Edit: Although what you are saying was the plan for the reusable launcher in "The Rocket Company." And I've asked before on this form for an expert who has read that book to give it a critical review.
Quote from: modemeagle on 06/08/2012 09:12 pmI updated the sim with the new M1D data and therefore can't match previous data as since the performance has changed. Attached are graphs from a one run. The run performs a 180 degree burn and stays at that attitude until impact for ease of simulating. The velocity listed is "flight path" velocity. If you want velocity data from a point, then let me know which ones and I will supply that data.Total Mass at staging: 69.5 tonnesTotal Mass after return burn: 31.2 tonnesTotal horizontal velocity after burn: -466.6 m/sApogee: 178.5 kmRegarding the engine forward impact. The stage has the cross section of a dragon capsule, but almost 5 times the mass.I still don't understand your simulation. It doesn't seem to take account of the dynamic pressure slowing down the falling stage. In the diagram at around 40-50 km the DP starts increasing, however the velocity curve doesn't flatten out till around 18 km when it drastically slows. The drag would slow the stage down significantly as it got below 50km.At 30 km, the charts show a velocity of about 1700 m/s and Q of about 25kPa. Ignoring supersonic effects which would increase the drag, that means there would be a total force on the side of about 3.6mN, so an acceleration of about 117 m/sec^2 so it would be slowing down at about 107 m/sec^2. At 20 km, the charts show it speeding up to about 1750 m/s and Q of about 140kPa. The force would be about 20mN, and it would be slowing at about 620 m/sec^2. That can't be the case, it would have slowed to terminal velocity before it got to 20km.My guess is that under those start conditions, the stage falling sideways would slow to terminal velocity at about 30km and 500m/s. Falling end on it would get to terminal velocity at 14km and 486 m/s. All that ignores supersonic drag effects, so it would probably slow down a lot faster.WRT the Dragon. Since the drag goes up with the square of the velocity, a factor of 5 in mass just about doubles the terminal velocity at given altitude. That's not the difference between subsonic and 1900 m/s and anyway the terminal velocity keeps decreasing as it falls.
I updated the sim with the new M1D data and therefore can't match previous data as since the performance has changed. Attached are graphs from a one run. The run performs a 180 degree burn and stays at that attitude until impact for ease of simulating. The velocity listed is "flight path" velocity. If you want velocity data from a point, then let me know which ones and I will supply that data.Total Mass at staging: 69.5 tonnesTotal Mass after return burn: 31.2 tonnesTotal horizontal velocity after burn: -466.6 m/sApogee: 178.5 kmRegarding the engine forward impact. The stage has the cross section of a dragon capsule, but almost 5 times the mass.
You're right about the drag. My drag model is based on surface drag since during launch your out of the air stream by the time your high in mach. I will see if my drag calculation source has the information needed for supersonic effects on drag.My drag is calculated using dynamic pressure, live Cd calculations (not a constant), and area of cross section. Dynamic pressure is from velocity and air density (nasa model). Cd was fun to add into the model.
There are too many sources of quite speculative specifications for the SpaceX's hardware (even Wikipedia) and it's confusing. Maybe we should start a thread collecting only the most reliable data.
Quote from: modemeagle on 06/09/2012 12:41 amYou're right about the drag. My drag model is based on surface drag since during launch your out of the air stream by the time your high in mach. I will see if my drag calculation source has the information needed for supersonic effects on drag.My drag is calculated using dynamic pressure, live Cd calculations (not a constant), and area of cross section. Dynamic pressure is from velocity and air density (nasa model). Cd was fun to add into the model.But does it take account of ordinary frontal drag, that is Q * Area?
QuoteThere are too many sources of quite speculative specifications for the SpaceX's hardware (even Wikipedia) and it's confusing. Maybe we should start a thread collecting only the most reliable data.Did we ever make any headway on this question? The Falcon 9 v1.1 data at the bottom of this referencehttp://www.spacelaunchreport.com/falcon9.htmlis internally inconsistant by many tonnes. It givesLift Off mass 480 tonnesStage 1 fuel mass 411 tonnesStage 1 dry mass 28 tonnesStage 2 fuel mass 73.4 tonnesStage 2 dry mass 4.7 tonnes.As you can read, that doesn't add up, not even close. I get 517.7 tonnes Lift Off mass by adding up the stage and fuel masses. What is a good guess for the lift off mass? I could use Stage 1 Thrust divided by Lift Off acceleration, if I knew what that was. If I guess about 1.15 g, I get the Lift Off mass to be about 520 tonnes. Would someone with experience take a cut at these estimations? (oldAtlas_Eguy?) Without accurate knowledge of Lift Off mass it is hard to credit simulations that calculate velocity at MECO, 180.2 seconds into the flight.
