SpaceX 'Star series' simulations

[OneSpeed]

This topic is intended to be along the lines of the  SpaceX Falcon Mission Simulations https://forum.nasaspaceflight.com/index.php?topic=42389.0 thread, but for the SpaceX Star series: StarHopper, Starship and Super Heavy.

First cab off the rank is a simulation of the StarHopper hopping to about 5kms altitude, hovering for a while, and landing back on the same pad. This simulation was first posted a week ago in L2, but since then many of the assumptions like 9mØ propellant tanks have become public knowledge, so I thought I'd post a minor update here. A week is like dog years at SpaceX! If the three Raptor engines provide 600t of thrust, then a GLOW of around 500t would provide a feasible T/W ratio of 1.2 at liftoff. Wet and dry mass fractions are a total guess at this stage, mine is 100t dry.

Because I've never simulated a hover for such an extended period, I've not noticed the drift that you will see in the video before. The rocket climbs vertically, and there is no wind in SpaceSim, so I assume the drift is actually caused by the rotation of the earth. Anyway, for that reason I needed to translate the rocket back towards the landing pad by about 200m on descent, and this was more difficult than you might assume. Pitching the rocket worked initially, but as the sink rate increased, so did the lift, in the opposite direction to the pitch. It 'felt' like descending in an aircraft with no tailplane, and could lead to tumble. So, since the hopper has no grid fins attached, I think it will need an RCS, perhaps pressure fed methalox vernier thrusters, if it is to descend with any velocity.

[jpo234]

Can your simulation figure out a rough estimate how high the hopper can actually hop? Could it break the Karman line and officially reach space similar to New Shepard?

[OneSpeed]

Can your simulation figure out a rough estimate how high the hopper can actually hop? Could it break the Karman line and officially reach space similar to New Shepard?

That's a good question. The sim assumes an available ΔV of 5.2 km/s. That's easily enough to reach the Karman line as long as you are happy to go supersonic, but maintaining control while descending might be another matter. If I simply set the throttle to 100% and burn to depletion, then theoretically the ship gets to about 960kms.

[philw1776]

960Kms amazing! Typo.

I did a simple spreadsheet assuming dry mass, propellant tonnage 400 tonnes limited to decent T/W and get ~5.2Km/sec.  Maybe (I doubt this) it could fly over the Karman line and use some remaining propellant to reduce velocity striking the atmosphere to retain control.  Beyond my ken.

EDIT: this crude hopper is not designed to go supersonic so the point is moot.  MaxQ would be "interesting"

Appreciate your excellent far better sims.

[envy887]

Can your simulation figure out a rough estimate how high the hopper can actually hop? Could it break the Karman line and officially reach space similar to New Shepard?

That's a good question. The sim assumes an available ΔV of 5.2 km/s. That's easily enough to reach the Karman line as long as you are happy to go supersonic, but maintaining control while descending might be another matter. If I simply set the throttle to 100% and burn to depletion, then theoretically the ship gets to about 960kms.

How high can it go while staying subsonic, both up and down? How about at a realistic dynamic pressure of perhaps a few psf? Ignoring the control issues of flying backward...

[OneSpeed]

How high can it go while staying subsonic, both up and down? How about at a realistic dynamic pressure of perhaps a few psf? Ignoring the control issues of flying backward...

A replacement nosecone is being constructed for StarHopper, and it looks more substantial than the first. Descending fins first with any velocity sounds like a recipe for disaster to me, but since you asked for it, here's a StarHopper sim limited to about Mach 0.75. Since there is no sign yet of vernier thrusters or grid fins, it also assumes the engines must be kept running in order to provide control authority. It could go quite a bit higher if it went supersonic at high altitude, and if the engines could be shut down and restarted.

[envy887]

Interesting... It looks like you're running all three engines the whole time? I'm assuming they will want to test inflight restarts. Maybe on just the outer engines, but possible on all three.

How high can it go if it accelerates to Mach 0.8 as quick as possible, holds that velocity for a while, shuts down all the engines and coasts to apogee (should be aerodynamicly stable thanks to the fins), restarts one engine at low throttle shortly after apogee to control the vehicle in free fall and avoid breaking Mach 0.8 on decent?

[edzieba]

this crude hopper is not designed to go supersonic so the point is moot.

Unless plans have changed (not impossible), the hopper is intended to go supersonic.

[docmordrid]

this crude hopper is not designed to go supersonic so the point is moot.

Unless plans have changed (not impossible), the hopper is intended to go supersonic.

That tweet could easily be about Starship Mk-1 - due this summer.

[dubya]

That tweet could easily be about Starship Mk-1 - due this summer.

Indeed. The definitive NO at the beginning of the sentence makes that conclusion pretty inescapable.

