Blue Origin Thread (merged)

[kerbiloid]

23 minutes ago, wumpus said:

the acceleration is higher, but the mass is lower (100*5>25*9).  The force is therefore less and you have less stress on the structure (I'm surprised it is that close.  Note that it has ~500t of force** just sitting on the pad and then taking off with a TWR of 1.3? makes it go even higher).

(Probably I can't really get what does confuse you in these calculations).

According to ( http://spaceflight101.com/spacerockets/falcon-9-v1-1-f9r/ ), in metric tonnes (i.e. 1000 kg)
1st stage: dry = 23-25 t , fuel = 396 t
2nd stage: dry = 4 t , fuel = 93 t
payload = 13 t

Merlins 654 kN at sea level, 716 kN in vacuum, 801 kN vacuum modification in vacuum.

So, T/W
on start = 9 * 654000 / (24000 + 396000 + 4000 + 93000 + 13000) / 9.81 = 1.13 g
on 1st stage burnout = 9 * 654000 / (24000 + 4000 + 93000 + 13000) / 9.81 = 4.5 g

Then 1st stage separates and just 1 Merlin works.
on return and landing 1st stage = 1 * 654000 / 24000 / 9.81 = 2.8 g
If 3 Merlins are on, then = 2.8 * 3 = 8.4 g
This means that 3 engines mode makes 2-3 greater stress (and though deformations) than any another usage case of the 1st stage.

Btw, meanwhile, 2nd stage:
in the beginning = 1 * 801000 / (4000 + 93000 + 13000) / 9.81 = 0.74 g
near finish = 1 * 801000 / (4000 + 13000) / 9.81 = 4.8 g

Edited June 15, 2016 by kerbiloid

[Rakaydos]

2 minutes ago, Damien_The_Unbeliever said:

So, assume one of the engines is contributing 0% of the thrust it's meant to, and it's not the centre engine. If you boost the other two engines to compensate, you have quite a lot of asymmetry to deal with. During normal (ascent) usage, you don't have to deal with this because it's always assumed that (given we're not talking about the centre engine, again) there's another engine opposite each engine and so we don't have to deal with any torque issues.

Do you design your entire set of engines to have far more gimbal to cope with that situation, or accept the loss?

Or quickly change to a 5 engine burn just long enough to sort out which engines are underperforming.

[PB666]

37 minutes ago, wumpus said:

Huh?

You do realize that rockets weigh less when landing then when launching?  The only way the rocket could reach launch stress is if it weighed well over 1/10th the launch weight (and remember the whole upper stage isn't there when landing).

Price of error remains the same.  There are nine rocket engines on the booster regardless of however many are firing on the way down.  Loss of one booster either way.  My guess is that if there is room for the fuel for a single engine landing, they will go that way.  If not, then a three engine landing it is.

There are other problems. You have bending forces on the bottom because a small part of the bottom of the rocket has the full force of engine thrust where as the areas of the six other engines do not. The non weight bearing surfaces of the rocket are being accelerated into the higher g-range and could fail. Its not so simple as the engines are producing no more thrust than launch and the weight is lighter, there peak forces move to different parts of the rocket, and those parts may fail. Things like fuel lines at 8G could become detached or kink, electrical couplings could short or break circuit, you can have insulation shift, all kinds of problems.

There is force and there is acceleration two different metrics.

Maximum force is applied to the core right at the engines and diffuses upward into the frame of the rocket, the crosssectional average moment of force declines moving up the rocket at maximum thrust. At maximum dynamic pressure, which based on their telemetry is not that great; sandwiches the middle of the rocket between thrust and drag forces applied to both ends. The forces exerted by the boundary layer that expands with speed and overpressures down the side of the rocket pulls panels and pushes panels in different places as the rocket speeds up in the the air stream.

Imagine however if you were hanging by your heals in a space suit from the bottom of the RP1 tank the blood pressure in your head causing your face to turn red starts off with a maximum of about 1.8 meters x 1.2 g + static blood pressure lying down. As the rocket accelerates to a maximum of 3 g but is turning so you have about 3.3g x your incline of about so that the vertical is about 1 meters. So now you are landing on the barge at 8g of deceleration and the barge is applying another 8g forces, you are now experience 15g or so of force with 1.8 meters. You blood vessels in your brain have ruptured and you are bleeding from your nose and the vessels inside of your eyes have ruptures and filled with blood.

That's what the rocket experiences in a high g-force deceleration and contact landing. This is why I say the forces of deceleration and landing create an upper limit of decelerations which cannot be surpassed without creating more structural strength. 

 

Edited June 15, 2016 by PB666

[Green Baron]

35 minutes ago, wumpus said:

* at least one professor at my college refused to teach this equation (I didn't have him) and my later professors had to apologize about using it.  Any reason it might be controversial (I'm guessing it has to be taken as an axiom, where most other things in physics can be derived).

 

I'm curious. Why did he refuse to teach that formula ? Did he miss relativistic considerations ? Or some supernatural power ?

