SpaceX And/Or Rocketry In General

Mike Hewson
Mike Hewson
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I think they are doing a

I think they are doing a great job. I see all rockets on their pads as an accident waiting to happen. That's just the nature of the enterprise : managing rapid release of directed energy. It's the 'directed' part that distinguishes a rocket from a bomb ! The rest is extra ie. what do you go about doing if it doesn't explode ?

I've also watched the videos on the capture and docking. Relatively less exciting to watch but absolutely necessary. Here's a couple of questions I'll throw out for consideration ( I'll give my answers in a few days ):

(a) Given that it took several days for the Dragon to catch up to the ISS, and that the capture/docking etc was a very leisurely affair ( eg. they could have performed it before or after dinner or even in their next work period ), why was the launch window at the gantry only one second wide ?

(b) The Dragon's final approach to the ISS is from below in stages along 'r-bar' ie. the vector from the centre of the Earth to the ISS. To get the Dragon to go precisely along r-bar toward the station why is the thrust to achieve that not along the line of r-bar ?

In any event one does get quite tired of the schadenfreude of useless journalists ...

Cheers, Mike.

( edit ) I have just read an ( unconfirmed ) report that hydraulic fluid was lost from the relevant system(s), and so they couldn't properly operate/deploy some fins on the upper part of the stage and hence correctly brake the craft. If that is correct then the next question is where did the fluid go ?

( edit ) They don't do closed hydraulic fluid loops .....

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

robl
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RE: I think they are doing

Quote:

I think they are doing a great job. I see all rockets on their pads as an accident waiting to happen. That's just the nature of the enterprise : managing rapid release of directed energy. It's the 'directed' part that distinguishes a rocket from a bomb ! The rest is extra ie. what do you go about doing if it doesn't explode ?

I've also watched the videos on the capture and docking. Relatively less exciting to watch but absolutely necessary. Here's a couple of questions I'll throw out for consideration ( I'll give my answers in a few days ):

(a) Given that it took several days for the Dragon to catch up to the ISS, and that the capture/docking etc was a very leisurely affair ( eg. they could have performed it before or after dinner or even in their next work period ), why was the launch window at the gantry only one second wide ?

I was not aware of this fact. A one second window seems to be cutting it awfully close.

Quote:

(b) The Dragon's final approach to the ISS is from below in stages along 'r-bar' ie. the vector from the centre of the Earth to the ISS. To get the Dragon to go precisely along r-bar toward the station why is the thrust to achieve that not along the line of r-bar ?

This question is out of my league.

Quote:

In any event one does get quite tired of the schadenfreude of useless journalists ...

Cheers, Mike.

( edit ) I have just read an ( unconfirmed ) report that hydraulic fluid was lost from the relevant system(s), and so they couldn't properly operate/deploy some fins on the upper part of the stage and hence correctly brake the craft.

Here is a Link that will provide more info on this. In short they are now trying to figure out how they miscalculated the amount of hydraulics needed.

Here is a video of a protype launch from an entirely different perspective. It gives you a better understanding of how much this thing is moving around on that pencil point. You might notice in the "ground shadow" the heat trail following the shadow. Also it shows the rentry guidance planes that did not have enough hydraulics on this last mission. They have a unique name ([edit] hypersonic grid fins)but I cannot remember or find it. They act like "bow thrusters" in that they steer it back over the launch pad. [edit] Be sure to watch the 2nd video that follows of the same launch.

I would like to see the video of this last launch from this perspective.

[Edit] Here is a link explaining and showing what went wrong with the landing. Pictures also show a bit of burn on the platform.

Mike Hewson
Mike Hewson
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Yup, unlike say your local

Yup, unlike say your local ditch digger the hydraulic fluid is not re-used. So after some piston has been extended by expansion of a hydraulic fluid cavity ( converting high pressure to low and thus doing useful work ), upon reset of the cavity to original dimensions ( say by an opposing hydraulic element ) the fluid is simply ejected from the apparatus. They were 10% low on their estimate of the volume of fluid required for the given manoeuvres, and so are going to put another 50% in the reservoir beginning next launch. :-)

This is very heartening news ie. no basic 'major' fault. Evidently the rocket is trashed but the platform is good, or will be after a bit of a clean up.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Phil
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RE: (a) Given that it took

Quote:
(a) Given that it took several days for the Dragon to catch up to the ISS, and that the capture/docking etc was a very leisurely affair ( eg. they could have performed it before or after dinner or even in their next work period ), why was the launch window at the gantry only one second wide ?

I'm no rocket scientist, and orbital mechanics make my brain hurt, but here's my two cents. Maybe I'll learn something here.

This is a guess on my part about the one second launch window. I'm assuming this rocket is not nearly as powerful as the space shuttle. Considering how far the ISS travels in one second, and the current rocket has less muscle to throw itself around in orbit catching things (like space stations), wouldn't you need to be as accurate as possible on launch times? Let gravity/inertia do the work?

Just pondering.

Phil

David S
David S
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Sidebar: That's a relief.

Sidebar: That's a relief.

