Author Topic: SpaceX to begin testing on Reusable Falcon 9 technology this year  (Read 259512 times)

Offline modemeagle

  • Full Member
  • ***
  • Posts: 382
  • Liked: 19
  • Canton, GA
Here is my final recovery simulation. 

I used a base .75 Cd.
Boost back is with 4 engines 100% thrust
Min throttle on center engine was 35% thrust (below published minimum for M1D)

Edit:  Edited graphs for accuracy.
« Last Edit: 06/10/2012 05:24 PM by modemeagle »

Offline RDoc

  • Full Member
  • **
  • Posts: 246
  • Liked: 0
I calculate CD based on this document.  It was the best I could find when I was writing my simulator.
Everything I've read has said that until the mean free path of the medium is large compared to the size of the object, Cd is pretty much constant. As the velocity goes transonic it greatly increases, then decreases again but to a higher level at supersonic speeds.

Beyond that, I'm not sure how to make a convincing argument, although if you are interested you might want to do web searches linking Cd and density or altitude. You won't find many hits, it's generally considered a constant.

Anyway, these are my comments:

First, the paper never discusses Pressure Drag which is what Q*A is about. I think what he's talking about is additional factors added to the subsonic Pressure Drag, but it's hard to say since the paper doesn't define its terms. Base drag by the way is the drag due to the tail of the object creating a vacuum at high speed.

Second, that paper is from the Jackson Model Rocket Club (jmrc) which isn't really a particularly impressive source.

Third, the NASA papers never mention anything about Cd varying with altitude, and in fact just take it as a constant. Personally, I consider NASA a better source.

Fourth, demonstrably Cd isn't anywhere near zero at high altitude.

If you are making a model that varies Cd, test it by using the Mercury capsule data at least. The capsule had a frontal area of about 3m^2, mass of 1200km, and at 36km and 3000 m/s generated 9G. That is seriously inconsistent with a Cd approaching zero.

http://www.google.com/url?sa=t&rct=j&q=atmospheric%20reentry&source=web&cd=2&sqi=2&ved=0CFoQFjAB&url=http%3A%2F%2Fexoaviation.webs.com%2Fpdf_files%2FAtmospheric%2520Re-Entry.pdf&ei=tR7UT4DgHMTW6gHy6aS8Aw&usg=AFQjCNEtKyc-oXS5vEOK6FdMC8lYqe22OA&cad=rja

If a model is wildly off from measured reality, it's the model not the reality that needs correction.

Offline RDoc

  • Full Member
  • **
  • Posts: 246
  • Liked: 0
Here is my final recovery simulation. 

I used a base .75 Cd.
Boost back is with 4 engines 100% thrust
Min throttle on center engine was 35% thrust (below published minimum for M1D)
I'm sorry to keep disagreeing with you, but this chart is pretty clearly not correct.

It still shows no dynamic pressure above about 50km and a sudden peak at 20km. That cannot be correct.

What was it that broke up the Columbia well above that altitude and caused the Mercury capsule to decelerate at 2G at 60km and over 8 at 36km?

Also, is the G chart in absolute numbers? It shows positive G values both increasing and decreasing velocity.

Offline modemeagle

  • Full Member
  • ***
  • Posts: 382
  • Liked: 19
  • Canton, GA
Here is my final recovery simulation. 

I used a base .75 Cd.
Boost back is with 4 engines 100% thrust
Min throttle on center engine was 35% thrust (below published minimum for M1D)
I'm sorry to keep disagreeing with you, but this chart is pretty clearly not correct.

It still shows no dynamic pressure above about 50km and a sudden peak at 20km. That cannot be correct.

What was it that broke up the Columbia well above that altitude and caused the Mercury capsule to decelerate at 2G at 60km and over 8 at 36km?

Also, is the G chart in absolute numbers? It shows positive G values both increasing and decreasing velocity.

Substitute G for acceleration and that is really what that graph is for recovery.  I just forgot to relabel the chart and change the units from G to m/s.  Multiply the value by 9.80665.

As for dynamic pressure.  This stage is not entering at orbital velocity but much lower.  If I input 5900 m/s at 61km I get 4,806 pa for dynamic pressure.

Offline modemeagle

  • Full Member
  • ***
  • Posts: 382
  • Liked: 19
  • Canton, GA
I calculate CD based on this document.  It was the best I could find when I was writing my simulator.
Everything I've read has said that until the mean free path of the medium is large compared to the size of the object, Cd is pretty much constant. As the velocity goes transonic it greatly increases, then decreases again but to a higher level at supersonic speeds.

Beyond that, I'm not sure how to make a convincing argument, although if you are interested you might want to do web searches linking Cd and density or altitude. You won't find many hits, it's generally considered a constant.

Anyway, these are my comments:

First, the paper never discusses Pressure Drag which is what Q*A is about. I think what he's talking about is additional factors added to the subsonic Pressure Drag, but it's hard to say since the paper doesn't define its terms. Base drag by the way is the drag due to the tail of the object creating a vacuum at high speed.

