Author Topic: SpaceX Starship/Super Heavy Engineering General Thread 4  (Read 1497602 times)

Offline Nevyn72

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #20 on: 12/29/2019 08:32 pm »
The following is cropped from bocachicagal's photos, seems that each tile is supported by 3 bolts, corresponding to the 3 holes on the tile.

I'm quite sure this way of mounting the heat shield tiles won't be the final method. While it works and allows for quickly attaching and detaching the tiles, the holes actually create weak spots in the tile, will probably add tension due to different thermal expansion coefficients of the tiles and the underlying steel structure and also trap some of the super heated plasma inside the cavity, probably melting the bolt.

I disagree. To me this seems a good approach.

- The tiles have an outer covering for strength and toughness. Inside is insulation.

- Without knowing how the tile shell is designed, you can't say the holes will cause weak spots. They didn't just drill holes in the tile shell. I am sure they are designed to handle the loads.

- Thermal expansion was surely taken into consideration when engineering the fasteners and attachment. Why would they not?

- Attachment points are recessed and can be easily plugged with insulating material.

John

John,
If you inspect the left most tile from image above, you can see that it has fractured between the 3 bolt holes used to affix the tile.
That feature looks too uniform to have just "fractured", it looks intentional.

I would call it intentional but in a different way. Those tiles were fitted for test purposes to see how well the attachment process worked during an actual (albeit brief) flight.

I would expect they would have each been attached slightly differently to see what method (eg. bolt tension, backing material, spacers, etc.) worked or didn't. The one that appears 'fractured' is one that didn't.  ;)

Offline Mandella

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #21 on: 12/30/2019 12:10 am »
And the tiles themselves might be constructed somewhat differently themselves, not just the attachment system.

It's the SpaceX way.

Offline Ionmars

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #22 on: 12/30/2019 01:38 am »
Given that SpX is known for 3-D printing, they might try printing the ceramic tiles and bolts as one whole piece. The purpose would be to avoid different expansion coefficients between bolt and tile. Nuts could also be of same material. The bolt holes in the underlying steel sheet would be sized to accommodate greater expansion during heating.

Added: It is OK that holes will expand larger during heating, but the holes will also expand further apart, which could exert excessive force on the bolts.
« Last Edit: 12/30/2019 01:56 am by Ionmars »

Offline Coastal Ron

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #23 on: 12/30/2019 02:54 am »
Given that SpX is known for 3-D printing, they might try printing the ceramic tiles and bolts as one whole piece.

The additive manufacturing for metals and ceramics that I'm familiar with would not allow this, since they can only do one type of material at a time. Plus ceramic usually need to be baked after being formed, and that would not match up with the finishing requirements for the metal components.

I did find this article about ceramic 3D printing which may be of interest:

Ceramic 3D Printing: A Revolution within Additive Manufacturing?

Quote
The purpose would be to avoid different expansion coefficients between bolt and tile. Nuts could also be of same material. The bolt holes in the underlying steel sheet would be sized to accommodate greater expansion during heating.

Added: It is OK that holes will expand larger during heating, but the holes will also expand further apart, which could exert excessive force on the bolts.

The stainless steel of the vehicle body and the attachment studs are likely to expand more than the ceramic, and I'm thinking that there can be enough "slop" in the attachment points to allow for thermal expansion with metal studs and ceramic tiles.

Though just as with the Starship design, we don't know if the ceramic tiles and the attachment method we saw are the final configuration. But they are a data point...  :D
If we don't continuously lower the cost to access space, how are we ever going to afford to expand humanity out into space?

Offline envy887

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #24 on: 12/30/2019 02:16 pm »
Given that SpX is known for 3-D printing, they might try printing the ceramic tiles and bolts as one whole piece. The purpose would be to avoid different expansion coefficients between bolt and tile. Nuts could also be of same material. The bolt holes in the underlying steel sheet would be sized to accommodate greater expansion during heating.

Added: It is OK that holes will expand larger during heating, but the holes will also expand further apart, which could exert excessive force on the bolts.

