Author Topic: Operating the ITS spaceship or tanker as an SSTO launch vehicle.  (Read 10258 times)

Online rakaydos

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I fail to see how vertical or horizontal makes any difference there. The problem is the same.
I'd argue that Airlaunch (horizontal takeoff TSTO) is different enough from traditional rocketry to merit a distinction, but that Horizontal SSTO suffers from the worst of both worlds.

Now, if we could get an airllaunch stage that could compete with the capability of even a RTLS Falcon 9 first stage... Upper atmoshphere with a high mach number, and all that. SCRAMjet and SABER may be possibiities in the future... as is, amusingy enough, certian blimp concepts.

Online OneSpeed

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12 Raptor 80 how much propellant mass were you using?
What is the maximum propellant mass for the tanker for their propellant tank size?
SX PDF slide 36 show 2,500 mt propellant.
And shows 380 mt propellant delivered to orbit. I assume that was part of the 2,500 mt and not extra.

I was assuming the same propellant mass as the maximum for the ITS tanker, i.e. 2500mT, for a GLOW of 2600mT. And yes, that the 380mT ITS Tanker payload would be residual from its 2500mT of propellant.

Offline john smith 19

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I'd argue that Airlaunch (horizontal takeoff TSTO) is different enough from traditional rocketry to merit a distinction, but that Horizontal SSTO suffers from the worst of both worlds.
The classic question of air launch is "Does it let us remove a stage." Historically the answer has been "no," so you're down to other benefits like lower development costs (if you're using more or less existing stages, as Orbital did with Pegasus).

The upside of HTO is that wings let you build bigger vehicles as you need thrust which is 1/3 to 1/2 the GTOW. VTO needs thrust >1x GTOW (at least 1.1). Wheather that's enough of an advantage depends on how good your structural design is and if you can tuck the wing and other major parts in the structural budget. That's a known (but non trivial) problem for aircraft designers, but not so much for people used to the simpler shape of a rocket stage.
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Now, if we could get an airllaunch stage that could compete with the capability of even a RTLS Falcon 9 first stage... Upper atmoshphere with a high mach number, and all that. SCRAMjet and SABER may be possibiities in the future... as is, amusingy enough, certian blimp concepts.
That's SCramjet and SABRE.

SABRESkylon is the joker in the pack. It's the only plan that designed to deliver TSTO payload in an SSTO design. It also gives you about 6x the Isp of a rocket in air breathing mode, allowing you to afford the structural mass that can make the system reusable.

Blimp concepts ( historically called rockoons) suffer from the problem that once the rocket separates you have a very short time to either vent the lift gas or ("magically") compress (or liquefy) it before your balloon rises so high the pressure differential splits it (or burst height).

If you can solve that problem you can make a reusable launch system. If you can't you won't. The problem is as you go higher the mass you can lift falls so the balloon size you need gets radically bigger, so you are into dealing with very large volumes of fairly low pressure gases quickly
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Online Robotbeat

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Air launch does kind of help you remove a stage. Taurus used an extra stage as Pegasus.

But sometimes adding an extra stage may be easier than the complications of air launch.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Offline ZachF

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FWIW an SSTO ITS could be a pretty serious weapons platform. Even if it's payload is only 40 tons, 40 tons of GPS-guided tungsten rods at ~8km/s will be pretty deadly.

I kinda hope it doesn't though...

Online rakaydos

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Blimp concepts ( historically called rockoons) suffer from the problem that once the rocket separates you have a very short time to either vent the lift gas or ("magically") compress (or liquefy) it before your balloon rises so high the pressure differential splits it (or burst height).
There's one group workign with a "blimp relay race" concept, where they design an upper atmsphere blimp that could never fly below 10 miles high, but that has a low enough envelope pressure that it can hold it's thin helium even against vacuum. (it is of course huge- over a KM long, to support itself with low pressure helium. Fortunately turbulance is negligable at that height) They intend to use this upper atmoshere blimp as first a launch platform, and later potentially an orbital vehical itself, if they can get certian drag reduction technoligies out of the lab and into real life.
They need to load it using conventional ground-to-high atmosphere blimps, but cargo transfer is gradual enough that you can compress gas to avoid popping the ferry balloon.

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Online envy887

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To kind of return to the topic of ITS as an SSTO, I wonder how well the ITS design' mass fraction fare if it's scaled down. I think everything goes down pretty linearly

Pressure vessel mass scales directly with volume, which is linear with lift-off mass.
Engine mass scales with thrust which is linear with lift-off mass.
Landing legs scale with landing mass, so if everything else affecting mass fraction scales linearly they will as well.
TPS is a function of area/mass. A smaller vehicle has a better area/volume ratio and so should have a better area/mass ratio.

