Author Topic: Elon Musk Reddit AMA on BFR  (Read 66159 times)

Offline hkultala

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Re: Elon Musk Reddit AMA on BFR
« Reply #140 on: 10/17/2017 01:53 pm »
Whats interesting is that if they switch to one engine to land then having the 3 engines in a row might make more sense so that one engine is dead center in the middle. That way the one engine does not need to gimbal off center.

No, it would just make the engine-out redundancy much harder, because the 2 other engines would be much further away from the centerline.

Offline hkultala

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Re: Elon Musk Reddit AMA on BFR very soon.
« Reply #141 on: 10/17/2017 01:59 pm »
Hmmmmm. Just realized we don't know if the tanker will also be updated to have three SL Raptors. I'd expect the answer to be yes if the main purpose is increased reliability and granularity.

With deep throttle capabilities of 20%, three Raptors rated for 1700kN (380 klbf), and a dry mass no more than 85t (187393 lb),

Dedicated tanker version will be lighter than the cargo and crew versions. Something like 70 tonnes maybe.

Quote
the minimum thrust with three engines firing is 228 klbf. With two, it's 152 klbf. So a three engined BFS definitely could hover briefly with 5-10% of fuel retained for landing, especially with a dedicated a considerably lighter tanker version.

No, the hovering is only interesting just before touchdown, not high in the air.

And just before touchdown it does not anymore have 5-10% of fuel left. It only has like 1% margin left.

So the numbers become like 70 tonnes vehicle weight, and 10 tonnes fuel weight, total ~ 80 tonnes.
Two engines at 20% is 68 tonnes, three engines at 20% is 102 tonnes.

Offline meekGee

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Re: Elon Musk Reddit AMA on BFR
« Reply #142 on: 10/17/2017 03:53 pm »
They're going to have to have a very full-proof mechanism against fueling mistakes...  Like aforementioned landing tanks.

When an airliner runs out of fuel, it's one thing...  But if you're 100 gallons short on a VTVL landing, engine redundancy is not going to be helpful...

Since atmospheric free fall is "the great equalizer", it erases any fuel consumption deviations that occurred prior to reentry. But the hoverslam happens after that, and so the landing tanks need to be 100% full before reentry, and used only for the hoverslam.


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Offline speedevil

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Re: Elon Musk Reddit AMA on BFR
« Reply #143 on: 10/17/2017 04:01 pm »
When an airliner runs out of fuel, it's one thing...  But if you're 100 gallons short on a VTVL landing, engine redundancy is not going to be helpful...

Which is why every landing area needs a ball-pit next to it for low fuel entries.
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Offline Peter.Colin

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Re: Elon Musk Reddit AMA on BFR
« Reply #144 on: 10/17/2017 05:28 pm »
Whats interesting is that if they switch to one engine to land then having the 3 engines in a row might make more sense so that one engine is dead center in the middle. That way the one engine does not need to gimbal off center.

No, it would just make the engine-out redundancy much harder, because the 2 other engines would be much further away from the centerline.

If the middle one fails it doesn’t matter if they are further away from the centerline both fire at 50% throttle and the force is balanced out.
If one of the outer ones fails you have one in the centerline at 100% throttle, so perfect balance.
In a triangular confuguration the force is never balanced out when one engine fails.


« Last Edit: 10/17/2017 05:36 pm by Peter.Colin »

Offline leetdan

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Re: Elon Musk Reddit AMA on BFR
« Reply #145 on: 10/17/2017 06:07 pm »
If the 'hover-slam' is a solvable control problem with a single engine, it isn't much of a stretch to envision a 'hover-tip' maneuver with multiple engines firing off-center.  The stack is tilted during descent to prevent XY acceleration, with Z velocity and now also deviation from vertical reaching 0 precisely at touchdown.  It's obviously harder, with additional trades needed WRT landing gear, but not unsolvable.
« Last Edit: 10/17/2017 06:08 pm by leetdan »

Offline Lar

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Re: Elon Musk Reddit AMA on BFR
« Reply #146 on: 10/17/2017 06:57 pm »
If the 'hover-slam' is a solvable control problem with a single engine, it isn't much of a stretch to envision a 'hover-tip' maneuver with multiple engines firing off-center.  The stack is tilted during descent to prevent XY acceleration, with Z velocity and now also deviation from vertical reaching 0 precisely at touchdown.  It's obviously harder, with additional trades needed WRT landing gear, but not unsolvable.

