Author Topic: Feasibility of a Raptor SSTO and Comparisons to A Fully Reusable Launch System  (Read 21319 times)

Offline dante2308

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Calculated the Isp for Raptor engine at 320s SL, 356s vac. Any objections?

Offline deltaV

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Calculated the Isp for Raptor engine at 320s SL, 356s vac. Any objections?
What sort of nozzle, first stage or vacuum?

Offline dante2308

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Calculated the Isp for Raptor engine at 320s SL, 356s vac. Any objections?
What sort of nozzle, first stage or vacuum?

First stage. 1m radius. It kind of depends on the camber pressure they achieve with staged combustion.
« Last Edit: 11/18/2013 06:45 pm by dante2308 »

Offline baldusi

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Reusable SSTO could be also tried with a full stage CH4/LOX rocket with composite tanks. Common bulkhead is trivial for this combo, the LNG part is already developed, you'd only need the LOX composites. A FS LNG/LOX with a chamber pressure of 60MPa would do something like 360s/385s SL/Vac! And have an amazing T/W for that technology, probably at 130 or even higher. Of course it would be an expensive rocket, but the mix of very high pmf and high isp should enable a reusable SSTO. BTW, it could well evolve from the Raptor family.

Offline Rabidpanda

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How would you get sea level Isp that high?

Offline dante2308

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Edit: Superseded by next post.
« Last Edit: 11/19/2013 06:45 am by dante2308 »

Offline dante2308

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Superseded by more accurate calculations.
« Last Edit: 12/15/2013 03:45 pm by dante2308 »

Offline hkultala

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Sorry about the roughness. Here is my first calculated orbit:

Launch from Florida:
The initial acceleration is 2.65101426361 m/s^2
The initial thrust percent is 100
The initial number of engines cut off are 0 out of 5
Main Engine Cut OFF
The time is 348.21 seconds
Velocity is 7.87330442438 km/s
Altitude is 134.127262879 km
The final acceleration is 9.44607675087 m/s^2
The final thrust percent is 99
The final number of engines cut off are 0 out of 5
Orbit Achieved!! Circular orbit velocity was 7.83321791163 km/s
Current mass is 18.2961705186 tons
The apogee (max Earth radius) is 524.819854367 km
The perigee (max Earth radius) is 117.608332424 km
The inclination is 27.8052772316 degrees

Obviously this isn't optimal yet and I can get a little more mass to an even higher orbit if i play with it a bit. What do you think about the numbers?

117 km perigee sound like it's going to drop after couple of orbits due atmospheric drag, not very good orbit.


Offline dante2308

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Sorry about the roughness. Here is my first calculated orbit:

Launch from Florida:
The initial acceleration is 2.65101426361 m/s^2
The initial thrust percent is 100
The initial number of engines cut off are 0 out of 5
Main Engine Cut OFF
The time is 348.21 seconds
Velocity is 7.87330442438 km/s
Altitude is 134.127262879 km
The final acceleration is 9.44607675087 m/s^2
The final thrust percent is 99
The final number of engines cut off are 0 out of 5
Orbit Achieved!! Circular orbit velocity was 7.83321791163 km/s
Current mass is 18.2961705186 tons
The apogee (max Earth radius) is 524.819854367 km
The perigee (max Earth radius) is 117.608332424 km
The inclination is 27.8052772316 degrees

Obviously this isn't optimal yet and I can get a little more mass to an even higher orbit if i play with it a bit. What do you think about the numbers?

117 km perigee sound like it's going to drop after couple of orbits due atmospheric drag, not very good orbit.
The slightest burn at apogee will fix that. This is about mass. The only variable I used was the number of Raptors. Everything else was optimized for that. The orbit has quite enough energy for LEO. I didn't have the time to go in and make the very fine corrections needed to put it in a higher perigee. Also note that the actual perigee is closer to 134 km about the time of maximum inclination.

I want to emphasize the lack of assumptions again. I optimized Isp straight from thermodynamics and chemistry. That drag you speak of is built into the code, the earth is oblate, initial conditions change with latitude and longitude, the Isp depends on the local density and pressure, and the drag force depends on relative velocity, Mach number, shock temperature, and altitude.
« Last Edit: 11/19/2013 12:47 pm by dante2308 »

Offline e of pi

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So the final mass placed into orbit is 18.3 tons, out of...how many? Back calculating from your earlier-stated ISp of 320 SL and 356 vac (which I assume you used here) and the initial 5 engines at (300,000 kgf) * (320/356),  I got that to have the initial acceleration of (9.81+2.65) m/s^2 you note, the vehicle would be about 400,000 kg on the pad. That means about 4.6% of the gross mass is tanks, engines, structure, TPS, and payload. That seems, in my opinion, to be more aggressive a target than is reasonable.

Offline baldusi

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How would you get sea level Isp that high?
Full staged combustion to enable a 60MPa chamber pressure. RD-191 is about 25, NK-33 and RD-0162 are about 17. So even for sea level you'd have enormous expansion ratio. Say, like 100:1 or even more. And yes, that would be for Sea Level.

