SpaceX Falcon 9 historic landing thread (1st landing attempt & most recent missions)

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I'm a little surprised that they are moving ahead with so many starlink satellites before they work out the brightness / visibility problems for astronomy.

Frankly I think there should be a full stop on these swarms of satellites until they can fix that. I think the destruction of the ability to make ground-based observations of the nighttime sky is tantamount to a crime against humanity until such time as they figure out proper reflectivity reducing coatings such as the one that they tested on a single satellite in a recent launch.
 
Won't they be visible in ground based IR telescopes regardless of coating?

Not there's a great deal of ground based IR astronomy - but isn't there some?
Or plane based? Like SOPHIA.
 
Ok, I have a question about the upcoming abort test. They keep saying that they don't expect the booster to survive.

But why not? The New Shepard did. I know the NS is only sub-orbital, but the test for both rockets is(was) supposed to be done near Max_Q - and I would think that's similar for both rockets. Near the speed of sound in (relatively) thick air.

Is the NS overbuilt in comparison to the F9-B5? The F9's got a hugely bigger task, so I can imagine that it runs closer to the margins than the NS.

Does anyone actually know?
 
I'm sure it has to do with the speed it will be going when the capsule detaches and the direction the booster will be traveling. It's no longer aerodynamic without the nosecone and will still be in quite a lot of atmosphere so it will tumble, and that will put enough stress on the booster it's likely to rip apart.
 
I was thinking the same thing as Mushtang and nytrunner. Also isn’t the pressure on an area equal to the square of the area or something like that so with faster speed comes a lot more force against a much larger diameter airframe.
 
Don't know if it's relevant but NS has fins and F9 doesn't, so NS may be aerodynamically stable. I thought I saw somewhere that the F9 was expected to start tumbling once the capsule makes its departure, and that will tear the booster and upper stage apart.
 
davdue said:
Also isn’t the pressure on an area equal to the square of the area or something like that so with faster speed comes a lot more force against a much larger diameter airframe.
Click to expand...

You're very close! Force is equal to pressure times area and for subsonic travel (incompressible flow) dynamic pressure is equal to 1/2(air density)(velocity^2) In this case, yes pressure goes up exponentially with speed, and the dynamic forces (including drag!) go up severely as well

For supersonic flow (compressible) things get nastier and some folks ditch the term "dynamic pressure" for "Impact Pressure" which involves specific heat ratios, Mach number squared and other fun quantities
 
I was hoping that as the rocket got to Max Q they'd trigger a small explosion in the booster as though something went wrong and see if the system detects it and pulls the capsule away. Programming the capsule to pull away at Max Q is still a test of part of the system but leaves out a very important step.

Perhaps this isn't the last test like this and they'll eventually test the whole thing with a big fireball.
 
Ok, so far I like the ‘fins’ answer.

Digging around, the NS Max Q is 45s into flight at 16000’. From commercial flight info, boost looks like a steady 1.5G thru Max Q - then a build to 3G. Free fall is 0G in the same graph, so I subtract 1 and get ~225m/s at Max Q (about 1/3 max V for the NS).

The Falcon 9 reaches Max Q at 74 sec (6 sec after Mach 1) and 44000 feet at at speed of about Mach 1.1, or 375m/s.

Not counting Mach effects, comparing the F9 to the NS, the ratio of the v^2s is 2.8 and the ratio of the density (standard atm) is 0.33. Both rockets are about 12’ in diameter.

So the force on the rockets at Max Q is actually pretty similar.

But the idea that fins stabilized the NS, and that the F9 won’t have enough RCS control authority seems plausible. During normal flight the F9 can gimbal its engines, but if they shut them all down, that won’t work so well.
 
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In 1-D compressible flow the isentropic gas dynamic pressure (delta P) is

delta p = .5 * gamma * p * M^2

where gamma = 1.4 = specific heat ratio for air
p on RHS = local ambient pressure
M = Mach number

If the flow is supersonic, shocks will have higher pressures. Keep in mind that if there is no tumbling, a structural column can have buckling problems. The inter-stage for the 1st/2nd stage connection may have its own issues. This would be a good problem for some program manager or project engineer to hand to the Space-X engineers, who do analysis.
 