Coslne losses suggest it might use more fuel than is absolutely necessary though, no?
"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 acsawdey

Re: Elon Musk Reddit AMA on BFR
« Reply #147 on: 10/17/2017 08:15 pm »
If the 'hover-slam' is a solvable control problem with a single engine, it isn't much of a stretch to envision a 'hover-tip' maneuver with multiple engines firing off-center.  The stack is tilted during descent to prevent XY acceleration, with Z velocity and now also deviation from vertical reaching 0 precisely at touchdown.  It's obviously harder, with additional trades needed WRT landing gear, but not unsolvable.

Coslne losses suggest it might use more fuel than is absolutely necessary though, no?

The gimbal angle can't be all that big and cos(12 degrees) is 0.978 so the losses are a few percent or less.

Offline Lar

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Re: Elon Musk Reddit AMA on BFR
« Reply #148 on: 10/17/2017 08:28 pm »
If the 'hover-slam' is a solvable control problem with a single engine, it isn't much of a stretch to envision a 'hover-tip' maneuver with multiple engines firing off-center.  The stack is tilted during descent to prevent XY acceleration, with Z velocity and now also deviation from vertical reaching 0 precisely at touchdown.  It's obviously harder, with additional trades needed WRT landing gear, but not unsolvable.

Coslne losses suggest it might use more fuel than is absolutely necessary though, no?

The gimbal angle can't be all that big and cos(12 degrees) is 0.978 so the losses are a few percent or less.


If you're landing on fumes, it does matter. but yeah.
"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 cppetrie

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Re: Elon Musk Reddit AMA on BFR
« Reply #149 on: 10/18/2017 01:34 am »
I got the impression that under nominal conditions all three landing engines will be firing. If there is an engine out the remaining two would throttle up to compensate vs only one running normally and another starting up in an engine out event. Startup would take way longer than just throttling up, which is very problematic when a failure occurs at the worst possible point in the landing. So with normal operation of all three engines running a triangular arrangement is symmetrical in all directions. Under engine out, the remaining two only need enough gimble to compensate for the failed engine. Worst case you provide enough gimble to compensate for two engines out. Since you can’t predict which engine will go out you want all three engines as close to center as possible, which the triangular arrangement also provides. Triangular is also the most centrally compact arrangement. My guess is a triangular arrangement for the landing engines.

Caveat: I’m only an arm-chair rocket engineer so I might have a flaw in my logic. If so, please feel free to kindly point it out. Thx.

Offline mgeagon

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Re: Elon Musk Reddit AMA on BFR
« Reply #150 on: 10/18/2017 05:30 am »
Take a brand new BFS (ship A) to Mars, in 2022 or 2024. That's what, 150 days travel then it sits while being unloaded, habitats established and proven, after that, it returns to earth. That's what,  another 150 days. The ship is now how old? At least a year old assuming establishing habitats is a priority, It could be much older.

How far will SpaceX advance the design of the currently new BFS's while ship A is making this round trip? Or another way of looking at it is, "How useful is a year or more old Falcon 9 these days?" Or, "Has SpaceX ever built a rocket that didn't undergo major evolutionary changes over the span of a year's time?"

In particular, we are addressing the first BFS's out of the box, not a mature, stable design as planned to exist by 2026.

I believe taking a look at the productuon runs of large transport category aircraft is instructive. The first few planes out of the factory are largely hand built, with tooling modified on the go and quality control standards yet to be developed. These aircraft are then added to the certification regimen and put through flight envelope testing. After a type certificate is issued by the FAA or other governmental agency, the test aircraft are refurbished and delivered to the launch customers.

These initial articles are typically overweight and are susceptible to long-term chronic maintenance, yet they continue to fly for decades. When major upgrades or airworthiness directives are issued, all aircraft of the same type are upgraded to the new standard.

It appears the business case for the BFR is predicated upon 1000 times reuse, meaning the development costs are spread between the number of ships built X the amount of flights they make. Counter to this would be a single flight to Mars and then becoming a museum piece in SITU. It would make much more fiscal sense to send the ship back to Earth for reuse. Even major upgrades to the engines or avionics would be far cheaper than scrapping the entire rocket. At least that is clearly true with atmospheric vehicles.

Offline aero

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Re: Elon Musk Reddit AMA on BFR
« Reply #151 on: 10/18/2017 06:51 am »
Take a brand new BFS (ship A) to Mars, in 2022 or 2024. That's what, 150 days travel then it sits while being unloaded, habitats established and proven, after that, it returns to earth. That's what,  another 150 days. The ship is now how old? At least a year old assuming establishing habitats is a priority, It could be much older.

How far will SpaceX advance the design of the currently new BFS's while ship A is making this round trip? Or another way of looking at it is, "How useful is a year or more old Falcon 9 these days?" Or, "Has SpaceX ever built a rocket that didn't undergo major evolutionary changes over the span of a year's time?"