Second, that paper is from the Jackson Model Rocket Club (jmrc) which isn't really a particularly impressive source.

Third, the NASA papers never mention anything about Cd varying with altitude, and in fact just take it as a constant. Personally, I consider NASA a better source.

Fourth, demonstrably Cd isn't anywhere near zero at high altitude.

If you are making a model that varies Cd, test it by using the Mercury capsule data at least. The capsule had a frontal area of about 3m^2, mass of 1200km, and at 36km and 3000 m/s generated 9G. That is seriously inconsistent with a Cd approaching zero.

http://www.google.com/url?sa=t&rct=j&q=atmospheric%20reentry&source=web&cd=2&sqi=2&ved=0CFoQFjAB&url=http%3A%2F%2Fexoaviation.webs.com%2Fpdf_files%2FAtmospheric%2520Re-Entry.pdf&ei=tR7UT4DgHMTW6gHy6aS8Aw&usg=AFQjCNEtKyc-oXS5vEOK6FdMC8lYqe22OA&cad=rja

If a model is wildly off from measured reality, it's the model not the reality that needs correction.

I went through that PDF and did not find a single chart that had dynamic pressure at 200kft.  Can you point out the relevant data.  Also, if you look at the graphs for mercury friendship 7 notice the velocity at 200kft and then see how that does not match the velocity on my graphs.  That mission was orbital and the F9 SI return to site is suborbital.  Please advise with your findings.  If you have the time can you make me a chart for dynamic pressure for that mission for comparison.

Offline charliem

  • Full Member
  • *
  • Posts: 146
  • Liked: 3
Here is my final recovery simulation. 

I used a base .75 Cd.
Boost back is with 4 engines 100% thrust
Min throttle on center engine was 35% thrust (below published minimum for M1D)
I'm sorry to keep disagreeing with you, but this chart is pretty clearly not correct.

It still shows no dynamic pressure above about 50km and a sudden peak at 20km. That cannot be correct.

What was it that broke up the Columbia well above that altitude and caused the Mercury capsule to decelerate at 2G at 60km and over 8 at 36km?

I see your objection is based in the striking difference between the numbers for F9 we are talking, and Mercury and Shuttle behaviors during reentry.

There's no contradiction. It's a question of weight, area, and velocity.

For Mercury you mentioned an speed of 3 km/s, area 3 m2, and mass 1.2 mT. That means a ballistic coefficient ~400 kg/m2 (if Cd ~1).

For a F9 first stage reentry is much slower, half of those 3 km/s. Moreover area is ~11 m2 and mass ~20 mT, so the ballistic coefficient is huge in comparison, ~1,800 kg/m2 (also supposing Cd near 1).

With a ballistic coefficient like that deceleration only gets high much lower into the atmosphere.

Here's a reference to an excel file showing it, by a source you might find more trustworthy.

Using it you find that for a F9R reentry like the one we are talking about dynamic pressure only starts to grow enough to count well under 40,000 m.

Executing that chart with our initial conditions it says that deceleration starts while crossing the 34000 m level, and max-Q is at 12300 m (deceleration 7.8 g), which is near the altitudes we were discussing.

EDIT: grammar
« Last Edit: 06/10/2012 03:28 PM by charliem »

Offline RDoc

  • Full Member
  • **
  • Posts: 246
  • Liked: 0

Here's a reference to an excel file showing it, by a source you might find more trustworthy.

Using it you find that for a F9R reentry like the one we are talking about dynamic pressure only starts to grow enough to count well under 40,000 m.

Executing that chart with our initial conditions it says that deceleration starts while crossing the 34000 m level, and max-Q is at 12300 m (deceleration 7.8 g), which is near the altitudes we were discussing.

Thanks for that reference, it's quite interesting to play with. One thing to keep in mind is that that's calculating a lifting reentry which requires keeping the vehicle under control and at the correct angle of attack.

The discussion modemeagle and I are having is WRT the stage going out of control after the retro burn and reentering essentially side on with no attitude control. It would almost certainly start to tumble, but we're assuming the same kinds of forces as a straight sideways flight.

In modemeagle's simulation, he's adjusting the Cd for altitude such that above about 30km the Cd is essentially zero. That in combination with low dynamic pressure results in essentially zero deceleration.

It is my understanding that for the stratosphere and lower, Cd is pretty much constant except for transonic and supersonic effects both of which increase Cd. That makes me think the stage will crush and start breaking up at high altitude, between 40 and 50 km.

If you set up your spreadsheet for a sidewise entry, Area = 1521, entry angle 45, Cd=1, Cl=.5, it will hit 1G at about 40km and rapidly go up to 6G at 30km.

That's probably pretty conservative since a better scenario is likely the stage hitting at 90 with no lift in which case it goes over 1G at 42km and over 5G at 33km.

WRT Cd, if you calculate the Cd at each altitude, it stays very close to the input value, although generally a bit higher. I imagine that's due to lift and compression effects.