And:

Of course error margins for reaction times, change in velocities, momentums, working pressures and i'm sure i forget something are much narrower at 8-9g than at 3g, at higher velocities than at lower velocities (and changes thereof). Plus the fact than one degree of freedom is nearly blocked when the system is already at 100%.

A little fault can have a higher impact than.

Edit: and i think firing up more engines won't help. First it takes a few seconds. Second all landings i watched had a little offset from the exact center. The whole process is a tradeoff between reaction time for corrections and braking power, though i don't know where the current limits are.

Edited June 15, 2016 by Green Baron
tiepoes

[Damien_The_Unbeliever]

5 minutes ago, Rakaydos said:

Or quickly change to a 5 engine burn just long enough to sort out which engines are underperforming.

So, you're already in a situation where you know that engines aren't restarting as you expected them to do. And your solution is to start more engines.

Added to which, these are often fuel-starved situations. To be able to start more engines, I believe you'd have to have primed the turbopumps on all of them, despite the fact that you don't expect to use them ~95% of the time.

[Kartoffelkuchen]

16 minutes ago, kerbiloid said:

Btw, meanwhile, 2nd stage:

in the beginning = 1 * 801000 / (4000 + 93000 + 13000) / 9.81 = 0.74 g
near finish = 1 * 801000 / (4000 + 13000) / 9.81 = 4.8 g

The "near finish" thing can't be true. They said during the webcast that they'd throttle the Mvac to keep an acceleration of 5g..

[kerbiloid]

11 minutes ago, Kartoffelkuchen said:

The "near finish" thing can't be true. They said during the webcast that they'd throttle the Mvac to keep an acceleration of 5g..

I just substitute the values given.
And throttled Merlin of 2nd stage is already 801 kN rather than "common" Merlin of the 1st (716 kN).

Oh, by the way, about even more modified Merlin ( http://spaceflight101.com/spacerockets/falcon-9-ft/ ):  "Sea Level: 845 kN – Vac: 914 kN"
But modified 2nd stage is also heavier (107.5 t of fuel instead of 93)

So,
in the beginning = 1 * 914000 / (4000 + 107500 + 13000) / 9.81 = 0.75 g
near finish = 1 * 914000 / (4000 + 13000) / 9.81 = 5.48 g
So, near finish they should throttle down the engine to prevent the crew from trauma.

Edited June 15, 2016 by kerbiloid

[Streetwind]

43 minutes ago, Damien_The_Unbeliever said:

So, assume one of the engines is contributing 0% of the thrust it's meant to, and it's not the centre engine. If you boost the other two engines to compensate, you have quite a lot of asymmetry to deal with. During normal (ascent) usage, you don't have to deal with this because it's always assumed that (given we're not talking about the centre engine, again) there's another engine opposite each engine and so we don't have to deal with any torque issues.

Do you design your entire set of engines to have far more gimbal to cope with that situation, or accept the loss?

At that point, you basically have an engine failure. Such a situation is not recoverable because you don't have enough deceleration. The Falcon 9 returning from a GTO mission requires a three-engine burn to land; it only shuts down the outer two when the flames are already licking the deck. Therefore, issues with limited gimbals aren't going to be your main concern when you're plowing into the ocean at a hundred meters per second

 

37 minutes ago, Rakaydos said:

Or quickly change to a 5 engine burn just long enough to sort out which engines are underperforming.

Not going to happen, because a.) the Merlins take two seconds to spool up to flight pressure, and by then it is probably too late; and b.) only the three designated engines have igniter fluid loaded for restarting.

(It's actually injected into all three everytime, regardless of whether you want to start three or one, which is why the very first landed F9 had odd white cristalline caking residue on the inside of two of the outer engine bells. It did its landing burn only with the center engine running, and the other two had the fluid just leaking out. This may also be the reason why they only load fluid in three engines, if the F9 can only do all-or-nothing fluid injection.)

Edited June 15, 2016 by Streetwind

[wumpus]

3 hours ago, Green Baron said:

I'm curious. Why did he refuse to teach that formula ? Did he miss relativistic considerations ? Or some supernatural power ?

I don't know.  I didn't have him and was wondering if it was a bit more common (similar to things like epsilons being controversial in calculus).

3 hours ago, PB666 said:

That's what the rocket experiences in a high g-force deceleration and contact landing. This is why I say the forces of deceleration and landing create an upper limit of decelerations which cannot be surpassed without creating more structural strength. 

This is all true, but generally the stress is pretty small.  Do you really think their isn't a tiny spike with a >20g acceleration when it lands?  Of course, you can typically just reattach a hose or something then (assuming you are willing to inspect it.  Which was never part of the original plan).

The stresses of the weight of the booster are pretty much locked in and unchangeable.  The stresses on tiny components attached to the booster are minimal and easier to design around.  I've designed electronics for mil-spec 810 and it wasn't a real issue (the mechanical guys who build heavy structures had to worry about it, I had to worry about 461 and EMI issues).  Small components are remarkably capable of handling 9g of acceleration (I'm guessing this is the force typically exerted by dropping a few inches.  Any mechanical engineers out there with shock/vibe experience?).

