Author Topic: Feasibility of a Raptor SSTO and Comparisons to A Fully Reusable Launch System  (Read 20972 times)

Offline meekGee

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Exactly - in a point-to-point scenario, the first stages are bound to their respective airports, and the second stages travel the world.

Mathematically speaking, since for every upper stage that lands there is an upper stage that leaves shortly thereafter, (since the number of upper stages at any airport can't accumulate) then it's equivalent to the case where the first stage arrives with the upper stage and departs with it (aka single-stage suborbital plane).  (The continuity condition for imaginary spaceplanes!)

But all in all - I am not sure point-to-point is indeed a coming market - and I don't think it affects SpaceX designs.

I am pretty convinced that in Elon's mind, you don't have a "rocket that is shedding parts on ascent", but rather the first stage, complete with legs, avionics, etc, is just a carrier aircraft.  That's how come he's speaking in terms of hours per turn-around, and why SSTO is not a buzz word with him.   SSTO was commonly conceived as the anti-thesis for a traditional staging rocket - but it loses its allure in the face of two-component (not stage!) launch system.
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Offline guckyfan

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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.

Actually, no. Gravity on Mars allows for SSTO and back. So no real need for a first stage.

Offline ArbitraryConstant

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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.
Might the second stage might make more sense in either case? Delta-v from Mars surface to Mars orbit is similar to second stage delta-v to Earth orbit, and the surface pressure is near vac so vac optimized engines are fine.

Earth first stages solve a problem that basically only exists on Earth.

Offline meekGee

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Yes - SSTO on Mars makes sense, and seems to come out similar to a second stage.

Whether they'll be the same thing exactly - if I can get that knowledge for Christmas, I'll be a happy little boy.

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Offline QuantumG

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Yes - SSTO on Mars makes sense, and seems to come out similar to a second stage.

Whether they'll be the same thing exactly - if I can get that knowledge for Christmas, I'll be a happy little boy.

5 km/s sounds like a nice round number. It'll get you to the surface of Mars from LEO on a ~6 month trajectory and, after refueling, back to Earth again, while also giving you global planetary access.

I hear those things are awfully loud. It glides as softly as a cloud. What's it called? Monowhale!

Offline guckyfan

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Yes - SSTO on Mars makes sense, and seems to come out similar to a second stage.

Whether they'll be the same thing exactly - if I can get that knowledge for Christmas, I'll be a happy little boy.

They may be designed for Mars duty but they will very likely begin their life as a second stage of some vehicle launching from earth. IMHO of course.

Offline dante2308

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

Offline e of pi

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I did a little optimization on my launch trajectory.
*snip*
"Main engine cut off! The mass fraction is 0.0354212260744"
*snip*
It's a bit more feasible.
Except that I'm not aware of any rocket stage that has 3.5% burnout mass--and to get any payload at all, you'de need less than that. Your old version only required 4.6%, this requires more aggressive structure, not less. Elon says FH is supposed to have 1/30th (or about 3.3%) but even that unprecedented number would only leave about 0.16% of the gross liftoff weight as payload. That's less than 1.6 tons for this thousand ton monster, and it's got to be expendable to manage that.

Offline dante2308

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I did a little optimization on my launch trajectory.
*snip*
"Main engine cut off! The mass fraction is 0.0354212260744"
*snip*
It's a bit more feasible.
Except that I'm not aware of any rocket stage that has 3.5% burnout mass--and to get any payload at all, you'de need less than that. Your old version only required 4.6%, this requires more aggressive structure, not less. Elon says FH is supposed to have 1/30th (or about 3.3%) but even that unprecedented number would only leave about 0.16% of the gross liftoff weight as payload. That's less than 1.6 tons for this thousand ton monster, and it's got to be expendable to manage that.

Your calculation of 4.6% was wrong. Each engine is about 2.94 MN. (326/365)*5*2.94MN/(9.81+2.85) ~=  1,100,000 kg. The mass percent was under 2 last time. I don't think we're there yet, but there is a slight mass savings from having a single stage and no interstage. Also no separate upper stage engine (not a trivial weight mind you). I think 4% would be achievable. We'll see.

Edit: I just looked at what you wrote a bit more carefully. 3.542%-3.3% = 0.242% = 2.42 tons.
« Last Edit: 12/07/2013 10:42 PM by dante2308 »

Offline dante2308

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Just one more comment to my dead thread on SSTO. I've (nearly) perfected my orbit calculator. It will target a specified circular orbit and produce a lot of information. Barring bugs and computer numerical integration error, it will produce a lot of very accurate information based on many dynamic variables including:

Drag as a function of pressure, altitude, Mach number, temperature
Initial conditions as a function of Latitude and Longitude
Thrust as a function of a maximum g-load with respect to the available thrust envelope
Earth size and shape as a function of oblateness
Dynamic Isp changes as a function of atmospheric pressure and other variables

My first outputs were off due to a few bugs, poor trajectory optimization, and a large time step. My most recent results are much more accurate, but not perfect. However, they are accurate enough to answer some questions.

