Author Topic: SpaceX Falcon 9 : CRS-16 (Dragon SpX-16) : December 5, 2018 - DISCUSSION  (Read 247846 times)

Online octavo

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I understand how just 1 engine is not able to counter a spin as it can only target one direction at a time, but having it gimble in certain way (counter clockwise?) could it not help counter some spin? And what is the effect of the jet hitting the water? Would it not feedback in some way that might also kill any movement relative to the water?
Or does the jet only give upward energy and no counter spin energy as feedback?

How could a center engine gimbal counterclockwise?

Huh? The engine can gimbal in any direction. If you gimbal from center to north and then north east, then east, then southeast, then south- you are in effect gimballing counterclockwise, no?

Offline Jakusb

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I understand how just 1 engine is not able to counter a spin as it can only target one direction at a time, but having it gimble in certain way (counter clockwise?) could it not help counter some spin? And what is the effect of the jet hitting the water? Would it not feedback in some way that might also kill any movement relative to the water?
Or does the jet only give upward energy and no counter spin energy as feedback?

How could a center engine gimbal counterclockwise?

Huh? The engine can gimbal in any direction. If you gimbal from center to north and then north east, then east, then southeast, then south- you are in effect gimballing counterclockwise, no?

That is indeed what I mean.. Also that energy should counter spin, would it not?
Only trouble would be that you need the gimbling to also steer the rocket to its intended landing spot..
So either they have foreseen this scenario and programmed to mitigate a spin (as much as possible), while steering at the same time...
The scenario is not that unthinkable, however countering a spin while steering does sound pretty complex, if not impossible... Then again, it clearly countered the spin in some way in the end.. ;)

My main question however is if it is theoratically possible there is feedback energy from the one jet hitting water? And such feedback that would also help counter spin...

Offline speedevil

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That is indeed what I mean.. Also that energy should counter spin, would it not?
No.
Take a screwdriver, place it in a screw, and move it in a cone around the screw. It does not tighten or loosen the screw.

Offline Jakusb

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That is indeed what I mean.. Also that energy should counter spin, would it not?
No.
Take a screwdriver, place it in a screw, and move it in a cone around the screw. It does not tighten or loosen the screw.

Ok, stand on a turning chair and sling something around... ;)
The engine gimbals outward not inward...
Some weight (engine bell) is effectively moved around the center axis..

Online octavo

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That is indeed what I mean.. Also that energy should counter spin, would it not?
No.
Take a screwdriver, place it in a screw, and move it in a cone around the screw. It does not tighten or loosen the screw.

In this scenario the gimballing engine would be the screw, not the screwdriver. I'm not sure how you would get a screwdriver straight and get a screw to wobble in a cone under the screwdriver. That's why it's not a great analogy.

Offline CorvusCorax

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He's actually right. You could use both the engine bell and the exhaust volume inside it as an impromptu "reaction wheel" by constantly actuating in a circular matter. And since the gas volume is being continuously expelled it even acts a bit like a yoyo despin-weight in fluid form, so it would be able to give a continuous rotational force without saturating. You rotate a gas volume relative to the core, then get rid of it. (The resulting gas jet would have spiral shape and expand away from the core)

The effect is going to be miniscule and orders of magnitude below the aerodynamic effect of the grid fins or the effects achievable if the rotation axis goes NOT through the gimbal bearing. But if you were in vacuum and this is the only effect you can use, it might actually work.

It has no practical application to this particular core landing, but I don't think you can completely neglect it in vacuum. ;)


Offline CorvusCorax

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« Last Edit: 12/11/2018 11:25 am by CorvusCorax »

Offline Jakusb

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He's actually right. You could use both the engine bell and the exhaust volume inside it as an impromptu "reaction wheel" by constantly actuating in a circular matter. And since the gas volume is being continuously expelled it even acts a bit like a yoyo despin-weight in fluid form, so it would be able to give a continuous rotational force without saturating. You rotate a gas volume relative to the core, then get rid of it. (The resulting gas jet would have spiral shape and expand away from the core)

The effect is going to be miniscule and orders of magnitude below the aerodynamic effect of the grid fins or the effects achievable if the rotation axis goes NOT through the gimbal bearing. But if you were in vacuum and this is the only effect you can use, it might actually work.