There are no bolts, or holes in the stainless skin. Those are studs welded directly to the skin - look up stud welding, it's simple, fast, cheap, repeatable, and durable. The tiles have 3 holes that the studs slip thorough, probably with a fixed head on the back side to keep the tile on, or possibly a nut for easier repair. Unlike the Shuttle's complex, sensitive, and incredibly time-consuming adhesive tile bonding process, welded studs would allow a small crew to tile a whole orbiter in a few days with simple equipment.

Offline FutureSpaceTourist

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #25 on: 12/30/2019 10:37 pm »
Continuing discussion on driving Starship flaps

[...]

twitter.com/erdayastronaut/status/1211550091414265856

Quote
It’d be so cool if Tesla motors were powerful enough to spin start! Have you moved onto direct drive / electromechanical on the body flaps with motors yet or still spinning a pump for SN1?

https://twitter.com/elonmusk/status/1211556824442425344

Quote
Direct drive using several Tesla Plaid motors in parallel for SN1. Simpler, lighter & more fault tolerant. Rear flaps each need ~1.5 megawatts. It’s like moving the entire wing of an aircraft!

twitter.com/john_gardi/status/1211567070808346624

Quote
How about worm gears for the rear flaps like the jack screws  in planes with an upper rear elevator on their tails?
Could you make them strong enough because getting enough torque to drive them at such a high gear ratio wouldn't be a issue.

https://twitter.com/elonmusk/status/1211784060705656832

Quote
Many ways to solve this problem, but the power requirements are much higher than aircraft control surfaces. When moving giant body flaps rapidly, achieving high power is much harder than high torque.

twitter.com/shaneappleton7/status/1211557213610733568

Quote
So we’re gonna see the plaid power train in action in a spacecraft before we see it in a production tesla? 🤣😂🤣

https://twitter.com/elonmusk/status/1211780010664742923

Quote
Building prototypes is relatively easy, volume production is hard
« Last Edit: 12/30/2019 10:44 pm by FutureSpaceTourist »

Offline _MECO

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #26 on: 12/30/2019 10:58 pm »
Continuing discussion on driving Starship flaps

[...]

twitter.com/erdayastronaut/status/1211550091414265856

Quote
It’d be so cool if Tesla motors were powerful enough to spin start! Have you moved onto direct drive / electromechanical on the body flaps with motors yet or still spinning a pump for SN1?

https://twitter.com/elonmusk/status/1211556824442425344

Quote
Direct drive using several Tesla Plaid motors in parallel for SN1. Simpler, lighter & more fault tolerant. Rear flaps each need ~1.5 megawatts. It’s like moving the entire wing of an aircraft!

twitter.com/john_gardi/status/1211567070808346624

Quote
How about worm gears for the rear flaps like the jack screws  in planes with an upper rear elevator on their tails?
Could you make them strong enough because getting enough torque to drive them at such a high gear ratio wouldn't be a issue.

https://twitter.com/elonmusk/status/1211784060705656832

Quote
Many ways to solve this problem, but the power requirements are much higher than aircraft control surfaces. When moving giant body flaps rapidly, achieving high power is much harder than high torque.

twitter.com/shaneappleton7/status/1211557213610733568

Quote
So we’re gonna see the plaid power train in action in a spacecraft before we see it in a production tesla? 🤣😂🤣

https://twitter.com/elonmusk/status/1211780010664742923

Quote
Building prototypes is relatively easy, volume production is hard

Holy cow, this newest tweet storm is entirely too much information. Last I saw on the engineering thread people were arguing about bulkhead thickness. Now we've got that, actuator power, almost the actuator design itself, pressurization details, manufacturing techniques, even projected manufacturing techniques and if I'm not reading too far into this, the fact that the bottleneck for production outside is in fact not thermal expansion.