In particular, we are addressing the first BFS's out of the box, not a mature, stable design as planned to exist by 2026.

I believe taking a look at the productuon runs of large transport category aircraft is instructive. The first few planes out of the factory are largely hand built, with tooling modified on the go and quality control standards yet to be developed. These aircraft are then added to the certification regimen and put through flight envelope testing. After a type certificate is issued by the FAA or other governmental agency, the test aircraft are refurbished and delivered to the launch customers.

These initial articles are typically overweight and are susceptible to long-term chronic maintenance, yet they continue to fly for decades. When major upgrades or airworthiness directives are issued, all aircraft of the same type are upgraded to the new standard.

It appears the business case for the BFR is predicated upon 1000 times reuse, meaning the development costs are spread between the number of ships built X the amount of flights they make. Counter to this would be a single flight to Mars and then becoming a museum piece in SITU. It would make much more fiscal sense to send the ship back to Earth for reuse. Even major upgrades to the engines or avionics would be far cheaper than scrapping the entire rocket. At least that is clearly true with atmospheric vehicles.

Your point is well taken. A significant difference between the initial aircraft articles and the initial BFS is that the weight penalty is much greater for the rocket than for the aircraft. Engineers developing aircraft and the SpaceX engineers both will do their best to eliminate excess weight, but overweight articles do happen in both cases.

It may be that Elon intends to deliberately over spec the initial ships for reliability (because of schedule pressure) then remove the excess based on flight data and tear down inspections. If that were the case then perhaps leaving the first two ships on Mars wouldn't be such a loss - that is - if the maintenance records, flight, and inspection data were gathered before the ship's departure for Mars.

Rocket developers have not had the luxury of using this WWII develop method but with the throw weight of the BFR/BFS, it is a possibility in this instance. I guess time will tell.
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Offline rsdavis9

Re: Elon Musk Reddit AMA on BFR
« Reply #152 on: 10/18/2017 11:23 am »
I got the impression that under nominal conditions all three landing engines will be firing. If there is an engine out the remaining two would throttle up to compensate vs only one running normally and another starting up in an engine out event. Startup would take way longer than just throttling up, which is very problematic when a failure occurs at the worst possible point in the landing. So with normal operation of all three engines running a triangular arrangement is symmetrical in all directions. Under engine out, the remaining two only need enough gimble to compensate for the failed engine. Worst case you provide enough gimble to compensate for two engines out. Since you can’t predict which engine will go out you want all three engines as close to center as possible, which the triangular arrangement also provides. Triangular is also the most centrally compact arrangement. My guess is a triangular arrangement for the landing engines.

Caveat: I’m only an arm-chair rocket engineer so I might have a flaw in my logic. If so, please feel free to kindly point it out. Thx.

could be that the engine reliability is mostly on startup. Once going the engines will be very likely to continue to work. I agree 3 at first is the best. But on the final landing approach it might be desirable to shut 2 down and throttle up the center.

So the question is what is easier to change throttle on?
1. 3 engines at low throttle setting.
2. 1 engine at high throttle setting.

I would guess 3 engines because the turbines are spinning slower and easier to change speed at a slower spin speed.
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Offline oldAtlas_Eguy

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Re: Elon Musk Reddit AMA on BFR
« Reply #153 on: 10/18/2017 10:00 pm »
If the 'hover-slam' is a solvable control problem with a single engine, it isn't much of a stretch to envision a 'hover-tip' maneuver with multiple engines firing off-center.  The stack is tilted during descent to prevent XY acceleration, with Z velocity and now also deviation from vertical reaching 0 precisely at touchdown.  It's obviously harder, with additional trades needed WRT landing gear, but not unsolvable.

Coslne losses suggest it might use more fuel than is absolutely necessary though, no?


The gimbal angle can't be all that big and cos(12 degrees) is 0.978 so the losses are a few percent or less.


If you're landing on fumes, it does matter. but yeah.
A fallacy.

Other than for maneuvering X-Y the angle of thrust is center-lined on the CG. So there is no cosine losses on engine out unless for some reason the engines are gimbaling not in unison but gimbling in opposition.

Offline aero

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Re: Elon Musk Reddit AMA on BFR
« Reply #154 on: 10/19/2017 03:59 am »
If the 'hover-slam' is a solvable control problem with a single engine, it isn't much of a stretch to envision a 'hover-tip' maneuver with multiple engines firing off-center.  The stack is tilted during descent to prevent XY acceleration, with Z velocity and now also deviation from vertical reaching 0 precisely at touchdown.  It's obviously harder, with additional trades needed WRT landing gear, but not unsolvable.