It has no practical application to this particular core landing, but I don't think you can completely neglect it in vacuum. ;)

What about the effect of the jet hitting (not rotating, thus counter moving) water?
Would it move the water in rotating motion, or would it have no effect at all?
If it would move the water, it would also feedback to the core, would it not?

Or would the rotating energy of the jet hitting the water have no (significant) effect at all?

Offline CorvusCorax

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He's actually right. You could use both the engine bell and the exhaust volume inside it as an impromptu "reaction wheel" by constantly actuating in a circular matter. And since the gas volume is being continuously expelled it even acts a bit like a yoyo despin-weight in fluid form, so it would be able to give a continuous rotational force without saturating. You rotate a gas volume relative to the core, then get rid of it. (The resulting gas jet would have spiral shape and expand away from the core)

The effect is going to be miniscule and orders of magnitude below the aerodynamic effect of the grid fins or the effects achievable if the rotation axis goes NOT through the gimbal bearing. But if you were in vacuum and this is the only effect you can use, it might actually work.

It has no practical application to this particular core landing, but I don't think you can completely neglect it in vacuum. ;)

What about the effect of the jet hitting (not rotating, thus counter moving) water?
Would it move the water in rotating motion, or would it have no effect at all?
If it would move the water, it would also feedback to the core, would it not?

Or would the rotating energy of the jet hitting the water have no (significant) effect at all?

None. Energy or torque exchanged between air and water has no effect whatsoever on the core. Since the exhaust is entirely supersonic, there is no force transfer "backwards" through the gas stream, and the inflicted forces against the engine bell and thrust chamber are exactly the same, regardless of whether the gas jet hits a shockwave during supersonic retropropulsion, goes into vacuum, or hits a water surface. There is no change in chamber pressure.

There might be a miniscule effect once the jet+steam+water droplets reflected by the water surface briefly hits parts of the rocket such as the legs, but that is very minor compared to the legs touching the liquid water itself only a fraction of a second later.




Offline meekGee

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For a rocket in vacuum, a central engine can't affect spin.

When there are other non axial forces, like gravity and fin forces, the central engine creates force pairs with all of then and so creates torques in all directions and affects axial spin.

If the control system didn't take that into account, they'd get unexpected spin during engine operation.



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ABCD: Always Be Counting Down
« Last Edit: 12/11/2018 01:11 pm by meekGee »
ABCD - Always Be Counting Down

Offline Kabloona

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Yes, agreed on the observations.

I can't find a good estimate on the force of the thrusters.  It's v*dm/dt, and v is probably 500-600, but I can't find dm/dt.   Given how quickly the thrusters flip a still-loaded stage around, they're not chump change, and 12 seconds is a lot of time.   From this, I draw the same conclusion you did - that during most of the time, the fins overwhelm the thrusters.

But the fins lose effectiveness towards the end (probably very non-linearly), and then both things happen - the legs deploy and cut the rotation speed as I calculated above, and the thrusters finally get their way.

As for the main engine - as explained by several people above - when there are external forces on the booster (such as gravity, aerodynamic loads, etc) a center engine can absolutely affect axial spin.  I think during that time, the fins are powerful enough that only something like the main engine can really fight them.

As an aside - I'm surprised by how much spare capacity the RCS thrusters had.

We might be able to get ballpark RCS thrust figures by considering the proposed application to Tesla roadsters. Everyday Astronaut did a writeup on that, in which he cited "speculation" that the thrusters were around 1,000-2,000 lbf.

https://everydayastronaut.com/spacex-rockets-on-the-new-tesla-roadster-seriously/

That seems reasonable in light of Musk's notion of putting 10 thrusters on a Tesla, and his somewhat tongue-in-cheek statement that the thrusters could enable "short hops," presumably meaning that if you put one downward-pointing thruster at each corner of the car, you could get T/W>1.

So let's assume as a minimum that the output of 4 thrusters > weight of one Tesla roadster. Add a driver and call the roadster 3,000 pounds loaded. Then if each thruster is 1,000 lbf, 4 thrusters gives you 4,000 lbf, for a T/W=4,000/3,000=1.33, definitely enough to achieve liftoff.

