Thursday, December 31, 2015

A bit of speculation yields one more reason to be optimistic about SpaceX

Speculation alert:  The following contains a bit of speculation, as I don't have the information about how SpaceX recovered its first stage recently.

If you followed SpaceX's progress over the years, you may have watched the video which illustrates their method to reuse the first stage.  The rocket flips and then fires retro to its movement.  This would eventually cause the rocket to slow down and then move in the opposite direction back to the launch site.  I think this is incorrect, from a technical standpoint.

What really happens, or should happen, is that gravity will stop the rocket eventually, so why use precious fuel?  However, the rocket isn't going straight up anymore, as it has gone downrange a considerable distance.  Some fuel may have to get expended in order to send the rocket back into vertical climbing mode, and then letting gravity bring it to a stop.

Once the rocket is climbing vertical, it should lose velocity rather quickly.  By the time it does, it will be almost in space.  In fact, from what I heard, it is already in space when it begins its journey back to the launch site.  This is important in terms of not having the aerodynamic pressure against the nozzles.  In space, there would be no atmosphere, and therefore, no pressure against the nozzles.

As the rocket slows, it can begin to move into position to flip, and then it will fire again.  At this point, it is just doing what it has already done.  The main challenge is to hit a second target with the accuracy required.

Accuracy in rocketry is a problem that has been solved for a long time.  I remember reading about this during the Cold War, as America's rockets were said to be more accurate than the Soviets rockets.  If this has been already perfected for decades, then hitting a spot on the ground could not be that big of a deal.  The rocket just executes mostly what it already demonstrated by getting to where it already was.

The main accomplishment then is just to get the thing to flip without damaging any components.  Since the rocket is moving rather slowly in space at this point, the danger is minimal.

The last part of landing the rocket had been practiced with the so-called Grasshopper rocket tests.  There's some discussion about how the rocket was guided, but it seems to me that the rocket's ability to compensate for wind is minimal.  Mostly, it is like how a military payload is delivered in war.  Whatever adjustments it needs to make would have to be limited.  It is just a dead weight hurtling downward to crash land to a precise spot, but the crash is averted at the last moment by firing the rockets to slow its descent.

Judging from what I have read, the rockets don't use gimbaling to adjust for wind.  Gimbaling the rocket nozzles would change the direction of force to compensate for the wind moving the rocket around.  Perhaps some other technique was employed?

Since there is some legitimate concern about retro firing of the rockets, one presumes that the rocket would not be allowed to gain too much speed.  The engines could be fired as a brake to prevent it from going too fast.  Since it must do that, I speculate, I'm guessing that rocket didn't have to undergo that much stress on the descent.

Much of the discoloration observed was most likely superheated atmosphere that scorched the side of the rocket.  This might be of some concern, but if the rocket survived, most likely it didn't cause much damage, except to the paint job.

Therefore, the rocket and its engines should be in good shape.  The chance of flying again with minimal refurbishing should be pretty good.

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