[edzieba]

The 'no' is in reference to the (now cancelled) S2 conversion mini-BFS vs. the now-actually-constructed BFS test article (AKA "BFR Test Ship"). That seems pretty definitive that the hopper will go supersonic, and I can see no reason why it could not do so. Its whole purpose is to replicate the supersonic-to-landing regime, with the follow-on Starship test vehicle then expanding that envelope to include the (sub)orbital through hypersonic re-entry regimes (replacing the previously proposed mini-BFS).

[Lar]

The 'no' is in reference to the (now cancelled) S2 conversion mini-BFS vs. the now-actually-constructed BFS test article (AKA "BFR Test Ship"). That seems pretty definitive that the hopper will go supersonic, and I can see no reason why it could not do so. Its whole purpose is to replicate the supersonic-to-landing regime, with the follow-on Starship test vehicle then expanding that envelope to include the (sub)orbital through hypersonic re-entry regimes (replacing the previously proposed mini-BFS).

I think that's an incorrect interpretation since the question asked was specific about hopper practicing the belly flop. His No seemed definitive to me.

[edzieba]

The 'no' is in reference to the (now cancelled) S2 conversion mini-BFS vs. the now-actually-constructed BFS test article (AKA "BFR Test Ship"). That seems pretty definitive that the hopper will go supersonic, and I can see no reason why it could not do so. Its whole purpose is to replicate the supersonic-to-landing regime, with the follow-on Starship test vehicle then expanding that envelope to include the (sub)orbital through hypersonic re-entry regimes (replacing the previously proposed mini-BFS).

I think that's an incorrect interpretation since the question asked was specific about hopper practicing the belly flop. His No seemed definitive to me.

The root question was on the modified S2 mini-BFS, not the hopper. Twitter is awful for linking with context, so a screenshot will have to do:

[OneSpeed]

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.

[livingjw]

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.

Did you hold AoA constant at 40 degrees? What was your L/D?

John

[envy887]

Did you hold AoA constant at 40 degrees? What was your L/D?

John

Lift and drag are both shown in the simulation. Hypersonic L/D looks like about 1.1.

[speedevil]

This reminds me of http://www.astronautix.com/s/saengerantipodalbomber.html - the description is somewhat fuzzy.
I have failed to find a source for "Concerning Rocket Propulsion for Long-Range Bombers" - and was wondering if the similarities might be more or less obvious with it.
Clearly the aimed for hypersonic L/D is much, much less with Starship.

[Keldor]

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.

What would a flight with 6-9 SL raptors and 3 vac raptors look like?  Or what about dual bell, for allowing deep throttling at landing while allowing high thrust semi-vacuum operation the rest of the time, in the same manner as SSME?  I know Elon Musk said duel bell didn't make sense, but I think that was in the context of Starship sitting on top of Superheavy, where the engines would always be firing in either a very high altitude as a second stage where TWR is less important or else when landing at sea level with a very low thrust.  Single Stage suborbital changes everything.

Is converting an engine like raptor to use a toroidal aerospike likely to be feasible without a drastic overhaul?  Aerospikes have limited utility for normal two stage flight, but again, single stage changes everything.

[OneSpeed]

Did you hold AoA constant at 40 degrees? What was your L/D?

John

Actually I held pitch constant at 42 degrees, much the same as a spinning stone would do, so the AoA varies a little bit. I modulated the pitch between runs to see what gave the best range. This sim was the best result, with L/D at around 1.2. The best trade was to maximise lift regardless of drag, which kept the ship out of the sensible atmosphere for the longest periods between 'bounces'. On average, this minimised the effect of the drag, and gave the best range.

What would a flight with 6-9 SL raptors and 3 vac raptors look like?

Perhaps counterintuitively, not as good. With 9 Raptors and a 3g limit on acceleration, I'm either throttling back or shutting down engines from the 90 second mark, when the ship is already at 34kms altitude and just over 1km/s. More than 9 engines would just add mass for no benefit. Vacuum engines would have less thrust at low altitude, which is when maximum thrust is most useable.

Is converting an engine like raptor to use a toroidal aerospike likely to be feasible without a drastic overhaul?  Aerospikes have limited utility for normal two stage flight, but again, single stage changes everything.

Even if it was an easy conversion, I'm not convinced there would be much advantage. Although not everyone agrees with me on this, once you have sufficient engines symmetrically arranged, then above a certain altitude, plume interaction generates recirculative flow against the dance floor. This gives the effect of a single 'virtual' vacuum nozzle, not unlike the toroidal aerospike. That's why I'm suggesting an engine arrangement as per below. There would still be plenty of room for cargo bins, but not that many would be needed with the 10t payload this sim suggests.

[Ultrafamicom]

Did you hold AoA constant at 40 degrees? What was your L/D?

John

Lift and drag are both shown in the simulation. Hypersonic L/D looks like about 1.1.

Isn't that a bit too high? Even shuttle has only a Hypersonic L/D of less than 1.5