Author Topic: Falcon 9 Question ...  (Read 1308 times)

Offline Glom

  • Jupiter
  • ***
  • Posts: 950
Re: Falcon 9 Question ...
« Reply #15 on: August 25, 2017, 08:52:12 AM »
We'd probably be talking once around with the second stage rather than boost back.

Offline raven

  • Saturn
  • ****
  • Posts: 1230
Re: Falcon 9 Question ...
« Reply #16 on: August 25, 2017, 03:38:19 PM »
Higher speeds and digging into the payload more, since you'd need a lot more fuel and/or shielding to slow down from orbital speeds. Still, if they can make it work, not just as a concept but also commercially, props to them of the highest order.
« Last Edit: August 25, 2017, 03:43:31 PM by raven »

Offline smartcooky

  • Saturn
  • ****
  • Posts: 1408
Re: Falcon 9 Question ...
« Reply #17 on: August 25, 2017, 04:58:20 PM »
Here is a nice little youtube video by Everyday Astronaut that uses a few Kerbal Space Program simulations to explain how much harder it is to recover a second stage. It graphically demonstrates raven's comment about speed and payload.

https://www.youtube.com/watch?v=4rC2Z5El-8E
► What you can assert without evidence, I can dismiss without evidence
► When you argue with idiots you risk being dragged down to their level and beaten with experience.
►"Conspiracism is a shortcut to the illusion of erudition

Offline Zakalwe

  • Saturn
  • ****
  • Posts: 1129
Re: Falcon 9 Question ...
« Reply #18 on: August 26, 2017, 12:35:04 PM »

I also read somewhere that Stage 1 burns fuel at a rate of about 250kg/s,
The whole stage or just the single Merlin that is used for booster landings?
"Quod gratis asseritur, gratis negatur"
“That which can be asserted without evidence, can be dismissed without evidence.” – Christopher Hitchens.

Offline bobdude11

  • Venus
  • **
  • Posts: 43
    • Aviall, A Boeing Company
Re: Falcon 9 Question ...
« Reply #19 on: August 26, 2017, 01:27:22 PM »
Once again, I am glad I joined this community. You not only answered my question, but also the other one I was going to ask!

I always learn something new and in detail.

I want to thank you all for allowing a space fan (since I was little and watched Neil Armstrong walk on the moon) to ask questions and not feel stupid doing so.

I know computers and InfoSec fairly well (been in that industry for over 30 years doing InfoSec for 20), but aviation and space exploration are my passions. I was not smart enough to be part of the groups helping to do those things, but I sure do enjoy watching the progress.

Thank you all who are part of that for all you do and allowing the little boy that watched Apollo to live again each and every day! The astronauts are typically the face of space, but all of you engineers and other support teams are the real heroes.
Robert Clark - InfoSec Analyst for Aviall, a Boeing Company
CISSP, MISM, MCSE and some other alphabet certifications.
I am moving to Theory ... everything works in Theory

Offline smartcooky

  • Saturn
  • ****
  • Posts: 1408
Re: Falcon 9 Question ...
« Reply #20 on: August 26, 2017, 06:53:03 PM »

I also read somewhere that Stage 1 burns fuel at a rate of about 250kg/s,
The whole stage or just the single Merlin that is used for booster landings?

AIUI, that is the fuel consumption rate for all 9 engines

Falcon 9 v 1.1 = 235 kg/s
Falcon 9 FT (Full Thrust) = 270 kg/s

The Stage actually uses different numbers of the 9 engines for different things

9 for lift off
3 for the boostback burn (if required)
3 for the re-entry burn
1 for the landing

► What you can assert without evidence, I can dismiss without evidence
► When you argue with idiots you risk being dragged down to their level and beaten with experience.
►"Conspiracism is a shortcut to the illusion of erudition

Offline Zakalwe

  • Saturn
  • ****
  • Posts: 1129
Re: Falcon 9 Question ...
« Reply #21 on: August 27, 2017, 06:44:04 PM »
Cool, thanks for the clarification.
Even with one engine burning there doesn't seem to be a massive margin for error!
"Quod gratis asseritur, gratis negatur"
“That which can be asserted without evidence, can be dismissed without evidence.” – Christopher Hitchens.

Offline jfb

  • Venus
  • **
  • Posts: 36
Re: Falcon 9 Question ...
« Reply #22 on: August 28, 2017, 11:57:24 AM »
Welp, I've tried running actual numbers based on what info I can find on the F9 FT 1st stage.  Grabbed most of my specs from the F9 Full Thrust and Merlin 1D Wikipedia pages.

