SpaceX Falcon 9 : CRS-16 (Dragon SpX-16) : December 5, 2018 - DISCUSSION

[CorvusCorax]

Yes, I think we all understand high school physics. can we move on now? 

Saltwater intrusion and re-use?

Better: Visible damage on various engine bells, potential origin and implications.

Assuming the engines are not affected by corrosion, which obviously caused surface discoloration.

Which damages can be banged out, ignored, or repaired.

Can the engine bell be easily replaced? It has regen cooling channels inside, right?

Aside from the massively dented in bell, at least one shows scratches/ dents which seem to come from the chains used during salvage.

How severe is thermal shock of being immereed in water while running? Would that crack it? center engine only or all?

[Rocket Science]

Yes, I think we all understand high school physics. can we move on now? 

Saltwater intrusion and re-use?

Better: Visible damage on various engine bells, potential origin and implications.

Assuming the engines are not affected by corrosion, which obviously caused surface discoloration.

Which damages can be banged out, ignored, or repaired.

Can the engine bell be easily replaced? It has regen cooling channels inside, right?

Aside from the massively dented in bell, at least one shows scratches/ dents which seem to come from the chains used during salvage.

How severe is thermal shock of being immereed in water while running? Would that crack it? center engine only or all?

Old H-1 test...
http://up-ship.com/blog/?p=5948

[meekGee]

>>> But increased MOI from leg deployment reduces effectiveness of RCS roll thrusters

This scratched my ears first time when I watched Scott's video and now it is on NASA forum.
IMO, this is not correct.

Remember momentum conservation law.

The speed of rotation has slowed down 1/2x, because MOI doubled after legs were deployed, so for the body to keep the same momentum, the speed of rotation should be halved.

RCS should fight against F9's momentum. It does not matter if legs are deployed or not. The momentum is the same (well provided we do not have any more forces adding to rotation, which in case of deployed legs is probabaly negative, that is the legs are slowing rotation down due to air resistance, and then due to water resistance).

My point is: RCS effectiveness in roll elimination have not changed with legs deployment.

Leg touching water, that I obviously buy.

Maybe that's why one leg was lost, too.

Did they ever fish it out?  I mean they have the exact surface coordinate, and a good estimate of surface currents...



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ABCD: Always Be Counting Down

Yes, the leg was removed by divers the first day that the booster was floating outside Port Canavera.

I forgot.  Much respect for the people who did that, btw, and on such short notice.

[codav]

Assuming the engines are not affected by corrosion, which obviously caused surface discoloration.
 of being immereed in water while running? Would that crack it? center engine only or all?

Which damages can be banged out, ignored, or repaired.

Ideally, salt water ingression into the plumbing should be quite minimal, as the Merlin engine uses pintle injectors. These stop both the RP-1 and LOX flows like a valve, so no water should be able to enter via the injector plate. The only way water might get into the engine is via the preburner exhaust. Even if salt water got inside, the materials used in the plumbing, turbopump etc. should not get corroded as they need to withstand the hot, almost pure oxygen from the preburner. Even the H-1 engine, which wasn't really built for reusability, could be refurbished for 5% the cost of a new engine. As SpaceX has a lot of experience with their engines, refurbishment will certainly be easier an cheaper.

As for the engine bells, those wwhich sustained heavier damage will be replaced for sure.

[Rocket Science]

Submerge the entire stage in a makeshift freshwater tank then attend to all the electrical systems... Be a great test for WD-40... Corporate sponsor for a test flight...

[whitelancer64]

Yes, I think we all understand high school physics. can we move on now? 

Saltwater intrusion and re-use?

Better: Visible damage on various engine bells, potential origin and implications.

Assuming the engines are not affected by corrosion, which obviously caused surface discoloration.

Which damages can be banged out, ignored, or repaired.

Can the engine bell be easily replaced? It has regen cooling channels inside, right?

Aside from the massively dented in bell, at least one shows scratches/ dents which seem to come from the chains used during salvage.