Coslne losses suggest it might use more fuel than is absolutely necessary though, no?


The gimbal angle can't be all that big and cos(12 degrees) is 0.978 so the losses are a few percent or less.


If you're landing on fumes, it does matter. but yeah.
A fallacy.

Other than for maneuvering X-Y the angle of thrust is center-lined on the CG. So there is no cosine losses on engine out unless for some reason the engines are gimbaling not in unison but gimbling in opposition.

It seems hardly worth the trouble but controlling the engines to gimbal in opposition would give an extra reduction in minimum thrust. The engines throttle down to 20% then if gimbaled in opposition by 12 degrees gives the vertical force of 19.56% of thrust. Does anyone know a number for the common maximum gimbal angle of rocket engines? Or is there even such a number outside of SpaceX? One half of one percent reduction in vertical force is likely less than vertical acceleration reduction due to the added mass needed to strengthen the engine gimbal supports.
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Offline Lars-J

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Re: Elon Musk Reddit AMA on BFR
« Reply #155 on: 10/19/2017 04:51 am »
It seems hardly worth the trouble but controlling the engines to gimbal in opposition would give an extra reduction in minimum thrust. The engines throttle down to 20% then if gimbaled in opposition by 12 degrees gives the vertical force of 19.56% of thrust. Does anyone know a number for the common maximum gimbal angle of rocket engines? Or is there even such a number outside of SpaceX? One half of one percent reduction in vertical force is likely less than vertical acceleration reduction due to the added mass needed to strengthen the engine gimbal supports.

So a whole 0.44% gain? That should make it clear for you why they are NOT doing it. If you are running that close to the margin where that makes all the difference, you will not have a reliable system.
« Last Edit: 10/19/2017 05:19 am by Lars-J »

Offline hkultala

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Re: Elon Musk Reddit AMA on BFR
« Reply #156 on: 10/19/2017 01:12 pm »
It seems hardly worth the trouble but controlling the engines to gimbal in opposition would give an extra reduction in minimum thrust. The engines throttle down to 20% then if gimbaled in opposition by 12 degrees gives the vertical force of 19.56% of thrust. Does anyone know a number for the common maximum gimbal angle of rocket engines? Or is there even such a number outside of SpaceX? One half of one percent reduction in vertical force is likely less than vertical acceleration reduction due to the added mass needed to strengthen the engine gimbal supports.

So a whole 0.44% gain? That should make it clear for you why they are NOT doing it. If you are running that close to the margin where that makes all the difference, you will not have a reliable system.

No, not 0.44% difference but 2.2% difference and 0.44 percentage point difference.

Offline meekGee

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Re: Elon Musk Reddit AMA on BFR
« Reply #157 on: 10/19/2017 03:41 pm »
It seems hardly worth the trouble but controlling the engines to gimbal in opposition would give an extra reduction in minimum thrust. The engines throttle down to 20% then if gimbaled in opposition by 12 degrees gives the vertical force of 19.56% of thrust. Does anyone know a number for the common maximum gimbal angle of rocket engines? Or is there even such a number outside of SpaceX? One half of one percent reduction in vertical force is likely less than vertical acceleration reduction due to the added mass needed to strengthen the engine gimbal supports.

So a whole 0.44% gain? That should make it clear for you why they are NOT doing it. If you are running that close to the margin where that makes all the difference, you will not have a reliable system.

No, not 0.44% difference but 2.2% difference and 0.44 percentage point difference.
The down side of such an arrangement is that an engine-out event is instantaneously catastrophic. (Well, probably, but much more so then when all engines are pointing through the c.g.)
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Offline intrepidpursuit

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Re: Elon Musk Reddit AMA on BFR
« Reply #158 on: 10/19/2017 07:37 pm »
I don't think the landing engines will gimbal separately but as a cluster like was suggested for the BFR IAC 2016. With 3 engines in a triangle I would cant them all slightly so they are all thrusting through CG so that a single engine failure would not require an immediate adjustment for stability. Then in a failure situation it just has to adjust to kill horizontal velocity, which should be minimal.

Offline rsdavis9

Re: Elon Musk Reddit AMA on BFR
« Reply #159 on: 10/19/2017 08:05 pm »
They have to be independently TVC so that you can get roll as well as left right top bottom.

EDIT: of course cold gas thrusters could do the roll correction.
« Last Edit: 10/19/2017 08:06 pm by rsdavis9 »
With ELV best efficiency was the paradigm. The new paradigm is reusable, good enough, and commonality of design.
Same engines. Design once. Same vehicle. Design once. Reusable. Build once.

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