Offline Eka

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #27 on: 12/31/2019 03:04 am »
Quote
Direct drive using several Tesla Plaid motors in parallel for SN1. Simpler, lighter & more fault tolerant. Rear flaps each need ~1.5 megawatts. It’s like moving the entire wing of an aircraft!
This tells me the motor case or it's shaft will be attached to the flap, and the other attached to the body. The motor won't even be able to make a half revolution. On the other hand it will be able to move that flap fast.
We talk about creating a Star Trek future, but will end up with The Expanse if radical change doesn't happen.

Offline envy887

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #28 on: 12/31/2019 01:55 pm »
Quote
Direct drive using several Tesla Plaid motors in parallel for SN1. Simpler, lighter & more fault tolerant. Rear flaps each need ~1.5 megawatts. It’s like moving the entire wing of an aircraft!
This tells me the motor case or it's shaft will be attached to the flap, and the other attached to the body. The motor won't even be able to make a half revolution. On the other hand it will be able to move that flap fast.

Can those motors produce the kN-m range torques needed to twist the flap against airflow? I think they will need extra gear reduction drives. Musk mentioned worm gears at the Mk.1 event, IIRC.

Offline niwax

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #29 on: 12/31/2019 02:16 pm »
Quote
Direct drive using several Tesla Plaid motors in parallel for SN1. Simpler, lighter & more fault tolerant. Rear flaps each need ~1.5 megawatts. It’s like moving the entire wing of an aircraft!
This tells me the motor case or it's shaft will be attached to the flap, and the other attached to the body. The motor won't even be able to make a half revolution. On the other hand it will be able to move that flap fast.

Can those motors produce the kN-m range torques needed to twist the flap against airflow? I think they will need extra gear reduction drives. Musk mentioned worm gears at the Mk.1 event, IIRC.

Wheel torque in the acceleration range they're targeting for the reminder plaid models is 10-15 kNm per wheel.
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Offline geza

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #30 on: 12/31/2019 03:13 pm »
Quote
Direct drive using several Tesla Plaid motors in parallel for SN1. Simpler, lighter & more fault tolerant. Rear flaps each need ~1.5 megawatts. It’s like moving the entire wing of an aircraft!
This tells me the motor case or it's shaft will be attached to the flap, and the other attached to the body. The motor won't even be able to make a half revolution. On the other hand it will be able to move that flap fast.

Can those motors produce the kN-m range torques needed to twist the flap against airflow? I think they will need extra gear reduction drives. Musk mentioned worm gears at the Mk.1 event, IIRC.

Wheel torque in the acceleration range they're targeting for the reminder plaid models is 10-15 kNm per wheel.
Elon specifies explicitly "direct drive", meaning no gear. Model 3 motor has peak power 250 kW and peak torque 450 Nm:
https://en.wikipedia.org/wiki/Tesla_Model_3
Powerwise, they need 6 motors. Unfortunately, my "back of envelope" implies torque at the MNm range. Either from Elon's 1.5 MW and assuming angular velocity in the order 1/sec. Or, from assuming 5g decelleration of a 100 metric ton spacecraft, which implyies 5 MN decelleration force. If one flap carries 1 MN, and we assume a moment arm in the order of 1 m, then we arrive to MNm, again. Which is in clear contrast to the cc. 3 kNm torque of 6 Model 3 motors. So, I don't know.

Offline niwax

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #31 on: 12/31/2019 03:29 pm »
Quote
Direct drive using several Tesla Plaid motors in parallel for SN1. Simpler, lighter & more fault tolerant. Rear flaps each need ~1.5 megawatts. It’s like moving the entire wing of an aircraft!
This tells me the motor case or it's shaft will be attached to the flap, and the other attached to the body. The motor won't even be able to make a half revolution. On the other hand it will be able to move that flap fast.

Can those motors produce the kN-m range torques needed to twist the flap against airflow? I think they will need extra gear reduction drives. Musk mentioned worm gears at the Mk.1 event, IIRC.