How severe is thermal shock of being immereed in water while running? Would that crack it? center engine only or all?

Old H-1 test...
http://up-ship.com/blog/?p=5948

Worthwhile to note that the salt water exposure times were 1, 2 and 9 hours for each test, and the engines were immediately cleaned afterwards.

Still, that's a reasonably good data point that could be hopeful for these Merlin engines.

Attached image from test.

[CorvusCorax]

Yes, I think we all understand high school physics. can we move on now? 

Saltwater intrusion and re-use?

Better: Visible damage on various engine bells, potential origin and implications.

Assuming the engines are not affected by corrosion, which obviously caused surface discoloration.

Which damages can be banged out, ignored, or repaired.

Can the engine bell be easily replaced? It has regen cooling channels inside, right?

Aside from the massively dented in bell, at least one shows scratches/ dents which seem to come from the chains used during salvage.

How severe is thermal shock of being immereed in water while running? Would that crack it? center engine only or all?

Old H-1 test...
http://up-ship.com/blog/?p=5948

Worthwhile to note that the salt water exposure times were 1, 2 and 9 hours for each test, and the engines were immediately cleaned afterwards.

Still, that's a reasonably good data point that could be hopeful for these Merlin engines.

Attached image from test.

Are you saying the times stated in the blog linked above are incorrect? 1, 2 and 9 matches the fully submerged, half submerged and on-water-surface times of test # 3 in 1962 in that document, but they also mentoon a 1 hour submerged, 2 hour half submerged, 2 hour surface and then a 12 hour waiting period before cleaning/flushing in test #2 1961. Was the same engine used for all 3 tests?

[OxCartMark]

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.

Are you assuming the cold gas thrusters are connected directly to the supply tank? I'm sure there is a pressure regulator between the supply tank and the thrusters. Providing a relatively constant pressure and thrust until the tank goes flat.

Why would you put a regulator in between the tank and thruster??  That would be a tremendous waste of energy, it would reduce thrust to its lowest planned level no matter what the available tank pressure.  Larger and heavier tanks would be needed.  Better to use whatever pressure is available and compensate in software ~ shorter bursts at high pressure and longer bursts at low pressure to obtain a desired impulse.

[envy887]

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.

Are you assuming the cold gas thrusters are connected directly to the supply tank? I'm sure there is a pressure regulator between the supply tank and the thrusters. Providing a relatively constant pressure and thrust until the tank goes flat.

Why would you put a regulator in between the tank and thruster??  That would be a tremendous waste of energy, it would reduce thrust to its lowest planned level no matter what the available tank pressure.  Larger and heavier tanks would be needed.  Better to use whatever pressure is available and compensate in software ~ shorter bursts at high pressure and longer bursts at low pressure to obtain a desired impulse.

In a vacuum where RCS is primarily used, exhaust velocity is constant for all chamber pressures, to first order. So using a regulator doesn't cost any energy, and allows the hardware to be optimized for a single chamber pressure rather than overbuilt for the max tank pressure.

[Brian45]

Watching the GPS III static fire video from US Launch Report, , at the end they added some "bonus" footage of the CRS-16 booster being processed for transportation.

They show the legs being removed. Was this done in this specific instance (because of the landing issues) and is not part of the normal post landing processing for the Block 5 booster? I thought that the legs would not need to be removed in normal circumstances.

[Lars-J]

Legs are still removed from all Block 5 boosters. They still have issues to work out before they can just fold them up.

[Jim]

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

Roll, which is a rotational on the longitudinal axis (body fixed coordinates).  Spin is not part of the discussion.  A spin is rotation in multiple axis.   

[Jim]

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.

Are you assuming the cold gas thrusters are connected directly to the supply tank? I'm sure there is a pressure regulator between the supply tank and the thrusters. Providing a relatively constant pressure and thrust until the tank goes flat.

Why would you put a regulator in between the tank and thruster??  That would be a tremendous waste of energy, it would reduce thrust to its lowest planned level no matter what the available tank pressure.  Larger and heavier tanks would be needed.  Better to use whatever pressure is available and compensate in software ~ shorter bursts at high pressure and longer bursts at low pressure to obtain a desired impulse.

