Falcon 9 Question ...

[bobdude11]

I have been watching the Falcon 9 videos and marveling at the progress made since Apollo; then a question came to mind:

Is the entry burn used as a form of heat shield for the 1st stage reentry or does it not get high enough to have to worry about the ionization heating during its return to terra firma?

Thanks in advance!

[smartcooky]

I have been watching the Falcon 9 videos and marveling at the progress made since Apollo; then a question came to mind:

Is the entry burn used as a form of heat shield for the 1st stage reentry or does it not get high enough to have to worry about the ionization heating during its return to terra firma?

Thanks in advance!

Its the former. Since Stage 1 is not equipped with a heat shield, it would burn up if it wasn't slowed down 
 
If you listen to the commentator on this youtube video of the latest flight (Auguit 14), you will hear him explain. I have cued to the correct time, but if cuing doesn't work, drag slider to 19m into video.

[smartcooky]

I would just like to add that if you watch the video from the point of 1st stage separation, the two telemetry boxes in the top right of the screen are the Stage 1 altitude and speed.

Note that at MECO/staging, the vehicle reaches 6,000Km at about 60km altitude. The speed then immediately starts to drop as Stage 1 continues to climb due to its momentum. It continues to climb another 60 km to about 120km at a speed of just over 1600 km/h, then the speed it starts to increase again as Stage 1 begins falling back to earth.  By the time it reaches the point of the re-entry burn its 52km up falling at over 4300 km/hr... it will either burn up or become severely damaged if its allowed to continue to accelerate. The re-entry burn sheds 1000 km/h off its rate of descent, bringing it down to about 3200 km/h, but as soon as the burn is done it starts accelerating again, but not as quickly because air-resistance starts to come into play, and ultimately, that air-resistance (assisted by the grid fins)  begins to slow the descent down because its falling faster than its terminal velocity. At 4km about the ground its still plummeting earthwards at over 1100 km/h, when the landing burn kicks in, and slow it from 1100 km/h to zero in 30 seconds and a perfect, pinpoint landing.

[bknight]

A slight sidetrack, I have asked others in CQ approximately how much fuel remains at touch down.  Does anyone know?

[bobdude11]

I have been watching the Falcon 9 videos and marveling at the progress made since Apollo; then a question came to mind:

Is the entry burn used as a form of heat shield for the 1st stage reentry or does it not get high enough to have to worry about the ionization heating during its return to terra firma?

Thanks in advance!

Its the former. Since Stage 1 is not equipped with a heat shield, it would burn up if it wasn't slowed down 
 
If you listen to the commentator on this youtube video of the latest flight (Auguit 14), you will hear him explain. I have cued to the correct time, but if cuing doesn't work, drag slider to 19m into video.

Thanks Smartcooky. I missed that initially. I think I was still under the impression that it was moving too fast to prevent burn up, but your explanations (this and the follow up) have cleared that up for me! Thanks!

[smartcooky]

A slight sidetrack, I have asked others in CQ approximately how much fuel remains at touch down.  Does anyone know?

AIUI, there is supposed to be no fuel left, or at least very little. Stage 1 performs a "hoverslam", its designed to have the landing burn terminate at 0 altitude 0 rate of descent as the fuel runs out. There is certainly not enough fuel left to hover if the timing is off.

[bknight]

A slight sidetrack, I have asked others in CQ approximately how much fuel remains at touch down.  Does anyone know?

AIUI, there is supposed to be no fuel left, or at least very little. Stage 1 performs a "hoverslam", its designed to have the landing burn terminate at 0 altitude 0 rate of descent as the fuel runs out. There is certainly not enough fuel left to hover if the timing is off.

I can believe that very little fuel is on board, but I find it difficult to believe zero and that is why I asked the question.  Remember back to the videos of the unsuccessful attempts and the large fireball as the first stage toppled over and impacted on the deck of the recovery barge.

[jfb]

A slight sidetrack, I have asked others in CQ approximately how much fuel remains at touch down.  Does anyone know?

AIUI, there is supposed to be no fuel left, or at least very little. Stage 1 performs a "hoverslam", its designed to have the landing burn terminate at 0 altitude 0 rate of descent as the fuel runs out. There is certainly not enough fuel left to hover if the timing is off.