Wheel torque in the acceleration range they're targeting for the reminder plaid models is 10-15 kNm per wheel.
Elon specifies explicitly "direct drive", meaning no gear. Model 3 motor has peak power 250 kW and peak torque 450 Nm:
https://en.wikipedia.org/wiki/Tesla_Model_3
Powerwise, they need 6 motors. Unfortunately, my "back of envelope" implies torque at the MNm range. Either from Elon's 1.5 MW and assuming angular velocity in the order 1/sec. Or, from assuming 5g decelleration of a 100 metric ton spacecraft, which implyies 5 MN decelleration force. If one flap carries 1 MN, and we assume a moment arm in the order of 1 m, then we arrive to MNm, again. Which is in clear contrast to the cc. 3 kNm torque of 6 Model 3 motors. So, I don't know.

Flaps will need to carry much, much less force than that, they make up a fraction of the body. They can also be folded back to limit peak deceleration or stresses on the flaps. In their skydive videos, they were never fully extended during reentry, that's not necessary as control surfaces.

His comment regarding direct drive was in response to a question about hydraulic actuators. I would expect them to have at least their standard integrated gear box.
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Offline SteveU

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #32 on: 12/31/2019 03:46 pm »

[snip]

His comment regarding direct drive was in response to a question about hydraulic actuators. I would expect them to have at least their standard integrated gear box.
There will have to be gearing if for no other reason to enable accurate & repeatable flap movements.  X shaft revolutions per degree of flap movement.  Can't get that kind of control from an electric motor over fractions of a revolution.

Not to mention the added benefit of trading motor revolutions for increased force.
[edit - grammar]
« Last Edit: 12/31/2019 03:47 pm by SteveU »
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Offline Eka

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #33 on: 12/31/2019 06:39 pm »

[snip]

His comment regarding direct drive was in response to a question about hydraulic actuators. I would expect them to have at least their standard integrated gear box.
There will have to be gearing if for no other reason to enable accurate & repeatable flap movements.  X shaft revolutions per degree of flap movement.  Can't get that kind of control from an electric motor over fractions of a revolution.

Not to mention the added benefit of trading motor revolutions for increased force.
[edit - grammar]
Do you need exactness? Or does the AI autopilot move the motor based on effect change wanted, feedback on what happened, and generally where flaps are?

Oh, I should mention, given the Tesla Model 3 motor teardown I saw, I have confidence it could be controlled to less than degree motions when pushing a flap against air. The real interesting thing is it will freely swing, and that can allow for consistent force despite going through thicker and thinner patches of the atmosphere.
We talk about creating a Star Trek future, but will end up with The Expanse if radical change doesn't happen.

Offline OTV Booster

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #34 on: 12/31/2019 06:46 pm »
Quote
Direct drive using several Tesla Plaid motors in parallel for SN1. Simpler, lighter & more fault tolerant. Rear flaps each need ~1.5 megawatts. It’s like moving the entire wing of an aircraft!
This tells me the motor case or it's shaft will be attached to the flap, and the other attached to the body. The motor won't even be able to make a half revolution. On the other hand it will be able to move that flap fast.

Can those motors produce the kN-m range torques needed to twist the flap against airflow? I think they will need extra gear reduction drives. Musk mentioned worm gears at the Mk.1 event, IIRC.


Musk said direct drive. An electric motor develops max torque at 0 rpm. From https://www.google.com/amp/s/www.motor1.com/news/384052/tesla-cybertruck-plaid-output-report/amp/

“According to Motor Trend, the Plaid-powered Cybertruck will likely produce 800 horsepower (597 kiloWatts) and 1,000 pound-feet (1,356 Nm) of torque.”

The article went on to say Cybertruck will have three motors. I think the 597kW is all three.

At 1.5mW per side and 199kW per motor this comes out to 7.5 motors per side. Did I get the decimals right? Didn’t see motor mass on a quick pass.

Phil

Edit: 452 Nm torque/motor.
« Last Edit: 12/31/2019 06:52 pm by OTV Booster »
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Offline scdavis

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #35 on: 01/01/2020 01:55 am »
I work for a company that is a leader in direct drive motors. By definition it means that the torque is directly coupled to the load - no gearing, pulleys or shafts. Servo motors can maintain fraction of arc second accuracy, much less degrees. That is not a problem.