In a vacuum where RCS is primarily used, exhaust velocity is constant for all chamber pressures, to first order. So using a regulator doesn't cost any energy, and allows the hardware to be optimized for a single chamber pressure rather than overbuilt for the max tank pressure.

And it provides a known thrust level. 

[meekGee]

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

Roll, which is a rotational on the longitudinal axis (body fixed coordinates).  Spin is not part of the discussion.  A spin is rotation in multiple axis.

Yes we clarified that before.  Lots of posts using the term spin informally, as in "spin about its axis" - meaning roll.  My posts sometimes too.

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ABCD: Always Be Counting Down

[meberbs]

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

Roll, which is a rotational on the longitudinal axis (body fixed coordinates).  Spin is not part of the discussion.  A spin is rotation in multiple axis.

Again, you cannot separate these concepts. Angular momentum (spin as you are calling it) is not fixed to body axes. The thing you are trying to discuss about a "roll" fixed to a body axis is not a thing that actually exists. The conversation is about reality, no matter how many times you claim otherwise, generic spin is inherent to the discussion.

Even if everything else was magically perfect, the engine itself could produce a torque which would make the spin off body axis, giving the engine the control authority to cancel the spin. (This may not be the best general solution when other control options exist, but it is a contribution.)

[kdhilliard]

Watching the GPS III static fire video from US Launch Report, at the end they added some "bonus" footage of the CRS-16 booster being processed for transportation.

Two other items of note in that video:
* At 3:57 the video fades between the "upper" leg being removed while the stage is resting on ground supports to the stage being secured to its transporter, and from the position of the dented engine nozzle it is clear that they rolled the stage about 135° in the process. (Link to 10s before the fade.)
* At 4:52 the video fades between the booster being secured to its transporter and an edge-on close-up of the engine bay showing a lot of plumbing where some shielding was removed.  (Link to 10s before the fade.)

Edit: Fixed link.  (Removed the "www." so the YouTube video doesn't embed.)

[AJW]

With aircraft, you are taught early on to do pre-flight control checks.  This is sometimes referred to as 'boxing the controls', because you run through each control and push it to the minimum and maximum range twice while also visually checking the control surface to verify proper movement.   You will see rocket engines go through a gimbal check to verify their movement for the same reasons.

Is SpaceX able to do a pre-flight control check of the grid fins?   This would require that they have the ability to retract which I haven't seen.   They were also originally built with an open hydraulic system.  If this is still the case, any tests would deplete the reservoir.

Anyone with knowledge on how or when this system is checked before a flight?

[Stan-1967]

The grid fins deploy when S1 is in free fall, and they are also carry some not insignificant mass.  I wonder if the hydraulics do not have power to deploy them when under 1g, therefore they can’t be ground checked when S1 is vertical under its own internal power.  Deployment also looks s bit non linear, like it is spring loaded or something.  Actual control inputs are rotational, and give 2 axis control only.

[Steven Pietrobon]

New Shepard tests its fins before launch. The fins on Falcon 9 now use closed loop hydraulics. It should be possible to test the fins using external power on Falcon 9 before launch. SpaceX may choose to do that in future.

[CorvusCorax]

The grid fins deploy when S1 is in free fall, and they are also carry some not insignificant mass.  I wonder if the hydraulics do not have power to deploy them when under 1g, therefore they can’t be ground checked when S1 is vertical under its own internal power.  Deployment also looks s bit non linear, like it is spring loaded or something.  Actual control inputs are rotational, and give 2 axis control only.

Afair, Falcon9 used to retract the fins on its own after landing, during saving procedure. They also extended them under 1 G on F9RDev1 flights, although those were lighter aluminum fins and a dev vehicle.

It could be something mundane like physically being in the way of the TE that prevents a full actuation test on the pad. Or they just didn't bother since the system wasnt mission critical until now.