Offline dante2308

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So the final mass placed into orbit is 18.3 tons, out of...how many? Back calculating from your earlier-stated ISp of 320 SL and 356 vac (which I assume you used here) and the initial 5 engines at (300,000 kgf) * (320/356),  I got that to have the initial acceleration of (9.81+2.65) m/s^2 you note, the vehicle would be about 400,000 kg on the pad. That means about 4.6% of the gross mass is tanks, engines, structure, TPS, and payload. That seems, in my opinion, to be more aggressive a target than is reasonable.

It's too aggressive. It's within Spacex's current architecture, but no, I don't support SSTO now. We need greater Isp engines with high thrust.
« Last Edit: 11/19/2013 10:11 pm by dante2308 »

Offline QuantumG

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It's too aggressive. It's within Spacex's current architecture, but no, I don't support SSTO. F = Isp*Veff*mdot so keeping mdot the same, you get a non-linear relationship between force and Isp.

.. but your turnaround time for the entire vehicle is vastly faster, possibly even just gas-and-go, and your ground infrastructure requirements are vastly less, possibly even just a refueling truck.

The question of fully reusable SSTO is one of plausibility, not utility. If you could get a reusable SSTO to work, with even a marginal payload, someone would find a use for it.

If you could build a first stage of a reusable TSTO that could also operate in reusable SSTO mode, you'd have the best of both worlds.
Human spaceflight is basically just LARPing now.

Offline go4mars

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It's too aggressive. It's within Spacex's current architecture, but no, I don't support SSTO. F = Isp*Veff*mdot so keeping mdot the same, you get a non-linear relationship between force and Isp.

.. but your turnaround time for the entire vehicle is vastly faster, possibly even just gas-and-go, and your ground infrastructure requirements are vastly less, possibly even just a refueling truck.

The question of fully reusable SSTO is one of plausibility, not utility. If you could get a reusable SSTO to work, with even a marginal payload, someone would find a use for it.

If you could build a first stage of a reusable TSTO that could also operate in reusable SSTO mode, you'd have the best of both worlds.
Or first stage lands, someone throws an ablative tarp on top, second stage lands on the first.  Or something else efficient (and more plausible like a crane and click-in connections).
« Last Edit: 11/19/2013 10:39 pm by go4mars »
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Offline QuantumG

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Or something else efficient (and more plausible like a crane and click-in connections).

We shall call it "click and go" TSTO reusability. Spread the word.

*cough*LEGO*cough*
Human spaceflight is basically just LARPing now.

Online meekGee

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Or something else efficient (and more plausible like a crane and click-in connections).

We shall call it "click and go" TSTO reusability. Spread the word.

*cough*LEGO*cough*

Integration between first and second stage should be LEGO, no coughing.

It's just that no rocket to date has been designed this way.

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Offline StephenB

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This is getting a little fairies in the flame trench ish.

Offline rst

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Or first stage lands, someone throws an ablative tarp on top, second stage lands on the first.  Or something else efficient (and more plausible like a crane and click-in connections).

Land on top?  And if the second stage thrusters get wonky on landing, it potentially takes the first stage with it.

Besides, I would have thought that the time-consuming parts of setting up the stack would be managing the connections between the stages, and between both and the support structure (electrical and fuel lines, etc.).   The crane is a whole lot more plausible.

Online meekGee

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Or first stage lands, someone throws an ablative tarp on top, second stage lands on the first.  Or something else efficient (and more plausible like a crane and click-in connections).

Land on top?  And if the second stage thrusters get wonky on landing, it potentially takes the first stage with it.

Besides, I would have thought that the time-consuming parts of setting up the stack would be managing the connections between the stages, and between both and the support structure (electrical and fuel lines, etc.).   The crane is a whole lot more plausible.

I'm sure the "land on top" comment was facetious.

However, with fully reusable components, there shouldn't be any connections between the stages.  The first stage should be a stand-alone flyer, no different than a carrier airplane".   This was never the case since expendable rockets need only one set of avionics, so the top of stack always drives the rocket.  But even with today's F9.1, the second stage has all the hardware necessary to do so. (and as far as I know, maybe it does).

In this case, the second stage only shadows before stage separation, but then becomes its own master at the moment of separation.

If you do that, the amount of communication necessary between the stages is minimal, and basically no electrical connection are necessary.

Connecting the two stages then is purely mechanical, and if you build a worthy latch system, that's all there is to it - "LEGO".

This kinda eliminates (or at least sharply reduces) one of SSTO's greatest selling points - the elimination of stage integration.

Payload integration, meanwhile, is pretty equivalent.


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Offline QuantumG

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If that were possible, I'd imagine the only other selling point of the SSTO would be the ability to land and relaunch from multiple launch sites. Aka, point to point. That'd be a minor advantage, though, as you could just have first stages at each launch site that return to their respective launch site after each launch.

Now, here's a thought: wouldn't it be nice to have one of these fully reusable TSTO first stages stationed at a base on Mars? Or the Moon.


Human spaceflight is basically just LARPing now.

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