But of course, thrust depends on chamber pressure, and the thrusters may have higher thrust on F9's and lower thrust on roadsters, simply by using lower-pressure gas on the roadster for convenience (higher safety margin, less energy required to repressurize tanks while driving, etc) But Musk did say the Tesla thrusters would run on "ultra high pressure air," and "ultra high" sounds to me like F9 pressure territory.

So if if we assume that Tesla thrusters would run at the same "ultra high pressure" as F9 thrusters, 1,000 lbf sounds like a reasonable minimum, given that 4 placed at the corners would, in fact, enable the "short hops" that Elon mentioned.
« Last Edit: 12/11/2018 01:20 pm by Kabloona »

Offline The Vorlon

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I would be REAL respectful of a tank that was at 1K PSI.  Just saying...

Offline Herb Schaltegger

I would be REAL respectful of a tank that was at 1K PSI.  Just saying...
Scuba tanks run 3 times higher than that and they get clanked around on docks and boat decks all over the world every minute of every day. Nothing special about 1,000 PSI per se. Only how robust the tank and fittings are (or aren’t) really matters.
« Last Edit: 12/11/2018 03:46 pm by Herb Schaltegger »
Ad astra per aspirin ...

Offline CorvusCorax

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For a rocket in vacuum, a central engine can't affect spin.

When there are other non axial forces, like gravity and fin forces, the central engine creates force pairs with all of then and so creates torques in all directions and affects axial spin.

If the control system didn't take that into account, they'd get unexpected spin during engine operation.
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ABCD: Always Be Counting Down

The thrust force of a engine does not  affect spin on an axis which goes through that engine's center of thrust (the way the roll axis goes through the center engine on F9)

ignore the thrust for a moment.
The engine has 2-axis TVC, which means the engine can be tilted in both pitch and yaw direction. This means the mass of the engine bell can be moved slightly off-axis in two directions.

If you gyrate the TVC around its center position in a circular fashion, you are moving (the) mass ( of the engine bell) in a circle. Doing so generates torque.

This is very similar to a reaction wheel. You can create a torque force if you accelerate or decelerate this gyrating movement, but the force is not continuous, only momentarily while you accelerate or decelerate the gyration.

Now enter thrust:

Under thrust the engine bell is filled with gas under pressure. This gas is mostly accelerating backwards, propelling the vehicle forward. But the gyration of the engine bell also imparts a circular motion on this gas volume, while it traverses the engine bell. As such it carries a small but measurable rotational momentum

This gas continuously exits the engine bell out the rear and takes this rotational momentum with it. This is visible to an outside observer in that the exhaust stream now forms an expanding spiral pattern instead of straight backwards.

That means, while under thrust, the gyrating engine bell imparts momentum continuously on the exhaust gas, resulting in a torque force (measurable in higher force needed to move the TVC in a circle than there would be if there was no thrust, thanks to the extra mass of the gas volume that needs to be accelerated) this torque force in turn adds or substracts from the vehicles rotational momentum, spinning it up or slowing it down.

the effect is very small due to the small time the gas spends within the engine bell and the small extra acceleration the movement of the engine bell due to TVC actuation imparts on the gas compared to the overall thrust. But in vacuum this effect would be measurable and possibly usable.


Offline edzieba

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Yep, 300 Bar (4350 PSI) is something I'd be happy to be around in a nice hefty steel tank. In a thin walled CoPV designed to be as light as possible without popping and making a big mess? Somewhat less happy.

On an unrelated note: Has anyone caught a photo of the inside of the interstage from the 'right' side (with the booster orientation assumed to be damaged-part-down)? John Kraus' fantastic photos (from the left side) during towing tantalisingly show the hydraulic feed lines vanishing under the left lip of the interstage, but hide the hydraulic equipment itself from view. There would be a lot of mysteries clarified by seeing how the grid-fin hydraulics are configured.

Offline OxCartMark

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If we're heading down the road of thruster thrust be mindful that its proportional (or at least related) to tank pressure and whatever pressure was available during the flip maneuver is likely to be significantly reduced by the time it gets close to landing in a normal landing and in this case where there was an ongoing battle between grid fins and thrusters I'd expect the pressure to be lower than what the designers had hoped for in their lowest pressure scenario.  Or not, maybe the tanks are vastly larger than necessary.
Actulus Ferociter!