A mostly-empty F9 booster can't hover - the T/W of the Merlin engine is too high.  The stage would start climbing again if they didn't touch down at the right instant (the Grasshopper test article was ballasted to allow hovering). 

They can't let the stage run completely dry at landing because the engine would not shut down cleanly, but they get close. 

[smartcooky]

A slight sidetrack, I have asked others in CQ approximately how much fuel remains at touch down.  Does anyone know?

AIUI, there is supposed to be no fuel left, or at least very little. Stage 1 performs a "hoverslam", its designed to have the landing burn terminate at 0 altitude 0 rate of descent as the fuel runs out. There is certainly not enough fuel left to hover if the timing is off.

A mostly-empty F9 booster can't hover - the T/W of the Merlin engine is too high.  The stage would start climbing again if they didn't touch down at the right instant (the Grasshopper test article was ballasted to allow hovering). 

They can't let the stage run completely dry at landing because the engine would not shut down cleanly, but they get close. 

Right, so part of the calculation to get a perfect hoverslam (achieving 0 VS at 0 ALT) must take into account the ever diminishing fuel weight?

[Glom]

A slight sidetrack, I have asked others in CQ approximately how much fuel remains at touch down.  Does anyone know?

AIUI, there is supposed to be no fuel left, or at least very little. Stage 1 performs a "hoverslam", its designed to have the landing burn terminate at 0 altitude 0 rate of descent as the fuel runs out. There is certainly not enough fuel left to hover if the timing is off.

A mostly-empty F9 booster can't hover - the T/W of the Merlin engine is too high.  The stage would start climbing again if they didn't touch down at the right instant (the Grasshopper test article was ballasted to allow hovering). 

They can't let the stage run completely dry at landing because the engine would not shut down cleanly, but they get close. 

Right, so part of the calculation to get a perfect hoverslam (achieving 0 VS at 0 ALT) must take into account the ever diminishing fuel weight?

That's pretty fundamental to any rocketry calculations.

[jfb]

A slight sidetrack, I have asked others in CQ approximately how much fuel remains at touch down.  Does anyone know?

AIUI, there is supposed to be no fuel left, or at least very little. Stage 1 performs a "hoverslam", its designed to have the landing burn terminate at 0 altitude 0 rate of descent as the fuel runs out. There is certainly not enough fuel left to hover if the timing is off.

A mostly-empty F9 booster can't hover - the T/W of the Merlin engine is too high.  The stage would start climbing again if they didn't touch down at the right instant (the Grasshopper test article was ballasted to allow hovering). 

They can't let the stage run completely dry at landing because the engine would not shut down cleanly, but they get close. 

Right, so part of the calculation to get a perfect hoverslam (achieving 0 VS at 0 ALT) must take into account the ever diminishing fuel weight?

Yup.

The fact that they're already making it look routine and easy is freaking insane.  This thing is as tall as an 11-story building, it falls roughly 100 km through the atmosphere (after having pushed the payload uphill), and lands on a postage stamp (sometimes on the water) with no hover time.  It's no wonder some people dismiss the landings as fake; between the size of the booster and the complexity of the operation, it's just brain-breaking to think about. 

Like I said, they have to leave some reserve to guarantee the engine shuts down cleanly (running out of LOX before RP-1 or vice versa would have bad effects, and may result in the engine shredding itself).  I don't have a reliable source, but my understanding is that it's on the order of a couple of hundred kg or so.

[smartcooky]

A slight sidetrack, I have asked others in CQ approximately how much fuel remains at touch down.  Does anyone know?

AIUI, there is supposed to be no fuel left, or at least very little. Stage 1 performs a "hoverslam", its designed to have the landing burn terminate at 0 altitude 0 rate of descent as the fuel runs out. There is certainly not enough fuel left to hover if the timing is off.

A mostly-empty F9 booster can't hover - the T/W of the Merlin engine is too high.  The stage would start climbing again if they didn't touch down at the right instant (the Grasshopper test article was ballasted to allow hovering). 

