is to add shielding to the spacecraft to block the debris. This will increase the mass of the lander and, therefore, reduce the mass of the payload by approximately 1 t."

So FOD is a significant issue. Other points to consider from this thread:

* Mars gravity only 38% of Earths so less thrust required on take off than on Earth
* Mars atmosphere only 1% of Earths so more gas dispersion than on Earth
* Longer landing legs move the engines further away from the surface
* Landing sites are likely to be covered with large quantities of very loose materials
* If a large enough angle can be achieved, engine gimbaling could be very useful in deflecting FOD
* Either berms will be needed or the BFS will need to land perhaps 1km from the habitat due to FOD
* Take off will require more thrust with full tanks and will be over a pit excavated by the decent engines
* It might be possible to place deflectors under the engines to mitigate problems at take off
* Difficult to test scenario due to Gravity / atmosphere differences between Mars and Earth

They'll bring a tiller, like from a vegetable garden.  Possibly a powerhead running that and a vibratory compactor. They will till in fiber and adhesive.  It is smoothed.  It sets, and problem solved.  Fiber, adhesive, and tools are all mass left behind.
« Last Edit: 03/20/2018 08:52 PM by tdperk »

Offline Lar

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Re: BFS Design Requirements
« Reply #70 on: 03/20/2018 11:34 PM »
This adhesive survives multiple landings and launches?  And can set up in a low pressure but high CO2 atmosphere?  Did you have a specific one in mind?

I think they will glassify things or put down mats. We'll find out.
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Offline rakaydos

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Re: BFS Design Requirements
« Reply #71 on: 03/21/2018 02:08 AM »
This adhesive survives multiple landings and launches?  And can set up in a low pressure but high CO2 atmosphere?  Did you have a specific one in mind?

I think they will glassify things or put down mats. We'll find out.
Wasnt there an old topic talking about marscrete? How hard is that stuff to make in an automated enviroment?

Offline Slarty1080

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Re: BFS Design Requirements
« Reply #72 on: 03/21/2018 10:41 AM »
The problem with marscrete would be too many uncertainties over the exact physical and chemical composition especially early on. I’m sure they will use that later, but on the first mission I’m not so sure.

One big issue is that the problem is hard to quantify. Where are we on the scale of 1 to 10 where 1 is take off from a smooth solid rock surface in a vacuum and 10 is take off from a desert demolition site on earth?

Other unknowns are how resilient the engines are to having rocks thrown at them at speed, the initial thrust and speed at lift off and the initial distance between the engines and the surface. Coupled with it being hard to test given 38% gravity and 1% atmosphere.

Anyone have any more ideas?
The first words spoken on Mars: "Humans have been wondering if there was any life on the planet Mars for many decades … well ... there is now!"

Offline speedevil

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Re: BFS Design Requirements
« Reply #73 on: 03/21/2018 05:51 PM »
Other unknowns are how resilient the engines are to having rocks thrown at them at speed, the initial thrust and speed at lift off and the initial distance between the engines and the surface. Coupled with it being hard to test given 38% gravity and 1% atmosphere.

Anyone have any more ideas?

You only care - for the initial vehicles - about 'will it explode or tip over'.
You do not care about relaunch, which makes things a little better.

There are many plausible approaches:
Pre-preparing the landing site using small robot vehicles or at least surveying in the synod previous to a landing.
Throwing out an ablative coated 9m diameter 'yoga mat' while traversing ballistically and then come back to it and land.
Land on parts of the various rover tracks that seem best.
Aerobrake into LMO and then throw out lots of sojourner class robots, in beagle 2 sized (70kg) entry packages, to do an immediate survey before landing.
Add landing engines at the top of the tanks of the vehicle canted out at 30 degrees, so nothing meaningful happens to the ground.
Yes, this would mean four engines, with a total thrust of some 100 tons for several seconds, and more complex piping.

There are no clearly obvious best solutions, especially with a cargo undefined beyond '150 tons of something'.


Offline wes_wilson

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Re: BFS Design Requirements
« Reply #74 on: 03/26/2018 12:30 AM »
The problem with marscrete would be too many uncertainties over the exact physical and chemical composition especially early on. I’m sure they will use that later, but on the first mission I’m not so sure.

One big issue is that the problem is hard to quantify. Where are we on the scale of 1 to 10 where 1 is take off from a smooth solid rock surface in a vacuum and 10 is take off from a desert demolition site on earth?

Other unknowns are how resilient the engines are to having rocks thrown at them at speed, the initial thrust and speed at lift off and the initial distance between the engines and the surface. Coupled with it being hard to test given 38% gravity and 1% atmosphere.

Anyone have any more ideas?

I wonder how much a launch cradle weighs?  Like the one used for BFR? And whether a BFS could carry a launch mount as a payload to Mars.  My assumption has always been that it would launch from the legs it lands on, but maybe that's not the case?  It's not using legs when it sits atop BFR before staging so maybe it takes off from a cradle at Mars. 
@SpaceX "When can I buy my ticket to Mars?"

Online Robotbeat

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Re: BFS Design Requirements
« Reply #75 on: 03/26/2018 03:41 AM »
Or just posts. Doesn’t have to be a full cradle at all. Could be really simple:

Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

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Online Nate_Trost

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Re: BFS Design Requirements
« Reply #76 on: 03/26/2018 03:29 PM »
Has the ramifications of extensive composite use in BFS been discussed in terms of long-term radiation exposure through multiple Mars round trips? It immediately sprang to mind after the Roadster mission when there was talk about just how fast the carbon-fiber body of the Roadster was going to break down in the space environment.

Offline speedevil

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Re: BFS Design Requirements
« Reply #77 on: 03/26/2018 03:36 PM »
Has the ramifications of extensive composite use in BFS been discussed in terms of long-term radiation exposure through multiple Mars round trips? It immediately sprang to mind after the Roadster mission when there was talk about just how fast the carbon-fiber body of the Roadster was going to break down in the space environment.

The proper paint pretty much solves the degradation problem.
The issue is vacuum UV degrading the epoxy badly once the non-UV-rated paint flakes off.
With proper coating, it's pretty much not an issue.
There is no significant damage from other radiation.

Online envy887

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Re: BFS Design Requirements
« Reply #78 on: 03/26/2018 03:53 PM »
Has the ramifications of extensive composite use in BFS been discussed in terms of long-term radiation exposure through multiple Mars round trips? It immediately sprang to mind after the Roadster mission when there was talk about just how fast the carbon-fiber body of the Roadster was going to break down in the space environment.

The proper paint pretty much solves the degradation problem.
The issue is vacuum UV degrading the epoxy badly once the non-UV-rated paint flakes off.
With proper coating, it's pretty much not an issue.
There is no significant damage from other radiation.
The entire BFS will be covered in TPS anyway. Even the back side will reach several hundred degrees C, so there will be little to no exposed or painted carbon fiber surfaces.

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