Author Topic: SpaceX Falcon Mission Simulations  (Read 25135 times)

Offline OneSpeed

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... I was also starting to feel like maybe we should create a simulation thread for these topics if people wanted to continue debating the implications of different simulations.

Sounds like a good idea. I've got a plot for the CRS-10 first stage I'd like to post, but it makes no sense in this thread. There are Mars simulation threads, so perhaps a Falcon one would make sense?

Flight Club is a simulator. Myself and zlynn1990 (of SpaceSim fame) collaborated a bunch on tweaking our atmospheric models and such when we were independently building our simulators. Just wanted to clear that up. Let me know if you have any questions about how Flight Club does things - happy to help in any way.

Thanks for setting the record straight!

Thanks for clearing that up and thanks for creating FlightClub!  I wonder if the apparent differences are more to do with the model or the assumptions for various inputs.  I was also starting to feel like maybe we should create a simulation thread for these topics if people wanted to continue debating the implications of different simulations.

No problem, and my pleasure! The apparent differences probably arise from a couple of things. I've tried to make Flight Club as generic as possible for different launch vehicles, which has possibly (though I have no reason to believe this is the case) made some of the logic around landing leg and grid fin deployments a bit wonky. You know, vehicle-specific things.

Then my atmospheric model is sound, as it is based on interpolated NASA data. However, the vehicle-specific drag models may need some fine-tuning. I have created a drag coefficient vs Mach number curve for the F9, but it's all educated guessing. I'm sure zlynn1990 has created his own educated guess, independently of mine.

... I was also starting to feel like maybe we should create a simulation thread for these topics if people wanted to continue debating the implications of different simulations.

Sounds like a good idea. I've got a plot for the CRS-10 first stage I'd like to post, but it makes no sense in this thread. There are Mars simulation threads, so perhaps a Falcon one would make sense?

Yeah sounds like a plan. I've already gotten my first stage portion for CRS-10 sufficiently matching reality (not perfectly - about 97%) and I'm working on my stage 2 portion now (see link below for both). I'll happily submit some discussion to that thread if it exists!

https://www.flightclub.io/results/?id=06466dd9-3a0a-492c-8bd5-58248ff31960&code=CR10
« Last Edit: 02/21/2017 01:09 AM by TheVehicleDestroyer »

Offline TOG

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Then my atmospheric model is sound, as it is based on interpolated NASA data. However, the vehicle-specific drag models may need some fine-tuning. I have created a drag coefficient vs Mach number curve for the F9, but it's all educated guessing. I'm sure zlynn1990 has created his own educated guess, independently of mine.


First off, great respect for the work involved in developing the process.  It shows a great love of the math involved in modeling a complex process.

And a question:  When you formulate the drag coefficient vs. Mach number curve, are you also taking into account the atmospheric density based on the altitude of the reading?  When I did some similar simulators, years ago for shuttle reentry and landing, that was one area that bit me in the back-side.  That and the effect of heating on the surface drag for localized atmospheric turbulence.  I had several calculations relating to the heat loss as the spacecraft moved through the atmosphere and the effect on the local density.

Out of mathematical curiosity, of course.

Thanks!
M's Laws of Aerodynamics:                                    On Physics Exam:
1) if you push anything hard enough it will fly          Q)The allegory of Schrödinger's cat shows what?
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Offline OneSpeed

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Re: SpaceX Falcon Mission Simulations
« Reply #23 on: 02/23/2017 01:19 PM »
Special thanks to gongora for setting up this thread. As promised, here is the CRS-10 first stage profile. The velocity is in m/s, but for the altitude, divide by 10 to get kms. For the acceleration, divide by 100 to get relative gs. A full simulation is coming soon.
« Last Edit: 02/23/2017 01:20 PM by OneSpeed »

Re: SpaceX Falcon Mission Simulations
« Reply #24 on: 02/24/2017 05:21 PM »
Flight Club simulation has also been finished

Data: https://www.flightclub.io/results/?code=CR10
Live: https://www.flightclub.io/world/?code=CR10&w=2
Live synced with webcast:

The timing of events in the webcast included quite a few delays. I analyzed them in my post to r/SpaceX here: https://www.reddit.com/r/spacex/comments/5vk3e8/slug/de2nqc2

I would copy and paste over but I don't know if it's possible to do tables on this forum? Sorry, NSF noob
« Last Edit: 02/24/2017 05:25 PM by TheVehicleDestroyer »

Offline OneSpeed

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Re: SpaceX Falcon Mission Simulations
« Reply #25 on: 02/25/2017 01:48 AM »
I would copy and paste over but I don't know if it's possible to do tables on this forum? Sorry, NSF noob

You can do tables, but it requires quite a bit of manual formatting. It's probably easier to just attach a spreadsheet.

Offline OneSpeed

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Re: SpaceX Falcon Mission Simulations
« Reply #26 on: 03/26/2017 02:38 AM »
Here is the latest simulation I have been working on in collaboration with NSF user ZLynn1990. This mission demonstrates how the Falcon Heavy could launch a crewed Dragon V2 around the moon. Unlike the Apollo missions, this is not a free return trajectory. Elon has stated that dragon will pass by the moon and coast out to 400,000 miles (640,000km). Few details are known about this mission, so what we created is a best guess based on the current known specification of the Falcon Heavy and Dragon V2 capsule.


Offline Proponent

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Re: SpaceX Falcon Mission Simulations
« Reply #27 on: 03/26/2017 01:04 PM »
That's amazing, OneSpeed -- thank you!

I'm surprised that the lift is so large during ascent.  It sometimes exceeds the drag by an order of magnitude.  Would Falcon Heavy actually fly at angles of attack large enough to generate such lift?  I'd have thought that from pitch kick to a low dynamic pressure, FH would be flying a gravity turn.

If it were up to me, I would probably let negative perigees be shown, because it gives you a feeling for the shape of the orbit.

A couple of really minor points/questions.  Somewhere in cis-lunar space, the atmospheric density rises to 0.001 kg/m3, and drag becomes postive -- a minor bug in the atmospheric model?

Again, that was really cool!


Offline manoweb

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Re: SpaceX Falcon Mission Simulations
« Reply #28 on: 03/26/2017 06:37 PM »
This is by far the best way to show this mission I've found. They should show your work in schools across the nation!

A couple of question: will the Dragon only get so far from the lunar surface at minimum, three or four lunar diameters? I hope for all that money the those astronauts will get to fly few km from the surface :)

Also: upon re-entry in Earth's atmosphere, I see that heating rate goes up to over one MW/m^2 but air density remains 0.000kg/m^3. Is this expected?

Offline OneSpeed

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Re: SpaceX Falcon Mission Simulations
« Reply #29 on: 03/27/2017 11:02 AM »
Thanks for the feedback!

I'm surprised that the lift is so large during ascent.  It sometimes exceeds the drag by an order of magnitude.  Would Falcon Heavy actually fly at angles of attack large enough to generate such lift?  I'd have thought that from pitch kick to a low dynamic pressure, FH would be flying a gravity turn.

It would certainly start out as a gravity turn. I've tried a few times now to match the velocity and altitude data provided in the SpaceX webcasts to simulations. There seems to be a trend. Once the dynamic pressure drops sufficiently, they introduce a small negative alpha, which favours horizontal acceleration over vertical. The F9 and FH have a very high fineness ratio, so lift/drag ratio is surprisingly high, the angle of attack doesn't need to be large to generate significant (negative) lift.

If it were up to me, I would probably let negative perigees be shown, because it gives you a feeling for the shape of the orbit.

I understand where you are coming from, but the predictive traces are not just showing the newtonian orbit, they include aerodynamic effects. So, if there is an atmosphere the trace may well stop completely at the planet's surface. Unfortunately, the traces can become quite erratic when approaching a planet's atmosphere from a large distance at a high update speed, but I figure that is better than just ignoring atmospherics completely.

A couple of really minor points/questions.  Somewhere in cis-lunar space, the atmospheric density rises to 0.001 kg/m3, and drag becomes postive -- a minor bug in the atmospheric model?

That definitely sounds like a bug. Thanks, I'll have a closer look.

A couple of question: will the Dragon only get so far from the lunar surface at minimum, three or four lunar diameters? I hope for all that money the those astronauts will get to fly few km from the surface :)

The closer you get to the moon, the larger the gravity assist. This profile gives about the right kick for a 400k mile apogee. For a 300k mile apogee, the closest approach would be even higher, about 17,000 kms. If you come too close, the kick is enough to reach Earth escape velocity, and the Grey Dragon would not have enough ΔV to return to Earth. Ever.

Also: upon re-entry in Earth's atmosphere, I see that heating rate goes up to over one MW/m^2 but air density remains 0.000kg/m^3. Is this expected?

Yes, it is. I'll see if I can push a code change for the display of really low values of density (this issue is even more marked on Mars).

Offline Proponent

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Re: SpaceX Falcon Mission Simulations
« Reply #30 on: 03/27/2017 01:09 PM »
A couple of question: will the Dragon only get so far from the lunar surface at minimum, three or four lunar diameters? I hope for all that money the those astronauts will get to fly few km from the surface :)

This whole thing really puzzles me.  I'd have thought that most passengers would want to approach the moon as closely as possible, and that SpaceX would want to keep the mission profile as simple as possible, especially for the first mission.  Put those two factors together, and doesn't it add up to an Apollo-style figure-of-eight free-return trajectory (an in-plane, co-rotating, circumlunar free-return trajectory, in the terminology used in the plot attached to this post)?  Then the only burns needed after TLI are small course course corrections.

Offline envy887

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Re: SpaceX Falcon Mission Simulations
« Reply #31 on: 03/27/2017 02:54 PM »
A couple of question: will the Dragon only get so far from the lunar surface at minimum, three or four lunar diameters? I hope for all that money the those astronauts will get to fly few km from the surface :)

This whole thing really puzzles me.  I'd have thought that most passengers would want to approach the moon as closely as possible, and that SpaceX would want to keep the mission profile as simple as possible, especially for the first mission.  Put those two factors together, and doesn't it add up to an Apollo-style figure-of-eight free-return trajectory (an in-plane, co-rotating, circumlunar free-return trajectory, in the terminology used in the plot attached to this post)?  Then the only burns needed after TLI are small course course corrections.

It makes a lot more sense if Elon meant 400,000 km when he said 400,000 miles.

Offline OneSpeed

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Re: SpaceX Falcon Mission Simulations
« Reply #32 on: 03/27/2017 08:32 PM »
This whole thing really puzzles me.  I'd have thought that most passengers would want to approach the moon as closely as possible, and that SpaceX would want to keep the mission profile as simple as possible, especially for the first mission.  Put those two factors together, and doesn't it add up to an Apollo-style figure-of-eight free-return trajectory (an in-plane, co-rotating, circumlunar free-return trajectory, in the terminology used in the plot attached to this post)?  Then the only burns needed after TLI are small course course corrections.

It makes a lot more sense if Elon meant 400,000 km when he said 400,000 miles.

Yes, I get that, but from https://phys.org/news/2017-02-spacex-people-moon-year.html

"The paying passengers would make a long loop around the moon, skimming the lunar surface then going well beyond, perhaps 300,000 or 400,000 miles distance altogether. It's about 240,000 miles distance to the moon alone, one way."

There appears to be no confusion between miles and kilometers in this statement. The inconsistency I see is in the mission times quoted so far. From the sim, it will be three days to the moon, and another week to the 400k mile apogee. It would be a further week for the return to earth, nearly 17 days altogether.

The Grey Dragon is going to be limited in available ΔV compared to Apollo, so I suspect this 'rollercoaster' mission profile is designed to do something spectacular (a new altitude record) with limited resources. This won't be the first Dragon 2 mission, so although certainly risky, it would be a logical incremental step.

Offline envy887

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Re: SpaceX Falcon Mission Simulations
« Reply #33 on: 03/27/2017 08:42 PM »
This whole thing really puzzles me.  I'd have thought that most passengers would want to approach the moon as closely as possible, and that SpaceX would want to keep the mission profile as simple as possible, especially for the first mission.  Put those two factors together, and doesn't it add up to an Apollo-style figure-of-eight free-return trajectory (an in-plane, co-rotating, circumlunar free-return trajectory, in the terminology used in the plot attached to this post)?  Then the only burns needed after TLI are small course course corrections.

It makes a lot more sense if Elon meant 400,000 km when he said 400,000 miles.

Yes, I get that, but from https://phys.org/news/2017-02-spacex-people-moon-year.html

"The paying passengers would make a long loop around the moon, skimming the lunar surface then going well beyond, perhaps 300,000 or 400,000 miles distance altogether. It's about 240,000 miles distance to the moon alone, one way."

There appears to be no confusion between miles and kilometers in this statement. The inconsistency I see is in the mission times quoted so far. From the sim, it will be three days to the moon, and another week to the 400k mile apogee. It would be a further week for the return to earth, nearly 17 days altogether.

The Grey Dragon is going to be limited in available ΔV compared to Apollo, so I suspect this 'rollercoaster' mission profile is designed to do something spectacular (a new altitude record) with limited resources. This won't be the first Dragon 2 mission, so although certainly risky, it would be a logical incremental step.

How long does it take to get to a 300k mile apogee and back?

“This would be approximately a weeklong mission, and it would skim the surface of the moon, go quite a bit farther out into deep space and then loop back to Earth,” Musk said. “I’m guessing, probably distance wise, maybe 300,000 or 400,000 miles.”

http://www.cbsnews.com/news/spacex-moon-mission-planned-for-2018/

Offline Proponent

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Re: SpaceX Falcon Mission Simulations
« Reply #34 on: 03/27/2017 09:24 PM »
How long does it take to get to a 300k mile apogee and back?

The orbital period of a 0 x 300,000-mi orbit is about 9.9 days.

Quote
“This would be approximately a weeklong mission, and it would skim the surface of the moon, go quite a bit farther out into deep space and then loop back to Earth,” Musk said. “I’m guessing, probably distance wise, maybe 300,000 or 400,000 miles.”

http://www.cbsnews.com/news/spacex-moon-mission-planned-for-2018/

Both that CBS article and Jeff Foust's piece in SpaceNews specifically mention a free-return trajectory and a week-long trip.  Foust does not mention 300,000 miles.

My guess is that the 300,000-mile figure is just wrong (I know Musk said it, but....), and the intended profile is a low-altitude free-return.
« Last Edit: 03/27/2017 09:50 PM by Proponent »

Offline deruch

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Re: SpaceX Falcon Mission Simulations
« Reply #35 on: 03/28/2017 04:25 AM »
Could the discrepancy between miles vs. km be explained by a mix up between "altitude" and total distance traveled?
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Offline OneSpeed

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Re: SpaceX Falcon Mission Simulations
« Reply #36 on: 03/28/2017 11:25 AM »
How long does it take to get to a 300k mile apogee and back?

I've refactored the sim for a 300k mile (480,000km) apogee. If Grey Dragon arrives at the moon in 3 days, at a pericynthion of about 16,500kms, the gravity assist then kicks it to 480,000kms right on 7 days after TLI. At that point, there needs to be a 'boostback' burn to shorten the perigee of the Earth orbit for re-entry, which would take place in another 5 days.

Offline OneSpeed

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Re: SpaceX Falcon Mission Simulations
« Reply #37 on: 04/01/2017 05:00 AM »
I've analysed the SES-10 launch profile as best I can from the broadcast footage. The S2 velocity and altitude is complete up to 620 seconds, but the rest has only a few data points.

Firstly, to get the simulation to match the data, I had to throttle back the first stage slightly more than the previous three missions. Assuming a nominal 7,607 kN thrust at sea level, the burn started at 88.5%, (CRS-10 91.5%), dropped slightly for max Q, then increased to 90% (c.f. 92.5%) up until the 150 second mark, where acceleration was limited to 4Gs.

For the second stage, I had the same issue that LouScheffer noted recently, i.e. that 100% thrust is not sufficient to produce the second stage performance we are seeing recently. I had to throttle to 106%, and even then had trouble matching the curve for a while. From the 330 second mark, 100% thrust seemed correct, until the 500 second mark, where acceleration was again limited to 4Gs until SECO.

The first stage landing was interesting because there was no boostback burn. From the video I had a fairly good idea of the orientation of the booster, when the entry burn started and finished, and when the landing burn started. I also knew when the booster went transonic. When I plugged what data I had into the sim, I got the profile attached below. It was interesting that Elon was talking after the launch about using the new titanium grid fins to enhance the glide of the booster (which would also give it more time to slow aerodynamically), because if the sim is correct, then to some extent that's what they've done here. The barge would have had to have been about 600kms downrange for this profile to be feasible.

Edit: after closer inspection of the broadcast videos from the 1:40 mark, SpaceX have introduced a first stage pitch up manoeuvre. If I include this manoeuvre in the simulation, the altitude and velocity match the broadcast data. However, most of the glide on descent now occurs between the entry and landing burns.
« Last Edit: 04/02/2017 11:33 AM by OneSpeed »

Re: SpaceX Falcon Mission Simulations
« Reply #38 on: 04/03/2017 03:59 PM »
Flight Club's recreation of SES-10 is up here:

All data (plots and 3D view) are here: https://www.flightclub.io/results/?code=SS10
The live replay can be watched here: https://www.flightclub.io/world/?code=SS10&w=2

The big difference between my simulation and OneSpeed's above is that I'm not using the uprated Merlin 1D thrust figures - my 1D's are still at 756kN instead of 845kN. However, with this thrust figure at close to 100% (dropped to 90% for MaxQ), my trajectory has still matched the webcast data closely.

So who knows what to think. Do we have a collection of Merlin thrust vs. Mission anywhere? Did this mission definitely use the 845kN version?

Offline envy887

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Re: SpaceX Falcon Mission Simulations
« Reply #39 on: 04/03/2017 11:38 PM »
Flight Club's recreation of SES-10 is up here:

All data (plots and 3D view) are here: https://www.flightclub.io/results/?code=SS10
The live replay can be watched here: https://www.flightclub.io/world/?code=SS10&w=2

The big difference between my simulation and OneSpeed's above is that I'm not using the uprated Merlin 1D thrust figures - my 1D's are still at 756kN instead of 845kN. However, with this thrust figure at close to 100% (dropped to 90% for MaxQ), my trajectory has still matched the webcast data closely.

So who knows what to think. Do we have a collection of Merlin thrust vs. Mission anywhere? Did this mission definitely use the 845kN version?

I have seen no evidence at all that they are actually flying the uprated Merlin yet, and both your and OneSpeed's simulations are good evidence that they are not.

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