Author Topic: General Falcon and Dragon discussion (Thread 14)  (Read 469162 times)

Offline meekGee

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #120 on: 01/24/2017 02:58 pm »
Begin by increasing accuracy for current F9 RTLS. Paint four small boxes on the landing pad and try to bullseye the leg tips. Explore enchancements to the landing radar + software, rinse, repeat. A low-cost way to start.

If successful, graduate to four receptacles for the leg tips. Then, maybe, consider a portable on-pad cradle, at which point you build a core with fins/cradle guides instead of legs.
My guess is that first attempts will be on a dedicated, non-pad cradle.

Either at spaceport America, or a new pad area at the cape.

Clocking of the stage is controlled today I believe, and is easy to do with the grid fins. (Low inertia, non-coupled dof)

Vids have minor discrepancies, always, but they generally show the plan.  Nobody invented the cradle just for the movie.  They probably figured that handling that size vehicle after landing is nuts.

The idea of a rocket remaining vertical throughout its lifetime is very appealing to me.  I don't like the constant rotation to horizontal.

I also like the integrated second stage / payload approach, loaded together on top of the first stage. Fewer first stages on a short turn around cycle, more second stages /payloads on a longer turn around cycle.

I was advocating both ideas for the eventual evolution of the F9, but clearly SpaceX development focus will move on from F9 to ITS, and I can't be more excited.
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Online Robotbeat

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #121 on: 01/24/2017 03:03 pm »
Masten Space Systems achieved the required accuracy as part of the lunar lander challenge. It's possible SpaceX may have to forgo hover slamming in order to get accurate enough, so the system has time to null all the rates. For the first flights, at least. The bad part of that isn't so much the performance loss as the wear and tear of hovering a rocket right about the cradle mechanism.

Full throttlable control so that you can take some time to set down gently is essential.  The F9 style hover slam is not going to get it there.

Edit: spelling
It may, eventually. But getting it to work at all is going to be more important at first. And honestly, I think F9 can actually hover, they just don't do it for every flight.
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Offline Jim

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #122 on: 01/24/2017 03:08 pm »
ITS doesn't have a separable payload or fairing.  The second stage is the payload.  Just like Corona/Discoverer and Agena.  It is not the same as placing a placing a payload on a second stage, enclosing it in fairing and then placing it all on the first stage.

Grid fins have no control during the last few seconds
« Last Edit: 01/24/2017 03:09 pm by Jim »

Offline meekGee

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #123 on: 01/24/2017 03:46 pm »
Masten Space Systems achieved the required accuracy as part of the lunar lander challenge. It's possible SpaceX may have to forgo hover slamming in order to get accurate enough, so the system has time to null all the rates. For the first flights, at least. The bad part of that isn't so much the performance loss as the wear and tear of hovering a rocket right about the cradle mechanism.

Full throttlable control so that you can take some time to set down gently is essential.  The F9 style hover slam is not going to get it there.

Edit: spelling
It may, eventually. But getting it to work at all is going to be more important at first. And honestly, I think F9 can actually hover, they just don't do it for every flight.
"Hovering is for humans"

If flight control computers could choose a bumper sticker, that'd be it.

When you hover you just give winds and other unknowns more time to influence​ your position.

The best way to increase X-y accuracy is to hit the pad at higher velocity.  Less time for unknown forces to act and unlike a human, a computer doesn't need to hover nearby to estimate the IIP.

The only problem is that a you try to hit the pad at higher decelerations, you increase your Z uncertainty.

The remedy for this is increase Z travel on capture.  Which means a heavier mechanism.

This is the gain of the cradle - you don't have to carry the landing gear with you, you leave it on the ground.  Make it as tall and heavy as you want, to absorb as much residual vertical velocity as you need.

My bet is that we'll see more slam, and less hover, but that because of the increased physical size, it'll look about the same.

----

As for integrated second stage /cargo hold - that's the only way you can achieve multiple-per-day launches.

Separate payload/fairing integration works for one launch per week, tops.  Any kind of rapid reuse must have a "payload bay mentality",which means the payload bay is part of the vehicle.  (Whether tanker, cargo, or people)
« Last Edit: 01/25/2017 02:06 pm by meekGee »
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Offline Lar

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #124 on: 01/24/2017 04:21 pm »
honestly, I think F9 can actually hover, they just don't do it for every flight.
I thought conventional wisdom was that even one engine would not throttle down far enough to hover with the kinds of fuel loads anticipated to be remaining when returning... did I miss something?

As for integrated second stage /cargo hold - that's the only way you can achieve multiple-per-day launches.

Separate payload/fairing integration works for one launch per week, tops.  Any kind of rapid reuse must have a "payload bay mentality",which means the payload bay is part of the vehicle.  (Whether tanker, cargo, or people)
Not sure I agree for cargo. Turnaround time for container flats is WAY faster than for boxcars.... it's not a payload bay (alone) that will give you the fastest turnaround, at least for loose cargo or bulk cargo, it's containerized lading where the container, preloaded, fits into the waiting receiver (payload bay)

Yes you pay a weight penalty. So what? Optimize for cost not weight.