Yes, the drag is based on frontal area and dynamic pressure.
QuoteThere are too many sources of quite speculative specifications for the SpaceX's hardware (even Wikipedia) and it's confusing. Maybe we should start a thread collecting only the most reliable data.Did we ever make any headway on this question? The Falcon 9 v1.1 data at the bottom of this referencehttp://www.spacelaunchreport.com/falcon9.htmlis internally inconsistant by many tonnes. It givesLift Off mass 480 tonnesStage 1 fuel mass 411 tonnesStage 1 dry mass 28 tonnesStage 2 fuel mass 73.4 tonnesStage 2 dry mass 4.7 tonnes.
@modemeagle, my simulation of the boost-back is different than yours.Initial conditions at MECO are:Speed relative to the pad: 2,600 m/sAngle of ascent: 23 degAltitude: 78,100 mDistance from the pad: 111,000 mPropellant unburned: 40.9 mTI leave 25 secs for separation and maneuvers to reorient the stage to a 161 deg heading.Ignition of 3 engines at 100% thrust for 77 secs, throttling down when necessary to keep the acceleration under 6 g.That puts the stage in a ballistic trajectory that, without more corrections, would take it less than 10 km from the pad.Apogee is 222 s after staging, at 243.000 m of altitude.The stage crosses down the Karman line 400 s after staging, doing 1.780 m/s and accelerating.These numbers are for an engines first trajectory, no try to slow down.Max speed: 2,060 m/s at 33,600.Max-Q: 2,542 kPa at 12,200 m.Max-g: 136 m/s2, same altitude.Subsonic at 1,400 m high.Crashes at 187 m/s, 469 s after staging. Both dynamic pressure and g load look difficult to survive.
OK - so the fuel tanks are not full. Instead of 411 tonnes and 73.4 tonnes of fuel in the first and second stage at lift off, we have something less than that. Great. Two things, 1 - Why would they build a rocket if they didn't expect to fill the tanks, and 2 - How does that help me evaluate the "boost back to pad" requirements?
Quote from: modemeagle on 06/09/2012 01:31 amYes, the drag is based on frontal area and dynamic pressure.Then why does it show the stage accelerating at 30km when the drag produces a deceleration of over 10G.
Can it somehow generate enough lift to stay at the same altitude? maybe fire multiple short bursts. Eventually the speed slow down and the pad caught up, by than F9 just need to descent.
Do we have numbers on max-q, velocity and acceleration at max-q of the rocket going up?...The question now will be if the engines nozzles would survive 25atm acting on them when falling down?
Charliem, why point at 161 degrees instead of 180 to cancel all of the horizontal thrust. At 161 your adding more to the apogee which will kill the reentry conditions.
Quote from: cordor on 06/09/2012 07:36 amCan it somehow generate enough lift to stay at the same altitude? maybe fire multiple short bursts. Eventually the speed slow down and the pad caught up, by than F9 just need to descent.Yes, certainly. In that simulation the burn to start the ballistic way back doesn't consume all the propellants, 4.6 mT remain so there's the possibility of slowing down using them.I'm still thinking about what would be the best way to do reentry because if you start one engine above the atmosphere there might not be enough fuel to reach the surface and do the landing, and if we wait until latter then there's the difficulty of ignition against a supersonic flow.And you can't wait till the stage is subsonic (1.400 m) either, because then there's not enough time to brake, even at max thrust (one engine).I see two ways. First, reorient the stage sideways to do reentry so the atmospheric drag slows it down much higher. Second, deploying a supersonic parachute at about 10 km up (mach 3).Quote from: hrissan on 06/09/2012 08:16 amDo we have numbers on max-q, velocity and acceleration at max-q of the rocket going up?...The question now will be if the engines nozzles would survive 25atm acting on them when falling down?For the going up max-Q was 30 kPa and max acceleration about 5.5 g.About max-Q on the way down I made a mistake on my previous post, was not 2,542 kPas but only 295 kPa (3 atm). I just edited the post.Quote from: modemeagle on 06/09/2012 12:43 pmCharliem, why point at 161 degrees instead of 180 to cancel all of the horizontal thrust. At 161 your adding more to the apogee which will kill the reentry conditions.You are right about the reentry conditions, but that trajectory was the one that burned less fuel, having also the best range.