David

Miserable old git
Patiently waiting for the asteroid with my name on it.

robl
robl
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ok, Here it is: actual

ok, Here it is: actual footage of the failed SpaceX landing on the floating platform.

Mike Hewson
Mike Hewson
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I am really taking to this

I am really taking to this Elon guy ( tweets ):

Quote:
Full RUD (rapid unscheduled disassembly) event. Ship is fine minor repairs. Exciting day!

Quote:
Next rocket landing on drone ship in 2 to 3 weeks w way more hydraulic fluid. At least it shd explode for a diff reason.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Phil
Phil
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I just told my roommate about

I just told my roommate about the "Full RUD" comment. He's still laughing 10 minutes later.

Phil

Mike Hewson
Mike Hewson
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RE: RE: (a) Given that it

Quote:
Quote:
(a) Given that it took several days for the Dragon to catch up to the ISS, and that the capture/docking etc was a very leisurely affair ( eg. they could have performed it before or after dinner or even in their next work period ), why was the launch window at the gantry only one second wide ?

I'm no rocket scientist, and orbital mechanics make my brain hurt, but here's my two cents. Maybe I'll learn something here.

This is a guess on my part about the one second launch window. I'm assuming this rocket is not nearly as powerful as the space shuttle. Considering how far the ISS travels in one second, and the current rocket has less muscle to throw itself around in orbit catching things (like space stations), wouldn't you need to be as accurate as possible on launch times? Let gravity/inertia do the work?

Just pondering.

Phil


That's more or less the case. If one's rocket had an arbitrary/unlimited amount of fuel then you could just take off any old time, turn and burn until you get wherever you want and whenever you want . It comes back to the good old power to weight ratio if you like, keeping in mind that fuel is part of the weight and payload is the useful bit ( for some purpose ) that we don't burn.

One especial reason is that like all freely* orbiting bodies in this type of system, the ISS is executing motion in a plane ( or near enough to over short time periods at least ) centred on the Earth. Another orbiting body on it's own but different orbiting plane - think of two LP records** with the same centre but mutually tilted - has to spend rather more energy to change planes. Much easier to be established on the ISS's plane from launch or soon after. All the rest is maneuvering in their co-plane.

Now the supply ship gets shot off a catapult called the Earth, with a velocity ( a directed quantity called a vector ) as per it's launch point. The ISS is looping around it's orbit which has a ~ fixed orientation with regards to the distant stars while the Earth spins around underneath. It would help for the above efficiency purposes if the Falcon waited, but not too long, for the Earth to rotate around so that the ship could have it's immediate ascent on the ISS's plane.

The Falcon's velocity with respect to the ISS before it has launched is going due East. The ISS's orbital plane is at around 50 degrees to the North of East, the ISS circulates in that with a West to East sense, and so the initial tilts and rolls of the Falcon to get onto the right groove are crucial from the get-go. It becomes much more fuel-expensive later to correct for earlier mis-alignment.

There are other determinants of the launch time and window width but I think that's a major one. Alot of this stuff was initially pencil and paper that wasn't realised in practice until the Gemini program. During that there were launches of one craft to go and catch up with another craft in orbit. The Agena rocket was a 'drone' target for some of that. I think it was Gemini 6 and 7 that were launched a day or two apart for the purpose of demonstrating that manned craft could acquire and dock on the run as it were. Ever so crucial to display that for the Moon effort. Buzz Aldrin wrote a few theses discussing this stuff, including a long discussion about close-in operations and the relative merits of in-plane and out-of-plane manoeuvres. There was much concern about how much fuel was consumed by all those little thrusters in order to get it right, with every twitch of the joystick costing a precious amount. This was a big parameter during simulations : 'too much fuel on that one Neil, do it again'. Obviously preferable to nail it first time. And the clock was ticking for other important reasons too ...

So yes, if your timing is good then much comes for free without extra effort.

Now isn't that just true of life in general ? You know, choosing exactly when to put the balaclava on before entering the bank, that type of thing ..... :-) :-)

Cheers, Mike.

* Not firing off their rockets to do things, just letting their own inertia and gravity determine their path into the future.

** That's an ancient historical audio recording medium to you younglings ... insert the phrase 'long playing pizza' if you prefer.

( edit ) Yup, it was Gemini 6 and 7. This will get you Trivial Pursuit points : Gemini 7 was launched before Gemini 6 ... strictly speaking it was the Gemini 6A mission that flew, but that's another tale. :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Mike Hewson
Mike Hewson
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Reviewing the 'robust

Reviewing the 'robust landing' video :

Given the fault as disclosed I'm surprised that it got anywhere near the platform at all. You can see how much the autonomous system was attempting to correct a substantial transverse component ( toward the right side of view ) by the huge left side lean from vertical on the way in. That rocket was at full throttle when it bought the farm, as you can especially tell by it's momentary reversal of that to-the-right-side drift just before first contact -> projecting some of the upper part of the ( now disassembling ) rocket back towards the left side of view. I'd interpret this as the flight control system being handed a hopeless dynamic state to recover meaningfully from.

Cheers, Mike.

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

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