... but that's ITS, not F9. F9 no doubt will remain S1, S2, Payload
« Last Edit: 01/24/2017 04:25 pm by Lar »
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Online Robotbeat

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #125 on: 01/24/2017 04:41 pm »
Masten Space Systems achieved the required accuracy as part of the lunar lander challenge. It's possible SpaceX may have to forgo hover slamming in order to get accurate enough, so the system has time to null all the rates. For the first flights, at least. The bad part of that isn't so much the performance loss as the wear and tear of hovering a rocket right about the cradle mechanism.

Full throttlable control so that you can take some time to set down gently is essential.  The F9 style hover slam is not going to get it there.

Edit: spelling
It may, eventually. But getting it to work at all is going to be more important at first. And honestly, I think F9 can actually hover, they just don't do it for every flight.
"Hovering is for humans"

If flight control computers could goose a bumper sticker, that'd be it.

When you hover you just give winds and other unknowns more time to influence​ your position.

The best way to increase X-y accuracy is to hit the pad at higher velocity.  Less time for unknown forces to act and unlike a human, a computer doesn't need to hover nearby to estimate the IIP.

The only problem is that a you try to hit the pad at higher decelerations, you increase your Z uncertainty.

The remedy for this is increase Z travel on capture.  Which means a heavier mechanism.

This is the gain of the cradle - you don't have to carry the landing gear with you, you leave it on the ground.  Make it as tall and heavy as you want, to absorb as much residual vertical velocity as you need.

My bet is that we'll see more slam, and less hover, but that because of the increased physical size, it'll look about the same.

----

As for integrated second stage /cargo hold - that's the only way you can achieve multiple-per-day launches.

Separate payload/fairing integration works for one launch per week, tops.  Any kind of rapid reuse must have a "payload bay mentality",which means the payload bay is part of the vehicle.  (Whether tanker, cargo, or people)
Landing gear are heavy, and it's nice to leave the mechanism on the ground, however I think the primary benefit here is the fact that you don't have to process the landed stage. You don't need to hoist that huge stage with cranes, you don't have to reset the legs, you don't have to integrate it on to the launch hold-downs. It's a giant jig to make integration faster and easier.

A primary enabler for truly rapid turnaround. That's the biggest benefit, not the reduction in dry mass.
« Last Edit: 01/24/2017 04:43 pm by Robotbeat »
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Offline Jim

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #126 on: 01/24/2017 04:46 pm »
Cradle and hold downs are feasibly exclusive

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #127 on: 01/24/2017 04:47 pm »
How many days does it take SpaceX to take a landed stage, remove the legs, put it on a carrier, and bring it back to the HIF, let alone the launch pad?

ITS will be bigger and so would take even longer if it used legs. But they want to reduce the turnaround time to just hours. Almost the only way to do that would be with a landing cradle.
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Offline Lar

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #128 on: 01/24/2017 04:50 pm »
Cradle and hold downs are feasibly exclusive
I only partied with Mechanical Engineers in college so I might be missing something but why? If there's a flame trench below the cradle, why can't the cradle have hold down pins that are driven home after the stage is in the cradle, and that are released in the normal way?
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Offline Lars-J

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #129 on: 01/24/2017 04:57 pm »
Masten Space Systems achieved the required accuracy as part of the lunar lander challenge. It's possible SpaceX may have to forgo hover slamming in order to get accurate enough, so the system has time to null all the rates. For the first flights, at least. The bad part of that isn't so much the performance loss as the wear and tear of hovering a rocket right about the cradle mechanism.

Full throttlable control so that you can take some time to set down gently is essential.  The F9 style hover slam is not going to get it there.

Not this fallacy again...

EDIT: To clarify, I don't think "full throttlable control" is necessary to improve landing accuracy by adding hover time (*that* is the fallacy), but softening the impact will be useful.
« Last Edit: 01/24/2017 06:30 pm by Lars-J »

Offline Jim

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #130 on: 01/24/2017 05:04 pm »
Cradle and hold downs are feasibly exclusive
I only partied with Mechanical Engineers in college so I might be missing something but why? If there's a flame trench below the cradle, why can't the cradle have hold down pins that are driven home after the stage is in the cradle, and that are released in the normal way?

There aren't pins.  They are clamps and they have to hold a vehicle during static fire.  Just the opposite catching a vehicle during hoverslam.

Offline Lar

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #131 on: 01/24/2017 05:07 pm »
Cradle and hold downs are feasibly exclusive
I only partied with Mechanical Engineers in college so I might be missing something but why? If there's a flame trench below the cradle, why can't the cradle have hold down pins that are driven home after the stage is in the cradle, and that are released in the normal way?