Offline meekGee

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If we're heading down the road of thruster thrust be mindful that its proportional (or at least related) to tank pressure and whatever pressure was available during the flip maneuver is likely to be significantly reduced by the time it gets close to landing in a normal landing and in this case where there was an ongoing battle between grid fins and thrusters I'd expect the pressure to be lower than what the designers had hoped for in their lowest pressure scenario.  Or not, maybe the tanks are vastly larger than necessary.
Good point.

Damn that system is complicated.

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ABCD: Always Be Counting Down

ABCD - Always Be Counting Down

Offline Johnnyhinbos

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I would be REAL respectful of a tank that was at 1K PSI.  Just saying...
Scuba tanks run 3 times higher than that and they get clanked around on docks and boat decks all over the world every minute of every day. Nothing special about 1,000 PSI per se. Only how robust the tank and fittings are (or aren’t) really matters.


Well - true, and yet sadly people do die or get maimed by SCUBA tanks every year. An acquaintance of mine is in possession of the tank from the following incident from January.

Quote
Yesterday evening,There was an explosion of a dive tank on the Utila Cays,causing Mr Newton Diamond to lose his leg and Keily Pineda whom loss her life.

Mr Newton suffered a massive amount of blood loss and needed to be taken to La Ceiba immediately. Utila is a small island and news travels very fast,and being our airport doesn’t have lights,everyone with a vehicle got together and went up on the airport to shine the way for the airplane to takeoff.
He made it to La Ceiba safe where he was interned in the hospital for surgery. We’re happy to announce that he’s doing good,he will have a long and slow recovery but we have faith that he will be good.

Keily was only 16 years old and she loss her life in this tragic accident. Her family is from the mainland but she’s born and raised on the Utila Cays and her body will be laid to rest here.

We’d like to thank the entire community for coming together when called upon to help in time of need and also a special thanks to Captain Clint Gerner for your rapid response and generous contribution to saving Mr Newton’s life. We will forever be grateful for what you did for our community.

Thank you all.
-Utila 911 team
« Last Edit: 12/11/2018 04:02 pm by Johnnyhinbos »
John Hanzl. Author, action / adventure www.johnhanzl.com

Offline OxCartMark

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So if if we assume that Tesla thrusters would run at the same "ultra high pressure" as F9 thrusters, 1,000 lbf sounds like a reasonable minimum...

My guess would be that the Tesla thrusters run at whatever pressure your local scuba shop is willing to sell you, at least for the first shot.  Second shot at a lower pressure.  Third shot at a lower pressure.  And so on until you visit the scuba shop again. So maybe you go out onto an empty mall parking lot and try some really sharp high speed corners and are amazed at the 3G corners that it'll achieve.  Then after doing a few and getting the feel you go get your friend to show him / her (think of all the videos of passengers being shown Ludicrous acceleration) but by now the tank pressure is a small portion of what it was and you go off the road and into the forest and you and your friend get killed then re-killed by the COPV bursting.

That is one tank fill situation, fill it with air intermittently at the scuba shop.  But that would require that a) Tesla supplies a very long hose connect your car in the parking lot with the compressor in the shop, and b) that the tank be metal (vs. plastic composite) or that Tesla / SpaceX is OK with air (oxygen) in the composite tank, and c) that the tank be filled without being immersed in cooling water which most shops would probably not go along with, or d) that you would need to take the tank into the shop.

Other potential fill scenarios would be if the car had an onboard compressor (energy consumptive, inefficient, slow, potentially noisy), or you would be given a high pressure compressor at home.  And in in both cases they could choose to fill the tanks with air or with nitrogen separated from air.

Hmm, just thought of another option which seems to me very likely - You obtain liquid nitrogen locally and its stored in a dewar in the car to be warmed (hopefully with heat from a heat exchanger rather than battery) and the resulting high pressure gas stored for the next shot or two.

Now that I've given it some thought I've convinced me that the Spacex option package on Roadster is the case of Elon getting ahead of himself without thinking it through.  May the force be with the engineers he's assigned to make it happen.

Some may argue that I've digressed from strictly CRS-16 content in this post and that may be true.
Actulus Ferociter!

Offline Chris Bergin

Yeah, people are reporting to mod over this, so back on topic please (as in from this post onwards).
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