Whether they can actually get sufficient torque in this manner is a very interesting question.

Offline Jcc

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #36 on: 01/01/2020 08:03 pm »
I work for a company that is a leader in direct drive motors. By definition it means that the torque is directly coupled to the load - no gearing, pulleys or shafts. Servo motors can maintain fraction of arc second accuracy, much less degrees. That is not a problem.

Whether they can actually get sufficient torque in this manner is a very interesting question.

Ok but physically how do you couple six motors directly inline with the shaft of the flap? Unless it is a split shaft in several segments.

Offline Norm38

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #37 on: 01/01/2020 08:10 pm »
SpaceX will have the budget for the highest accuracy encoders and drives, but from a motor control standpoint, control is much, much easier if there is some gear reduction between the motor and the load.

On a car or a washing machine, you don't need stepper motor behavior, don't need to control wheel rotation accurately to degrees to drive the car. Direct drive can work fine.
But on a control surface, position must be accurate, and aero forces on the flaps pushing back against the motor can create a lot of encoder noise.  When encoder values are changing the opposite direction of what the drive is commanding, the drive either has to shut down or the drive has to be able to interpret and ride through the disturbance.  Even a 10:1 gearbox makes a big difference on reducing backlash to the encoder.

I would not be surprised if they end up with some gearing.

Offline garcianc

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #38 on: 01/01/2020 09:15 pm »
Perhaps this paper is relevant to this discussion.

It discusses the use of electric linear actuators in submarine control surfaces.

Also, regarding the force and torque requirements, in submarine control surfaces we use mechanical linkages that act as force amplifiers to allow these very large surfaces to move very quickly underwater. The submarines where I served used hydraulic linear actuators, but I have no reason to believe that the same could not be achieved with electric actuators these days, though I no longer have first-hand knowledge of what is current (plus I would probably not be able to discuss).

I am not an expert in aerodynamics or rockets, so please forgive my constant submarine analogies, it is one of the few things that I feel somewhat knowledgeable about and hopefully I am contributing to this discussion at least a bit.

Offline OTV Booster

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Re: SpaceX Starship/Super Heavy Engineering General Thread 4
« Reply #39 on: 01/01/2020 09:36 pm »
SpaceX will have the budget for the highest accuracy encoders and drives, but from a motor control standpoint, control is much, much easier if there is some gear reduction between the motor and the load.

On a car or a washing machine, you don't need stepper motor behavior, don't need to control wheel rotation accurately to degrees to drive the car. Direct drive can work fine.
But on a control surface, position must be accurate, and aero forces on the flaps pushing back against the motor can create a lot of encoder noise.  When encoder values are changing the opposite direction of what the drive is commanding, the drive either has to shut down or the drive has to be able to interpret and ride through the disturbance.  Even a 10:1 gearbox makes a big difference on reducing backlash to the encoder.

I would not be surprised if they end up with some gearing.

Gut response without numbers: the control will not need to be all that tight. Once the fins have aero loading all the backlash, if any, will be taken up. Direct drive has little or no backlash.

To hold the fins fixed against load will require power. Slippage should result in some detectable back EMF (probably the wrong term). If the slippage is too slow to detect this way there will eventually be an impact on attitude. The computer will not care why the attitude is changing. It will throw in a correction.

Encoders will be there for coarse positioning and maybe limit switches for ‘if all else fails’.

In small aircraft there are degree settings for flaps. For everything else there is airspeed, rate of descent and maybe pitch angle. The pilot doesn’t know or care what angle the control surfaces are at.

It would be interesting if anybody, active or lurking, has experience programming autopilots. The old analog autopilots had to work with what the bird was doing, not control surface angles.

Phil
We are on the cusp of revolutionary access to space. One hallmark of a revolution is that there is a disjuncture through which projections do not work. The thread must be picked up anew and the tapestry of history woven with a fresh pattern.

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