For those of us who don't have access to STK, I would be happy to calculate the performance of hypothetical rockets. Here is the most recent output for my program based on the question in the OP:

The lift off mass is 1000 tons
The initial acceleration is 3.25979758456 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 mass fraction is 0.0371414054242
The time is 316.76 seconds
Velocity is 7.8138971901 km/s
Altitude is 166.578020591 km
Altitude is 150.058766559 km (max Earth radius)
The final thrust percent is 70
The final number of engines cut off are 4 out of 5
Orbit Achieved!! Circular orbit velocity was 7.81376010016 km/s
Current mass is 37.1414054242 tons
The apogee (max Earth radius) is 154.235545544 km
The perigee (max Earth radius) is 147.93978061 km
The inclination is 28.5722681456 degrees

Attached is a sample local altitude versus time graph during the ascent.

To answer the question of SSTO for Raptor, you can get at least 3.7% of the take off mass into a 150kmx150kmX28.6 orbit as measured by the maximum radius of the Earth. The delta v back to the atmosphere from this orbit is not particularly onerous at 39.27m/s or a few hundred kg of fuel. It is not impossible, but it is an engineering nightmare. Much more useful would be calculations from the Martian surface.


Offline Avron

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Yes - SSTO on Mars makes sense, and seems to come out similar to a second stage.

Whether they'll be the same thing exactly - if I can get that knowledge for Christmas, I'll be a happy little boy.

5 km/s sounds like a nice round number. It'll get you to the surface of Mars from LEO on a ~6 month trajectory and, after refueling, back to Earth again, while also giving you global planetary access.



I think we are getting closer. Trying to find the factual comments from Elon and not what the reporters interpreted .. one is   I think you could land with the entire thing, said Musk (Royal Aeronautical Society ) he also noted -  "Probably not a Mars cycler; the thing with the cyclers is, you need a lot of them," Musk ( SPACE.com)

I was looking for where Elon mentions  Raptor in connection with a vehicle, and all I can find is "spaceship" (Royal Aeronautical Society ) . I cannot find Elon mention Raptor and launch vehicle in the same context.

What we do know is Elon wants the "spacecraft" back . and 'ship back because those things are expensive'

Offline meekGee

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I think we are getting closer. Trying to find the factual comments from Elon and not what the reporters interpreted .. one is   I think you could land with the entire thing, said Musk (Royal Aeronautical Society ) he also noted -  "Probably not a Mars cycler; the thing with the cyclers is, you need a lot of them," Musk ( SPACE.com)
That meshes well with the 3-month transit statement.
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Offline dante2308

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Yes - SSTO on Mars makes sense, and seems to come out similar to a second stage.

Whether they'll be the same thing exactly - if I can get that knowledge for Christmas, I'll be a happy little boy.

5 km/s sounds like a nice round number. It'll get you to the surface of Mars from LEO on a ~6 month trajectory and, after refueling, back to Earth again, while also giving you global planetary access.



I think we are getting closer. Trying to find the factual comments from Elon and not what the reporters interpreted .. one is   I think you could land with the entire thing, said Musk (Royal Aeronautical Society ) he also noted -  "Probably not a Mars cycler; the thing with the cyclers is, you need a lot of them," Musk ( SPACE.com)

I was looking for where Elon mentions  Raptor in connection with a vehicle, and all I can find is "spaceship" (Royal Aeronautical Society ) . I cannot find Elon mention Raptor and launch vehicle in the same context.

What we do know is Elon wants the "spacecraft" back . and 'ship back because those things are expensive'

Musk also stated that a cycler would have an inclination issue.

Offline dante2308

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Yes - SSTO on Mars makes sense, and seems to come out similar to a second stage.

Whether they'll be the same thing exactly - if I can get that knowledge for Christmas, I'll be a happy little boy.

Merry Christmas! Not only does SSTO work on Mars, you can escape Mars with more mass than a two stage rocket could put in LEO with the same lift-off mass.

The very same SSTO:

The lift off mass is 1000 tons
The initial acceleration is 43.6676675604 m/s^2
The initial thrust percent is 88
The initial number of engines cut off are 2 out of 6
Main Engine Cut OFF
The time is 123.085 seconds
Velocity is 4.98660320793 km/s
Altitude is 65.3562801306 km
Altitude is 49.9313317088 km (max Mars radius)
The final thrust percent is 70
The final number of engines cut off are 5 out of 6
Mars Escape!! Escape orbit velocity was 4.98601698629 km/s
Current mass is 124.7255312 tons

Major problem trying to get into LMO. You need some really deep throttle.

Quote
Design details of Falcon Heavy, and of Merlin 1D performance, have not been divulged.  In order to achieve the payload capability claimed by SpaceX, the new rocket engine will have to provide improved specific impulse and the stages will have to provide very high propellant mass ratios.  SpaceX claimed that the two "first stage" strap-on units will achieve a 30 to 1 gross mass to dry mass ratio, implying an unprecedented propellant mass fraction of better than 0.966

If one assumes that Elon landed 50 tons on Mars and 34 tons of that would need to be structural to get up to 1000 tons with the quoted fraction. Of course this is silly. You don't need anything that size.

Point is that SSTO works wonderfully on Mars. We should move there...

« Last Edit: 12/17/2013 02:28 AM by dante2308 »

Online Robotbeat

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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.

Yeah, I'm not sure how comfortable I am with hand-waving away how difficult integrating two stages is, but I certainly think it's an /entirely/ plausible alternative to the much more technically difficult reusable SSTO problem. If there's a need for such rapid integration, it's certainly technically feasible.
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