There aren't pins.  They are clamps and they have to hold a vehicle during static fire.  Just the opposite catching a vehicle during hoverslam.
So land with the clamps wide open. (think baby bird bills waiting to receive their gifts from momma) Once nestled in the cradle, engage the clamps. Might need more distance of travel on the clamp jaws than in the non cradle case but not undoable..

Might be veering offtopic, I think there's a thread here somewhere for this so I'll stop.
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Online Robotbeat

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #132 on: 01/24/2017 05:18 pm »
The cradle will do doubt be a GSE nightmare. But if you can solve it....
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Offline raketa

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #133 on: 01/24/2017 07:00 pm »
Their landing is accurate, that I have no doubt, that will start to test their cradle technology.

Based on what?  Current accuracy is not enough for a cradle.

I think the final version of the cradle will be like a cone that will be 50% - 100% wider than rocket width. And on the side of the rocket will have fins to adjust the final position. Bottom part of rocket will be design to dump vertical speed.
They couldn't expect every time to be inch precision landing. But base on the previous landing precision by 5-10 feet is already achieved.
« Last Edit: 01/24/2017 07:09 pm by raketa »

Offline guckyfan

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #134 on: 01/24/2017 09:46 pm »
The cradle will do doubt be a GSE nightmare. But if you can solve it....

Maybe something like this.


Offline DanielW

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #135 on: 01/24/2017 10:45 pm »
The cradle will do doubt be a GSE nightmare. But if you can solve it....

Maybe something like this.



The whole point of a this exercise is to NOT throw the boosters away.  ;D

Offline IainMcClatchie

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #136 on: 01/25/2017 08:44 am »
The catch mechanism is gonna be awesome.  The ITS booster is slated to have 275 tonnes dry mass, and three legs.  So each mechanism has to halt something like 100 tonnes.

Tilt is significant, but less than the Falcon 9 first stage.  The Falcon 9 first stage has 26 tonnes spread over 175 m^2 side cross section (149 kg/m^2).  ITS booster has around 300 tonnes spread over 930 m^2 (323 kg/m^2).  So we should expect the ITS booster to land with half the crosswind-induced tilt of the Falcon 9 stage 1.  Note that this the the required tilt once crosswind velocity has been nulled out.  While cancelling that velocity, the tilt will be just as bad as Falcon, but that happens a second or two before touchdown.

If the vehicle is landing in e.g. a 40 knot crosswind, it's going to have a lateral force on it of 120 kN = 0.4 m/s^2.  Assume a landing thrust of 2 Gs, that's a tilt of 1.1 degrees.  Across 12 meters diameter, that's a tilt of 24 cm.  Assume the Cg is 28 m from the bottom (those Raptors have 200:1 thrust to weight), that's a 93 cm lateral shift of the Cg relative to the landing pads.  If that's towards one landing pad, that pad will have to halt 130 tonnes.  What is more significant is that the wind directly away from that pad produces a torque which will lift up 1555 kN on the pad, enough to overturn the vehicle.  The vehicle tips over in 31 knots of wind.  This means the landing catch may have to pull down hard (~500 kN) on one of the landing pins before the vehicle has come to a complete halt.  That means the pin capture mechanism has to actuate before the braking mechanism finishes.  Pin capture has to finish in something like 200 milliseconds!

DOT maximum descent rate for aircraft landings is 10 feet/second (25.473).  Decelerating at 2 Gs requires 0.31 sec requires 47 cm of travel.

Righting the vehicle during the 0.31 sec stop from tilt due to 40 knot crosswind requires 0.9 rad/s^2 angular acceleration.  Hardly any of this is going to happen, as the forces required on the landing pins are an order of magnitude larger than the braking forces.  Instead, the vehicle will halt while tilted and then straighten up over the next few seconds.  I'm not clear how the Falcon 9 manages tilt when landing, as it seems the legs would want to right the vehicle while stopping, and that's just terrible for structures this large.  This might even be the single biggest reason to get rid of the landing legs.

It seems 1 meter of vertical travel after catch should be sufficient to accomodate tilt, catch, and braking.

Offline Herb Schaltegger

Re: General Falcon and Dragon discussion (Thread 14)
« Reply #137 on: 01/25/2017 12:35 pm »
This is fascinating and all, but since this thread is "General Falcon and Dragon Discussion," can we please get back to that? Thanks.
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Offline Jim

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #138 on: 01/25/2017 12:54 pm »
The catch mechanism is gonna be awesome. 

Yes, fantasy items always are

Online Robotbeat

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Re: General Falcon and Dragon discussion (Thread 14)
« Reply #139 on: 01/25/2017 01:54 pm »
The catch mechanism is gonna be awesome. 

Yes, fantasy items always are

Is someone collecting posts like this so we can compare them with reality in the future?
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