The "uncontrolled reentry" of the 2nd stage

[adiggs]

Reading this week's rocket report I was reminded of this:

A Falcon 9 rocket making an uncontrolled re-entry looked like an alien armada

Typically, a Falcon 9 rocket makes a more controlled return to Earth.

arstechnica.com

The interesting bit of this story, to me, is that a cousin of mine lives in Washington and found a piece of carbon fiber in her front yard that she knew wasn't there a few days earlier (too big to mow over accidentally, and that part of the lawn is mowed regularly).

With a little help from Jonathan McDowell who apparently knows people at SpaceX

they think it's part of a composite-weave "raceway cover" that covers cables on the outside of the second stage

My cousin ends up with a stage 2 souvenir.

 

[Nikxice]

Lucky find! To some this might appear to be Space (X) junk, but this flown piece circled the globe for 3 weeks. Plus, perhaps it is unique. I haven't come across evidence of other debris from that second stage re-entry. Keeping photo documentation and relevant news articles will give the item provenance. If your cousin is interested in parting with her "souvenir" there might be someone willing to make an offer. Just asking for a friend who collects SpaceX artifacts.

 

[HVM]

Second stage's controlled re-entry would look exactly same but would take place over Indian Ocean. Also highly oxidized carbon fiber composites can be as dangerous as Asbestos fibers. Dropping space-hardware to populated area is bit $hitty, whatever it's done by China or a private company. Some parts like COPVs and turbopump parts can survive re-entry and can cause some hazard at ground.

 

[HVM]

...And not only can, it did:

https://twitter.com/x/status/1378062115248996353

 

[adiggs]

Lucky find! To some this might appear to be Space (X) junk, but this flown piece circled the globe for 3 weeks. Plus, perhaps it is unique. I haven't come across evidence of other debris from that second stage re-entry. Keeping photo documentation and relevant news articles will give the item provenance. If your cousin is interested in parting with her "souvenir" there might be someone willing to make an offer. Just asking for a friend who collects SpaceX artifacts.

I'll keep that in mind (that you might be interested .. err your friend ). She hasn't said one way or the other, though its been mentioned. I could easily be wrong (and I know I'm in the right crowd to make me wrong) - I suspect that it'll be far more valuable as a prop for future story telling (remember that day when a piece of a rocket fell out of the sky?) than it will be in an auction or similar.

 

[adiggs]

Second stage's controlled re-entry would look exactly same but would take place over Indian Ocean. Also highly oxidized carbon fiber composites can be as dangerous as Asbestos fibers. Dropping space-hardware to populated area is bit $hitty, whatever it's done by China or a private company. Some parts like COPVs and turbopump parts can survive re-entry and can cause some hazard at ground.

I wonder about this. I suspect that the controlled re-entries look far more vertical in their flight path. Slow down enough under thrust for gravity to take over and pull it reasonably straight down and with more accuracy.

What I read about how this stage came down - there wasn't enough propellant for the normal controlled re-entry so the decision was made to vent the remaining propellant and allow it to reenter the atmosphere when it was ready. The problem being that the physics isn't so easy to calculate and you end up with a possible landing footprint that was at least 1 full rotation of the earth long.

But letting it re-enter on a flatter and faster trajectory also increased the degree to which it broke up as well as how completely the pieces burned up.


Maybe somebody who actually knows something (as opposed to me who is guessing and hasn't worked in the industry) can comment

 

[e-FTW]

...And not only can, it did:

https://twitter.com/x/status/1378062115248996353

That engineering team should get this back to hang on their wall. They built one heck of a tough beast!

Or they make a beer cooler out of it:

https://twitter.com/x/status/1354666562851135489

 

[bxr140]

I wonder about this. I suspect that the controlled re-entries look far more vertical in their flight path. Slow down enough under thrust for gravity to take over and pull it reasonably straight down and with more accuracy.

While not an unreasonable thought process, the difficult-to-fathom physics of an object in orbit end up quite different. In the context of something in reasonably stable LEO orbit (like a Starlink second stage) there's no major difference between the re-entry angle in a controlled vs uncontrolled situation, and they're both very shallow.

Its really all about energy (as opposed to speed). An object with thrust capability will most certainly thrust into its velocity vector to decrease altitude (as would be the case with a re-entry burn) which intuitively is very akin to 'hitting the brakes'...but in doing so its actually speeding up in the process. Only once it actually hits the atmosphere and drag pulls it out of actual orbit does the object lose speed.

What that burn is really doing is removing energy from the object/orbit (the thrust is directly opposed to [m * V^2]) and that loss of energy is really what makes The Thing "come down" faster. What's also useful to consider is that there's a massive discrepancy between the energy of the object and its available thrust...so the re-entry burn is less like slamming on the brakes in a car to slow down on the freeway and maybe more like lifting the throttle slow down on the freeway.

 

[Electroman]

While not an unreasonable thought process, the difficult-to-fathom physics of an object in orbit end up quite different. In the context of something in reasonably stable LEO orbit (like a Starlink second stage) there's no major difference between the re-entry angle in a controlled vs uncontrolled situation, and they're both very shallow.

Its really all about energy (as opposed to speed). An object with thrust capability will most certainly thrust into its velocity vector to decrease altitude (as would be the case with a re-entry burn) which intuitively is very akin to 'hitting the brakes'...but in doing so its actually speeding up in the process. Only once it actually hits the atmosphere and drag pulls it out of actual orbit does the object lose speed.

What that burn is really doing is removing energy from the object/orbit (the thrust is directly opposed to [m * V^2]) and that loss of energy is really what makes The Thing "come down" faster. What's also useful to consider is that there's a massive discrepancy between the energy of the object and its available thrust...so the re-entry burn is less like slamming on the brakes in a car to slow down on the freeway and maybe more like lifting the throttle slow down on the freeway.

The way I understood orbital mechanics - Every object in orbit has two components:

- a velocity vector that is tangential to the elliptical (lets assume circular for simplicity) orbit. This is the speed the 2nd stage imparts on the satellite. If an object is orbiting at say 10 km/second, it is this speed that is measured tangential to the orbit.

- a 2nd component, gravity, that continuously pulls the object towards the center of the earth. In this case it not imparting any increase in speed, but changes the direction of the object to keep it in circular motion. It changes the velocity vector continuously by changing direction but NOT changing the speed. Without gravity it will simply go straight and away from earth.

When a reentry burn is done, it reduces the tangential speed (component 1 above), and that results in the object now gaining speed in the direction towards the center of earth. Now for simplicity if you assume the reentry burn is extremely powerful and cancels instantaneously all of the tangential speed it imparted , such that it loses all of the 10km/sec speed in just say 1 second. Then the object will do a free fall from its orbit height (say 250 km) straight down like an rock dropped from a tall building. When it hits the top of atmosphere at 100 km, it would have gained enough vertical (downward speed) of 150 km of freefall.

 

[bxr140]

The way I understood orbital mechanics - Every object in orbit has two components:

I think at least the spirit of what you're trying to say is sound.

At a summary level, the easiest place to start is Newton's laws. An object in an orbit has constant energy (which is more or less the 1st) and then there's a bunch of good old F=ma things (the 2nd), some of which add or subtract energy (like a thruster firing or atmospheric drag) and some of which do not (like gravity). The result of course is the 3rd.

What's most relevant is that the available forces (more pedantically, their resultant work) is orders and orders of magnitude smaller than the object's energy. In context of the conversation there's simply not anywhere near enough cumulative force available to "turn" any LEO orbiting object into a materially different heading on re-entry, let alone the book-end thought experiment of 'full stop, then drop straight down'.