So a 1/10 mass scale of the tanker would be 1/101/3 in diameter and length, which is 5.6 m diameter by 23 m length. It could dry mass at 9 tonnes, hold 250 tonnes of subcooled methalox, and have 1 Raptor engine which could be optimized for SL to vacuum. Despite more air drag reach the ISS from the Cape with about 6.5 tonnes of payload while reserving 2% of its fuel for landing. 4 methalox RCS thrusters could provide a unity TWR at landing.

That's right about the performance of Falcon 5 for ISS missions, and it's also similar to SSO orbits and other LEO destinations, all in a 265 tonne at liftoff fully reusable SSTO. Could be very useful for transporting small payloads to a space station quite cheaply.


Online OneSpeed

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I did a similar simulation recently of a half scale BFS with two Raptor 80 engines. I got 7mT to 400 x 400 kms, perhaps I was more conservative on mass fractions and ullage.

Anyway, we're starting to build up quite a range of ITS alternative configurations. Some of these figures are projections from 300 x 300km simulations.

NameConfigurationMass to 400 x 400 kmsVehicle massPayload
Micro BFS SSTO1 x Raptor 5025.5 mT9 mT6.5 mT
Mini BFS SSTO2 x Raptor 8043 mT18 mT7 mT
BFS SSTO9 x Raptor 50187 mT90 mT25 mT
BFS SSTO3 x Raptor 50 + 6 x Raptor 80193 mT92 mT29 mT
FH Booster / Mini BFS27 x Merlin 1D + 2 x Raptor 5068 mT73 + 18 mT36.5 mT
BFS SSTO12 x Raptor 80236 mT100 mT63 mT
Mini ITS7 x Raptor 50 + 2 x Raptor 80116 mT50 + 18 mT80 mT
ITS Crew42 x Raptor 40 + 3 x 50 + 6 Vac578 mT275 + 150 mT300 mT

Edit: added FH Booster / Mini BFS

« Last Edit: 03/07/2017 12:40 AM by OneSpeed »

Online Robotbeat

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Nothing that HMXHMX hasn't talked about numerous times.
Chris  Whoever loves correction loves knowledge, but he who hates reproof is stupid.

To the maximum extent practicable, the Federal Government shall plan missions to accommodate the space transportation services capabilities of United States commercial providers. US law http://goo.gl/YZYNt0

Online Chris Bergin

Nothing that HMXHMX hasn't talked about numerous times.

True and Robot is now replying to a removed post, because we have rules about civility. Uncivil, post gets removed.

Offline Manabu

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Here are the Raptor figures I'm using in SpaceSim. Most of these figures have been published by SpaceX or mentioned in the Reddit AMA. I've simply interpolated to get my Raptor 80 figures. The mass estimates are envy's, they sound pretty reasonable to me.

   Cycle            Full-flow staged combustion
   Oxidiser         Subcooled liquid oxygen
   Fuel            Subcooled liquid methane
   Chamber pressure   300 bar or 30 MPa
   Throttle capability 20% to 100% thrust

   Sea-level Nozzle
      Expansion Ratio   40
      Thrust (SL)      2,842 kN
      Thrust (Vac)   3,061 kN
      Isp (SL)      333s
      Isp (Vac)      360s
      Diameter      1.51m
      Mass         2400 kg
      
      Expansion Ratio   50
      Thrust (SL)      3,094 kN
      Thrust (Vac)   3,333 kN
      Isp (SL)      334s
      Isp (Vac)      361s
      Diameter      1.78m
      Mass         2500 kg
      
      Expansion Ratio   80
      Thrust (SL)      3,200 kN
      Thrust (Vac)   3,400 kN
      Isp (SL)      336s
      Isp (Vac)      368s
      Diameter      2.4m
      Mass         2800 kg
      
   Vacuum Nozzle
      Expansion Ratio   200
      Thrust         3,500 kN
      Isp            382s
      Diameter      3.8m
      Mass          3000 kg
I don't know from where you got your Raptor 40 performance numbers. In the SpaceX presentation it is listed as SL 3,050 kN at 334 s while other people got via RPA numbers very close to that. Plus your nozzle diameter is also too small: it should be 1.7m to be consistent with the Raptor 200 and Raptor 80 nozzles. Although I admit it is inconsistent with measurements on the SpaceX slides/renders and 42 of those wouldn't fit under the booster... but it is better to be internally consistent with your numbers IMHO. The booster seem to be using a Raptor 32 while the BFS a Raptor 44...