NOTE: this doesn't really apply to "getting the dregs out of the fuel tanks" which as far as I know is the current speculation as to why the falcon9 [booster] crashed.  Running any engine down to zero is not recommended (if at all possible, not always possible in rocket science) , and then suddenly changing the situation from 3g to 9g can't help.

Edited June 15, 2016 by wumpus

[VirtualCLD]

5 minutes ago, wumpus said:

I don't know.  I didn't have him and was wondering if it was a bit more common (similar to things like epsilons being controversial in calculus).

I believe it's due to the fact that F=ma is a(n) (over)simplification of the "true" mathematical form of Newton's Second law: F=dp/dt or the rate of change in momentum is equal to the force applied. Change in momentum can be due to change in velocity and/or change in mass, especially for rockets. 

[Motokid600]

Why is it the grid fins deploy like that? I always thought it was an even deployment, but in the vid you see them partially open and then suddenly spring open on full deployment.

Edited June 15, 2016 by Motokid600

[SargeRho]

The fins are probably spring-loaded for reliability reasons, just like the legs.

[wumpus]

1 hour ago, VirtualCLD said:

I believe it's due to the fact that F=ma is a(n) (over)simplification of the "true" mathematical form of Newton's Second law: F=dp/dt or the rate of change in momentum is equal to the force applied. Change in momentum can be due to change in velocity and/or change in mass, especially for rockets. 

I bet that's it.  I always wondered about it, but had too much else to worry about when taking those classes.

[PB666]

1 hour ago, wumpus said:

This is all true, but generally the stress is pretty small.  Do you really think their isn't a tiny spike with a >20g acceleration when it lands?  Of course, you can typically just reattach a hose or something then (assuming you are willing to inspect it.  Which was never part of the original plan).

If one of the engines is underperforming, you bet. Those engines were supposed to cut down as the rocket was meters from the target, but if the rocket is in full deceleratoin as it hits the target first there is the shock wave from the landing struts and then the shockwave from the engines striking the deck. The effect on parts varies but a part supporting the engine bay could suffer over 30 g of deceleration.

 

[ProtoJeb21]

@Spaceception

So I heard there was a little... "accident" with the SpaceX first stage landing. What the Fridge happened?

[max_creative]

Just now, ProtoJeb21 said:

@Spaceception

So I heard there was a little... "accident" with the SpaceX first stage landing. What the Fridge happened?

Uhh... Well, there was a RUD. They happen. Especially when trying to land on a barge in the middle of the ocean and your rocket is coming back from GTO. At least the barge is ok.

[ProtoJeb21]

Just now, max_creative said:

Well, there was a RUD

What's a RUD?

[Andem]

2 minutes ago, ProtoJeb21 said:

What's a RUD?

Rapid Unintended Dissassembly.

[Spaceception]

Just now, ProtoJeb21 said:

What's a RUD?

Rapid Unplanned Disassembly.

[max_creative]

3 minutes ago, ProtoJeb21 said:

What's a RUD?

Rapid unsceduled dissambly. Aka it exploded. Maybe Elon Musk needs practice by using the powered landing scenario in ksp.  (except in the scenario it's landing back the ksc, not on a barge.)

[ProtoJeb21]

12 minutes ago, Spaceception said:

Rapid Unplanned Disassembly.

 

12 minutes ago, Andem said:

Rapid Unintended Dissassembly.

So, in other words, what all rockets did back in KSP v0.8

[kerbiloid]

Quote

Looks like thrust was low on 1 of 3 landing engines. High g landings v sensitive to all engines operating at max.

So, that's about the error cost on 8-9 g and 3 engines instead of 1. The least possible discoordination — and you get a boomerang instead of an arrow..
If it were a single engine, this case would not appear at all. If acceleration were low — it could be parried by engines control.

Edited June 16, 2016 by kerbiloid

[Hysterrics]

Was the webcast overall really confusing for anybody else? There were multiple mistakes made by the hosts, it seems. (eg: Calling the landing burn the entry burn)

[Green Baron]

I waited until it was over and then replayed the crucial parts, so i didn't even realize. The girl said she had something to do with the development of the gridfins ? They are probably no professional "talkers", can happen 'cause excitement :-)

 

Has anyone any news on what happened during landing ? I'm not in the social networks ... and the spacex-site doesn't say much ...

 

 

Edited June 16, 2016 by Green Baron

[PB666]

1 hour ago, Green Baron said:

I waited until it was over and then replayed the crucial parts, so i didn't even realize. The girl said she had something to do with the development of the gridfins ? They are probably no professional "talkers", can happen 'cause excitement :-)

 

Has anyone any news on what happened during landing ? I'm not in the social networks ... and the spacex-site doesn't say much ...

 

 

She kind of adds an atmosphere to the 'cast.

EDIT: Apparently the rocket landed, but the internal damage causes a fire. Rocket base is still upright on the barge. Barge appears to have been unaffected. So it does appear to have been some sort of high g-force acceleration damage.

 

 - - - - - - -

 

Edited June 16, 2016 by PB666