They can't let the stage run completely dry at landing because the engine would not shut down cleanly, but they get close. 

Right, so part of the calculation to get a perfect hoverslam (achieving 0 VS at 0 ALT) must take into account the ever diminishing fuel weight?

Yup.

The fact that they're already making it look routine and easy is freaking insane.  This thing is as tall as an 11-story building, it falls roughly 100 km through the atmosphere (after having pushed the payload uphill), and lands on a postage stamp (sometimes on the water) with no hover time.  It's no wonder some people dismiss the landings as fake; between the size of the booster and the complexity of the operation, it's just brain-breaking to think about. 

Like I said, they have to leave some reserve to guarantee the engine shuts down cleanly (running out of LOX before RP-1 or vice versa would have bad effects, and may result in the engine shredding itself).  I don't have a reliable source, but my understanding is that it's on the order of a couple of hundred kg or so.

OK, lets put some numbers on this. According to Wiki, Falcon 9 lifts of  with 146,000 litres of LOX and 94,000 litres of RP-1 in Stage 1

Lox is  1.41 kg/l, and RP-1 is 800g/l

- NOTE: I used https://www.aqua-calc.com/calculate/volume-to-weight for the conversions

Lox weight is 166,000 kg
RP-1 weight is 75,000 kg

Total is about 241,700 kg of fuel

If we take your figure of "on the order of a couple of hundred kg or so" as correct, then 200/242,700 = less than 0.1% of the starting fuel load left.

I also read somewhere that Stage 1 burns fuel at a rate of about 250kg/s, so if there really is only 200kg left on board, that is less than 1 second of fuel left on landing.

Holy Cow! Those are some razor thin margins for error!

[raven]

And they make it look so damn easy. Is there any idea when the next flight of a refurbished 1st stage will be?

[Geordie]

And they make it look so damn easy. Is there any idea when the next flight of a refurbished 1st stage will be?

"a reused Falcon 9 will launch the SES-11 communication satellite on early October."

http://www.launchphotography.com/Delta_4_Atlas_5_Falcon_9_Launch_Viewing.html

"The SES 11/EchoStar 105 satellite will likely ride a Falcon 9 first stage that first flew Feb. 19 with a Dragon supply ship heading for the International Space Station, one source said, but a firm assignment has not been confirmed. That vehicle returned to a vertical touchdown at Landing Zone 1 at Cape Canaveral Air Force Station."

"Liftoff from a Florida launch pad is scheduled no sooner than around Sept. 27, a couple of days after a United Launch Alliance Atlas 5 rocket is set to haul a classified payload into orbit for the National Reconnaissance Office, the U.S. government’s spy satellite agency. The Atlas 5 flight with a U.S. national security mission is already booked on the Air Force’s Eastern Range for Sept. 25, and will receive priority to launch first if it remains on schedule."

https://spaceflightnow.com/2017/08/04/ses-agrees-to-launch-another-satellite-on-a-previously-flown-falcon-9-booster/

[smartcooky]

And they make it look so damn easy. Is there any idea when the next flight of a refurbished 1st stage will be?

As near as I can figure out...

► Aug. 24 (today) Falcon 9 • Formosat 5 - Vandenburg SLC-4E - First stage will return to landing on a platform downrange in the Pacific Ocean. (about 9 hours ago - 100% successful)

► Sept. 7 Falcon 9 • OTV-5 - Pad 39A - First stage will return to landing at Cape Canaveral

► Sept. 27 Falcon 9 • SES 11/EchoStar 105 - Pad 39A - The Falcon 9 rocket will launch with a previously-flown first stage (no mention of recovery type but IMO it looks like a heavy payload so perhaps it will return to land on a platform downrange in the Atlantic Ocean..

NOTE: With today's launch from Vandenburg, that is 12 successful missions this year so far, with nine Stage 1 landings attempted, all of them also successful. Two of the launches have used refurbished 1st stages.

So far, they really are making this look routine.

I read an interesting article a few days ago about Elon Musk wanting to work on a system to recover the 2nd Stage as well. IMO that is going to be a whole lot more difficult (by orders of magnitude) because we are talking much higher speeds.