Simple interpolation for getting Raptor 80 numbers is wrong, as it would be working much more over-expanded than the Raptor 40, that is already slightly over-expanded at SL. Envy887's numbers seem much better, but you seem to have used only his diameter and mass numbers, not the SL performance ones:
SL: 3005 kN at 323.2 s
Vacuum: 3479 kN at 372.2 s

I also don't know from where you got your Raptor 50 numbers, but they are suspicious as well, as any ER higher than about 32 should have smaller SL thrust and isp. Correct me if I'm wrong.

Plus, I'm doubtful if the BFS can throttle enough for landing with just Raptor 80 engines. I would leave the center cluster alone in those modifications.
« Last Edit: 03/05/2017 03:29 AM by Manabu »

Online OneSpeed

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I don't know from where you got your Raptor 40 performance numbers.

From the Reddit AMA, which was more recent than the IAC Mexico presentation. In the AMA Elon said the Raptor 40 has 290mT thrust at SL, less than the figure from the IAC presentation.

Plus your nozzle diameter is also too small: it should be 1.7m to be consistent with the Raptor 200 and Raptor 80 nozzles. Although I admit it is inconsistent with measurements on the SpaceX slides/renders and 42 of those wouldn't fit under the booster... but it is better to be internally consistent with your numbers IMHO. The booster seem to be using a Raptor 32 while the BFS a Raptor 44...

If the slides are from the CAD drawings, why would they be incorrect? It may be that the Raptor 40 has a slightly smaller throat, and the 1.51m and 40:1 ratio are correct.

Simple interpolation for getting Raptor 80 numbers is wrong, as it would be working much more over-expanded than the Raptor 40, that is already slightly over-expanded at SL. Envy887's numbers seem much better, but you seem to have used only his diameter and mass numbers, not the SL performance ones:
SL: 3005 kN at 323.2 s
Vacuum: 3479 kN at 372.2 s

Although I have great respect for Envy's expertise and analysis, I'm surprised that his Raptor 80 vacuum figure is only 21kN less than that for the Raptor 200, presumably for the same fuel flow. If that is accurate, there seems little reason for the larger 200:1 nozzle to exist. Perhaps he could respond separately?

I also don't know from where you got your Raptor 50 numbers, but they are suspicious as well, as any ER higher than about 32 should have smaller SL thrust and isp. Correct me if I'm wrong.

The Raptor 50 numbers are from the IAC Mexico slides. Where does your 32:1 figure come from? Wouldn't chamber pressure effect the point at which a nozzle becomes overexpanded?

Plus, I'm doubtful if the BFS can throttle enough for landing with just Raptor 80 engines. I would leave the center cluster alone in those modifications.

Leaving the centre three engines as Raptor 50s could certainly work, the difference in performance would be minimal. How many percent do you estimate the Raptor 80 could throttle to?

Offline Manabu

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From the Reddit AMA, which was more recent than the IAC Mexico presentation. In the AMA Elon said the Raptor 40 has 290mT thrust at SL, less than the figure from the IAC presentation.
In the reddit thread it is asked what are the *Vacuum* Thrust and Isp for the "Sea-Level Raptor" variant. So that 290mT is thrust at vacuum, not SL. But he does not specify if it is for Raptor 40 (that is indeed labeled as the SL variant in the IAC slides) or maybe the raptor variant used in the booster. Well, it doesn't add up for me anyway... This whole thing is strange, maybe result of fluctuating raptor performance figures...


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If the slides are from the CAD drawings, why would they be incorrect? It may be that the Raptor 40 has a slightly smaller throat, and the 1.51m and 40:1 ratio are correct.
Wouldn't that be a scaled down raptor then? A different engine?

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The Raptor 50 numbers are from the IAC Mexico slides. Where does your 32:1 figure come from? Wouldn't chamber pressure effect the point at which a nozzle becomes overexpanded?
I can't find any direct mention to a Raptor with 50 expansion ratio on the IAC slides. My 32:1 figure comes from analogy to other engines, like the Merlin 1D+ that has roughly a third of the chamber pressure and whose ideal SL expansion ratio would be 11.05. Scaling that linearly, you get around 30 for raptor actually. Or from AP3's analysis that I linked earlier, that estimates optimum expansion for the Raptor 40 is at 1.71km, 0.813 atm, so optimum SL expansion is something lower. I'm not sure about my number, but everything indicates something lower than 40 as optimum SL nozzle.

Just as a note, I understand that the optimum launcher nozzle is something bigger than that, because the average pressure it will work is much less than sea level.

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Plus, I'm doubtful if the BFS can throttle enough for landing with just Raptor 80 engines. I would leave the center cluster alone in those modifications.