Offline RDoc

  • Full Member
  • **
  • Posts: 246
  • Liked: 0
2. Engine forward configuration would not go above 1G until impact with the water (at over 1,900 m/s)..
If you use the spreadsheet charliem referenced, an engine first reentry would exceed 1G at about 26km, max out at about 9.5G at 11km and hit the ground at about 300m/s.

Edit - I reran this using an entry angle of 70 degrees to match your simulation, Cd of 1.2 (supersonic), and Cl of .1.
« Last Edit: 06/12/2012 02:46 PM by RDoc »

Offline malu5531

  • Full Member
  • *
  • Posts: 196
  • Liked: 73
  • Sweden
In the reusability video from SpaceX, we see the landing gear deploy right before landing.

Now, what if they where to be deployed (or "half-deployed") at apogee through supersonic phase, acting as some type of ballute, while keeping attitude of stage nose-first? It might help with the deceleration and mitigate high Gs?

Modemeagle; How would that influence your simulation?

I attach image to illustrate possible landing gear dual function as ballute.

(Edit; Ballute might be the wrong word, since it's not a balloon / parachute combo, but for lack of better word. :))
« Last Edit: 06/10/2012 10:17 PM by malu5531 »

Offline Joel

  • Full Member
  • ****
  • Posts: 480
  • Liked: 14
  • Barcelona, Spain
As per suggestion, I will forward a thought from a different thread (http://forum.nasaspaceflight.com/index.php?topic=29021.0).

For the moment, F9 does not have engine-out capability in the very beginning of the flight. But, if the first stage carries extra propellant for a propulsive landing, would it be possible to use this propellant to give engine-out capability throughout the flight?

That is, if an engine needs to be shut down early in the flight, could the rocket switch to an expendable mode and still make orbit? Thus sacrificing the launch vehicle to save the payload.
"Success is stumbling from failure to failure with no loss of enthusiasm." - Winston Churchill

Online meekGee

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 3250
  • Liked: 761
As per suggestion, I will forward a thought from a different thread (http://forum.nasaspaceflight.com/index.php?topic=29021.0).

For the moment, F9 does not have engine-out capability in the very beginning of the flight. But, if the first stage carries extra propellant for a propulsive landing, would it be possible to use this propellant to give engine-out capability throughout the flight?

That is, if an engine needs to be shut down early in the flight, could the rocket switch to an expendable mode and still make orbit? Thus sacrificing the launch vehicle to save the payload.

Only if the extra propellant doesn't kill the thrust margin.  If it does, you'll either settle back with a thud, or (less extremely) waste the extra propellant on gravity drag since you're ascending slower.

Offline modemeagle

  • Full Member
  • ***
  • Posts: 382
  • Liked: 19
  • Canton, GA
As per suggestion, I will forward a thought from a different thread (http://forum.nasaspaceflight.com/index.php?topic=29021.0).

For the moment, F9 does not have engine-out capability in the very beginning of the flight. But, if the first stage carries extra propellant for a propulsive landing, would it be possible to use this propellant to give engine-out capability throughout the flight?

That is, if an engine needs to be shut down early in the flight, could the rocket switch to an expendable mode and still make orbit? Thus sacrificing the launch vehicle to save the payload.

The F9V1.1 does have engine out from lift off ONLY if its carrying less than 10 tonnes to LEO, full load should be single engine capable around 60 seconds into flight.  This is to 51.6 inclination. 
Reusable version should always be able to sacrifice margin/recovery of SI to make sure mission success is priority.  (just my opinion without any insider information)

Offline Jim

  • Night Gator
  • Senior Member
  • *****
  • Posts: 22120
  • Liked: 981
  • Cape Canaveral Spaceport
But, if the first stage carries extra propellant for a propulsive landing,

Not a near term capability

Offline baldusi

  • Senior Member
  • *****
  • Posts: 4877
  • Liked: 361
  • Buenos Aires, Argentina
As per suggestion, I will forward a thought from a different thread (http://forum.nasaspaceflight.com/index.php?topic=29021.0).

For the moment, F9 does not have engine-out capability in the very beginning of the flight. But, if the first stage carries extra propellant for a propulsive landing, would it be possible to use this propellant to give engine-out capability throughout the flight?

That is, if an engine needs to be shut down early in the flight, could the rocket switch to an expendable mode and still make orbit? Thus sacrificing the launch vehicle to save the payload.

The F9V1.1 does have engine out from lift off ONLY if its carrying less than 10 tonnes to LEO, full load should be single engine capable around 60 seconds into flight.  This is to 51.6 inclination. 
Reusable version should always be able to sacrifice margin/recovery of SI to make sure mission success is priority.  (just my opinion without any insider information)
Or switch to RLV and get back after a failure. Intact abort capability is one of the reasons airplanes are so safe.

Offline RDoc

  • Full Member
  • **
  • Posts: 246
  • Liked: 0
Or switch to RLV and get back after a failure. Intact abort capability is one of the reasons airplanes are so safe.
Dumping the fuel is going to be a bit tricky isn't it?

Landing with over 100mT of propellent in the first stage, not to mention the entire second stage on top sounds dicey to me.

Tags: