SpaceX 'Star series' simulations

[OneSpeed]

Here is a simulation of the upcoming Starship Number 8 flight to 12.5km altitude and return. I didn't post a simulation of the projected 15km flight, because it seemed likely that the ship would go supersonic unless an engine was shut down prematurely, or there was a lot of propellant as ballast.

A 12.5km apogee makes a subsonic flight more likely, reducing the risk in what is already a risky enough proposition.

[Oersted]

Initial T/W of only 1.16? - Wauw, I had no idea it would lift off so slowly. That is going to be extremely impressive to watch. Saturn V vibes. Literally!

[leetdan]

The exclusion area extends a fair amount offshore, and isn't centered on the launch pad.  Isn't it reasonable that some amount of fuel would be spent in translating away from the ground infrastructure?  Wouldn't that translate into less deep throttling and higher TWR on liftoff?

[daveglo]

Here is a simulation of the upcoming Starship Number 8 flight to 12.5km altitude and return. I didn't post a simulation of the projected 15km flight, because it seemed likely that the ship would go supersonic unless an engine was shut down prematurely, or there was a lot of propellant as ballast.

A 12.5km apogee makes a subsonic flight more likely, reducing the risk in what is already a risky enough proposition.

THIS is the best explanation I've seen regarding the change from 15km to 12.5.  No dreamed-up safety rules or FAA administrative garbage, just pure aerospace engineering.

Nice work, OneSpeed (as usual)!

[OneSpeed]

The exclusion area extends a fair amount offshore, and isn't centered on the launch pad.  Isn't it reasonable that some amount of fuel would be spent in translating away from the ground infrastructure?  Wouldn't that translate into less deep throttling and higher TWR on liftoff?

It's certainly possible that the flight won't be straight up and down, as per some of the Grasshopper flights. As you can see from the sim, there is still about 40t of propellant remaining after landing. This ballast is required to keep the profile subsonic. So, there should be plenty of prop available to translate offshore a bit if they want, and literally glide back.

However, the T/W also needs to be quite low for the three engine hoverslam. From the sim, it will be quite sporty at around 1.7g versus Falcon 9 at around 1.3g. On that basis, I'm expecting a launch quite like Saturn V, as Oersted suggests.

[eriblo]

The exclusion area extends a fair amount offshore, and isn't centered on the launch pad.  Isn't it reasonable that some amount of fuel would be spent in translating away from the ground infrastructure?  Wouldn't that translate into less deep throttling and higher TWR on liftoff?

It's certainly possible that the flight won't be straight up and down, as per some of the Grasshopper flights. As you can see from the sim, there is still about 40t of propellant remaining after landing. This ballast is required to keep the profile subsonic. So, there should be plenty of prop available to translate offshore a bit if they want, and literally glide back.

However, the T/W also needs to be quite low for the three engine hoverslam. From the sim, it will be quite sporty at around 1.7g versus Falcon 9 at around 1.3g. On that basis, I'm expecting a launch quite like Saturn V, as Oersted suggests.

It is worth noting that the FCC permits that still apply specify a 2 km radius of operation from the pad which is significantly more restrictive than the exclusion zone or the TFR.

I wonder if they intend to have any significant propellant ballast - 40 t extra would be ~20 t sloshing around in the main tanks during the various flips and flops. I modified flightclubs 15 km profile to 12.5 km with a 3 Raptor ascent and 2 Raptor landing and adjusted the propellant load to 90 t have 25-30 t during descent (i.e. full headers and empty mains), ~7.5 of which are used for landing:

85% throttle, max velocity 350 m/s

and

50% throttle, max velocity 275 m/s

The 50% throttle case max velocity is the same as your simulation despite having 30 t of propellant less and 1 km higher cut off so I guess flightclub has higher ascent drag - I wonder if it might be related to the fact that it has a 9 m radius Starship Interestingly the propellant load works out to the same for both throttle settings suggesting that the increase in gravity losses are about the same as the decrease in drag losses.

[OneSpeed]

I wonder if they intend to have any significant propellant ballast - 40 t extra would be ~20 t sloshing around in the main tanks during the various flips and flops.

Perhaps, or maybe SN8 dry mass is more than my 120t estimate, and the required ballast is correspondingly less. For example, SN8 has been fabricated mostly from 4mm thick coil, and SpaceX is hoping to move to 3mm in the future. If her dry mass is actually 150t, then the prop load could be 30t less.

Either way, the sim tells us that the gross mass required to not exceed the speed of sound for a 12.5km apogee and 3 raptors at 50% throttle is around 240t. YMMV.

The 50% throttle case max velocity is the same as your simulation despite having 30 t of propellant less and 1 km higher cut off so I guess flightclub has higher ascent drag - I wonder if it might be related to the fact that it has a 9 m radius Starship

Interesting, why do they have a 9m radius Starship? Are they assuming skydiver orientation on the way up?

The exclusion area extends a fair amount offshore, and isn't centered on the launch pad.  Isn't it reasonable that some amount of fuel would be spent in translating away from the ground infrastructure?  Wouldn't that translate into less deep throttling and higher TWR on liftoff?

Thinking about this some more, the sub-tropical jetstream over Boca Chica could be as much as 25m/s on launch day, which would carry SN8 downrange somewhat for free.

[su27k]

Will there be attempts to simulate SN8's actual flight profile? It would be interesting to see how much performance they're holding back due to the need to burn all the way to apogee. I assume it's possible to get rough speed and altitude estimate from some of the amateur videos.

[RedLineTrain]

It would also be very interesting to see an updated Starship P2P flight based on what we know so far based on SN8.

This is a speculative simulation of a single stage Starship P2P flight. With 9 SL Raptors, and a full propellant load, the initial T/W is a healthy 1.6. So, throttle back for MaxQ occurs early, at the 36 second mark. If the ship were to continue to a purely ballistic trajectory, re-entry g forces would be prohibitive (~20gs). Instead, I've used negative pitch to flatten the trajectory, reducing the re-entry flight path angle. This allows the ship to skip like a stone on a pond, extending the range out to 10,000kms. The peak g force on the first 'bounce' is just over 4. If the Starship had larger (dragon?) wings, and hence a greater lift coefficient, the peak could be reduced further, and the range extended beyond 10,000kms.

[OneSpeed]

Will there be attempts to simulate SN8's actual flight profile? It would be interesting to see how much performance they're holding back due to the need to burn all the way to apogee. I assume it's possible to get rough speed and altitude estimate from some of the amateur videos.

I've started on one, it's tricky without the telemetry. From the frost lines, does anyone have a good number for how much propellant was on board?

It would also be very interesting to see an updated Starship P2P flight based on what we know so far based on SN8.

Perhaps unintuitively, SN8's flight changes the P2P model very little. So far, it actually appears to validate it.

[vaporcobra]

Will there be attempts to simulate SN8's actual flight profile? It would be interesting to see how much performance they're holding back due to the need to burn all the way to apogee. I assume it's possible to get rough speed and altitude estimate from some of the amateur videos.

I've started on one, it's tricky without the telemetry. From the frost lines, does anyone have a good number for how much propellant was on board?

Just knowing that SN8 was more or less hovering at apogee, that Raptor's nominal thrust range is 90-200 tons, and a maximum mass flow rate of ~550 kg, it should be relatively easy to calculate a reasonable range of propellant loads. Just splitting the difference and arbitrarily assuming an average Raptor mass flow rate of 400 kg/s (~72.5% thrust) during the test, I get a propellant mass of ~250 tons at liftoff.

If all Raptors were capped at either minimum or maximum throttle, propellant consumption would have been somewhere between 160 and 350 tons. Unfortunately, assuming SN8's empty weight is around 70 tons, I think any number in that range could be reasonable despite the slow acceleration observed at liftoff.

[OneSpeed]

Here is a simulation of the recent SN8 flight to 12.5km and return.

The simulation assumes a dry mass of 120t, and 230t of propellant. If that was the case, there would have been roughly 37t of residual propellant at landing.



Edit: changed plot to .jpg

[DreamyPickle]

SN8-telemetry.png fails to open unless extension is manually changes to BMP.

Also, the G-force meter seems to indicate passengers only experience 1.5G? The maneuver looks scary but might not be all that bad.

[wes_wilson]

SN8-telemetry.png fails to open unless extension is manually changes to BMP.

Also, the G-force meter seems to indicate passengers only experience 1.5G? The maneuver looks scary but might not be all that bad.

I was trying to imagine what that ride would feel like; would be interesting to hear it described in people terms.  You're weightless in orbit; you'd feel some semblance of gravity when re-entering and decelerating(?); back to zero gravity when you're free falling at terminal velocity; then a brief 1.5g as you rotate and back to full g as you land?

[steveleach]

SN8-telemetry.png fails to open unless extension is manually changes to BMP.

Also, the G-force meter seems to indicate passengers only experience 1.5G? The maneuver looks scary but might not be all that bad.

I was trying to imagine what that ride would feel like; would be interesting to hear it described in people terms.  You're weightless in orbit; you'd feel some semblance of gravity when re-entering and decelerating(?); back to zero gravity when you're free falling at terminal velocity; then a brief 1.5g as you rotate and back to full g as you land?

I suspect it would feel similar in many ways to being a passenger in a fast lap of a motor racing circuit. Terrifying, nauseating, exhilarating.

[cdebuhr]

SN8-telemetry.png fails to open unless extension is manually changes to BMP.

Also, the G-force meter seems to indicate passengers only experience 1.5G? The maneuver looks scary but might not be all that bad.

I was trying to imagine what that ride would feel like; would be interesting to hear it described in people terms.  You're weightless in orbit; you'd feel some semblance of gravity when re-entering and decelerating(?); back to zero gravity when you're free falling at terminal velocity; then a brief 1.5g as you rotate and back to full g as you land?

My understanding (FWIW - possibly not much!) is that you'd feel a heck of a lot more than "some semblance" of gravity during atmospheric entry and deceleration.  More like 3g to 4g, give or take.  Also, when falling at terminal velocity you will feel slightly more than 1g ... 1g for earths gravity, and slightly more because your actually decelerating all the way as the atmosphere get more dense as you fall.

[eriblo]

Since I have not yet seen it done: I took a shot at estimating the propellant load from SN8s flight from the visible frost.

For all flight attempts the LOX tank frost band reached just below the barrel weld above the aft bulkhead attachment weld. From Rafaels schematics the volume of the aft bulkhead is ~135 m^3 and the distance from attachment weld to barrel weld distance is ~0.5 m.

Combined main tank LOX volume (neglecting LCH4 piping): 166 m^3
Main tank LOX mass (density 1200 kg/m^3): 199 t
Total main tank propellant load (O/F = 3.6) : 254 t
Additional propellant in headers and down-comers: Up to 33 t

Rafael could give more exact numbers for the volumes but I estimate that the fuel would not quite fill the common bulkhead which is consistent with the lack of a corresponding frost line (making this close to an upper bound for propellant load). The other uncertainty is the height of the LOX frost band and how well it corresponds to liquid level - it looks to me like the barrel weld is visible so the height should be < 0.5 m. Using 0.4 m cuts total propellant by 10 t.

It is not too far of from the simulations but my feeling is that would be hard to get them to agree. I believe that they hovered at apogee specifically to minimize residuals (to prevent sloshing) and there is not much room for using more propellant - drag and throttle losses are minimal and the dry mass is already at the upper bound of the given numbers and best estimates.

[OneSpeed]

Here is a simulation of an orbital Starship launch, updated to have a 28 engine Super Heavy. The payload is 150t to LEO, and I'm assuming Super Heavy has 8 x 210t thrust Raptors with gimbal and throttle, and 20 x 300t thrust Raptors without either.

[vaporcobra]

Here is a simulation of an orbital Starship launch, updated to have a 28 engine Super Heavy. The payload is 150t to LEO, and I'm assuming Super Heavy has 8 x 210t thrust Raptors with gimbal and throttle, and 20 x 300t thrust Raptors without either.

Excellent work, as always! Out of curiosity, are you able to model performance to inclinations that would require dogleg maneuvers? Namely 53, 70, and 97.6 degrees for Starlink. Unclear if 97.6 degrees is even within the realm of possibility but that would be interesting to see

[OneSpeed]

Excellent work, as always! Out of curiosity, are you able to model performance to inclinations that would require dogleg maneuvers? Namely 53, 70, and 97.6 degrees for Starlink. Unclear if 97.6 degrees is even within the realm of possibility but that would be interesting to see

Thanks! Yes, SpaceSim can model any launch azimuth, as well as doglegs on top of those. For Starship, the range of allowable launch azimuths will depend on the launch site. Phobos and Deimos could potentially make nearly any azimuth achievable, and so doglegs might not be necessary. With that in mind, do you have a short list of scenarios?