  • Dry mass - 22200 kg
  • Liftoff mass - 433100 kg
  • Prop mass - 410900 kg
  • M1D chamber pressure - 97 bar
  • M1D Isp - 282 s
  • M1D thrust - 845 kN (sl)

Given those M1D numbers, the mass flow rate is roughly 305 kg/s per engine at full throttle. 

I got some loose timings from the CRS-12 launch, which can be divided into several phases:

  • Liftoff to Max Q (roughly 78 seconds), 9 engines at full throttle
  • Max Q to MECO (roughly 67 seconds), 9 engines throttled down to 80%(?)
  • Boostback burn (roughly 10 seconds), 1 engine full throttle
  • Re-entry burn (roughly 12 seconds), 3 engines full throttle?
  • Landing burn(roughly 30 seconds), 1 engine 70% throttle

I'm pulling the throttle amount for Max Q out of thin air.  I know they throttle down, but not sure by how much.  80% seems reasonable, but again, that's just pulled out of thin air. 

Based on those numbers, I come up with remaning propellant at the end of each phase:

  • ~196317 kg
  • ~48860 kg
  • ~45803 kg
  • ~38100 kg
  • ~31681 kg

So, definitely more than a couple of hundred kg left in the tanks at the end of the landing burn, so my assumption was pretty wrong.  Then again, that assumes the above calculations have any basis in reality. 

But it makes sense - given the T/W on the M1D, they need all the ballast they can get, even at 70% throttle. 

Offline Northern Lurker

  • Venus
  • **
  • Posts: 49
Re: Falcon 9 Question ...
« Reply #23 on: August 28, 2017, 06:48:15 PM »
  • Dry mass - 22200 kg
  • Prop mass - 410900 kg

So each kilogram of spacecraft holds 18,5 kg of propellant. That really drives home how difficult and technologically demading reaching orbit with reasonable payload actually is.

Lurky
« Last Edit: August 28, 2017, 06:50:14 PM by Northern Lurker »

Offline Glom

  • Jupiter
  • ***
  • Posts: 950
Re: Falcon 9 Question ...
« Reply #24 on: August 28, 2017, 07:28:49 PM »
They may throttle down for max q, but wouldn't they throttle up afterwards?

Offline jfb

  • Venus
  • **
  • Posts: 36
Re: Falcon 9 Question ...
« Reply #25 on: August 29, 2017, 12:27:34 PM »
They may throttle down for max q, but wouldn't they throttle up afterwards?

I doubt it - as the stage gets lighter, the G load gets higher for the same amount of thrust.  ISTR them shutting down two engines on the V1.0 F9 at some point after Max Q to manage the G load. 

Offline jfb

  • Venus
  • **
  • Posts: 36
Re: Falcon 9 Question ...
« Reply #26 on: August 29, 2017, 12:43:32 PM »
  • Dry mass - 22200 kg
  • Prop mass - 410900 kg

So each kilogram of spacecraft holds 18,5 kg of propellant. That really drives home how difficult and technologically demading reaching orbit with reasonable payload actually is.

Lurky

It ain't called "the tyranny of the Rocket Equation" for nothing.  A linear increase in delta-V requires an exponential increase in propellant mass.  And for a kerolox gas generator with a sea level Isp of 280-ish seconds, that's a lot of propellant mass to begin with. 

Offline raven

  • Saturn
  • ****
  • Posts: 1230
Re: Falcon 9 Question ...
« Reply #27 on: August 29, 2017, 08:52:58 PM »
Frankly, it's amazing humans have gotten into space at all. For big rockets like the Saturn V, we're talking a small nuke's worth of energy.

Offline Glom

  • Jupiter
  • ***
  • Posts: 950
Re: Falcon 9 Question ...
« Reply #28 on: September 01, 2017, 03:05:46 PM »
Imagine if we had a thicker atmosphere and more gravity.

Offline bknight

  • Uranus
  • ****
  • Posts: 2145
Re: Falcon 9 Question ...
« Reply #29 on: September 01, 2017, 04:22:35 PM »
Imagine if we had a thicker atmosphere and more gravity.

If one reads a few flat earth beliefs, rockets don't actually go into orbit, just over the horizon(into the ocean?) never attaining orbit. ::)
Truth needs no defense.  Nobody can take those footsteps I made on the surface of the moon away from me.
Eugene Cernan