I read somewhere that once the capsule separates the booster will have a open tube and the air forced inside will blow it apart.
 
The front of the airframe will just be a tube (think interstage coupler) and the dynamic pressure will be trying to blow that apart, and also putting large forces directly on the end of the tank. That will translate to extra forces that are likely outside what the airframe is designed to withstand during a normal flight. This is being done at MaxQ so this is the biggest force expected. I would think they would already know, from design calculations and simulations, where the likely failure of the vehicle will be as the capsule pulls away. I would think there would be bets on how it rekits itself around the SpaceX workforce :). Remember that the second stage has full tanks at this time and some of that force may be transferred south if the forward tank bulkhead collapses. Yes, buckling is a definite possibility as the capsule will pull away off the flight axis, creating asymmetric airflow on the following booster.

They are just shutting down the first stage engines, and the loss of thrust is being used as a trigger for the abort. There is no explosive trigger planned.

New Shepard is only 60' long, and the Falcon 9 is 230' long. Much more bending moments on the F9 airframe. I think the bending moments are proportional to the square of the length IIRC.

9xFMNv4WisNHfKik20w37yYzpaIsoUvX7-OlLY0Z8Xs.png
 
If it is supersonic, a blunt end or open tube will have a detached bow shock ahead of it. Pretty bad stuff, because a bow shock is almost like a normal shock. A normal shock is the worse, because the pressure rise is the highest, much worse than an attached oblique shock and much higher than simple dynamic pressure.
 
Nytrunner said:
You're very close! Force is equal to pressure times area and for subsonic travel (incompressible flow) dynamic pressure is equal to 1/2(air density)(velocity^2) In this case, yes pressure goes up exponentially with speed, and the dynamic forces (including drag!) go up severely as well

For supersonic flow (compressible) things get nastier and some folks ditch the term "dynamic pressure" for "Impact Pressure" which involves specific heat ratios, Mach number squared and other fun quantities
Click to expand...

Hey not bad for an electrical engineer that hasn’t touched fluid dynamics since college back in the 80’s.
 
Launch a model rocket without the nosecone and even with fins you'll see why SpaceX doesn't expect the booster to survive.

But, there's a HUGE difference between "not surviving" and "erupting in an enormous fireball, out of which the capsule rockets away safely". I know which one I'd rather see. :cool:
 
Mushtang said:
Launch a model rocket without the nosecone and even with fins you'll see why SpaceX doesn't expect the booster to survive.

But, there's a HUGE difference between "not surviving" and "erupting in an enormous fireball, out of which the capsule rockets away safely". I know which one I'd rather see. :cool:
Click to expand...

The boosters of my two stage scale sounding rockets fly with open interstages - after separation.

I get that the aerodynamics are harsh. But the first pass comparison of the NS and the F9 is more similar than I thought it would be. NS is 1/3 V^2, but 3X the air density. The NS is subsonic, while the F9 is transonic. Ok - that's worse. Maybe lots worse.

The NS booster wasn't expected to survive either - but did. The escape motor exhaust didn't burst the forward tank dome (Titan II staging, anyone?). The capsule mount didn't shred. It didn't tumble.

So, transonic is harsher than subsonic. And the F9 might not have enough RCS control authority with the main engines off. And the F9 is a lot fracking longer, so if it starts to tumble, it's just done. And it's built for a lot harder job than the NS booster - they may reach similar peak altitudes, but the F9 is going a WHOLE lot faster horizontally. To get than performance, the F9 might be running with a lot less material strength margin.

It will be interesting to watch. (!) Will it blow when the SuperDraco exhaust plume washes over it? Will it tumble and break up. Will it fly straight, against the odds, and they send a destruct? Even if the booster lucks out, I'm thinking it might be too heavy to attempt a landing. That's usually done mostly empty. which hadn't occurred to me before now.
 

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