Leaving the centre three engines as Raptor 50s could certainly work, the difference in performance would be minimal. How many percent do you estimate the Raptor 80 could throttle to?
I don't really know how to estimate this, but if linear linear extrapolation could be used, then I would guess around 40~50% throttle.
« Last Edit: 03/05/2017 02:54 PM by Manabu »

Offline Manabu

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Note: By Raptor 40 below I mean the 1.7m one for consistency with the other ones, but I think it doesn't really matters much if it is slightly more or less expanded because the way they are used.

My original idea for a BFS SSTO was to substitute 2 or 3 of the Raptor 200 by a cluster of 3 x Raptor 40. The downside of this approach is that the remaining Raptor 200 can't be lit on the ground, plus are dead weight up to a certain altitude. But I think they eventually pay for their weight in ISP and added thrust middle flight. Needs simulation. Of course, this approach needs a new thrust structure and tank design, but the tank can even be a bit bigger and more mass efficient than before IMHO, because it can be rounder at the base.

If SpaceX is willing to make one new type of nozzle, a cluster of 2 x Raptor 80 + 1 x Raptor 40 fits perfectly in the space left from removing one Raptor 200, and provides for an even more rounded tank and better average ISP and thrust. This should give a slightly better payload to orbit.

And I have some feature requests for the simulation: it would be interesting to show in the left bar the total delta-v expended to reach orbit, including losses, and what is the average isp of the flight. Those numbers can then be put in the traditional rocket equation to estimate small changes more easily.

Increasing expansion does eventually lead to seperation at sea level, at about 140:1, but SL thrust drops to 2715 kN per engine and ISP drops to 292s. On a thrust-limited design this is a poor trade-off, even though vacuum ISP is close to 380.

However, instead of nine 80:1 nozzles it might make sense to have a mix of 50:1 and 140:1. The former have much higher thrust and ISP at SL, and the latter have higher thrust and ISP in vacuum, but both are operable (if needed) across the entire flight. The 50:1 engines could be shut down to improve overall ISP once the vehicle is high enough (and light enough) that the 140:1 nozzles can maintain optimal acceleration.
SpaceX is reportedly using a 150 expansion ratio for Raptor testings, but maybe because the test stand is a few hundred meters more above SL.

There are several more interesting raptor configurations. One can do a circle of 9 x 3m diameter Raptor 125 around the gimbaling cluster. Or something not as symmetrical with 6 x Raptor 140 + 4 x Raptor 57. In all those examples there is space for up to 4 x Raptor 40 engines in the gimbaling cluster if desired because the surrounding engines are smaller than the original Raptor 200.

Online envy887

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Envy887's numbers seem much better, but you seem to have used only his diameter and mass numbers, not the SL performance ones:
SL: 3005 kN at 323.2 s
Vacuum: 3479 kN at 372.2 s

Although I have great respect for Envy's expertise and analysis, I'm surprised that his Raptor 80 vacuum figure is only 21kN less than that for the Raptor 200, presumably for the same fuel flow. If that is accurate, there seems little reason for the larger 200:1 nozzle to exist. Perhaps he could respond separately?
[/quote]

I got 3573 kN for the Raptor 200, because I was basing this on the R40 engine performance and not trying to match both... They aren't quite consistent with each other.

Offline Manabu

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I got 3573 kN for the Raptor 200, because I was basing this on the R40 engine performance and not trying to match both... They aren't quite consistent with each other.
From the thread I linked earlier they did the opposite, basing the Raptor 40 in the Raptor 200, and the next poster  said that if he varied the mixture ratio he would get better numbers for the SL version. Maybe that is the source of this inconsistency? Or maybe SpaceX numbers are just all over the place and we should round down to the lowest denominator...
« Last Edit: 03/05/2017 06:52 PM by Manabu »

Offline MikeAtkinson

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I would not be too concerned with consistency, the Raptor figures depend on various factors:

1. design aim (what performance the engineers are aiming to obtain).

2. estimated performance (what the design obtains on average).

3. measured performance (the current results obtained, perhaps extrapolated to larger engines or expansion ratios).

4. minimum guaranteed performance (below which an engine would be rejected).

5. performance after a set of known improvements.

6. ultimate performance after a set of known and unknown improvements.

Some of these performance figures will change over time as new data is gathered and the design of ITS is matured.

There is also the possibility that SpaceX are being deliberately misleading in giving consistent performance figures, perhaps for competitive reasons or to conform to ITAR.

Offline Manabu

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The fact that raptor performance is on flux don't precludes us from trying to be internally consistent when making our own predictions for fun over this paper rocket.

It also don't makes it right to claim that an ER of 80:1 is better than an ER of 40:1 for a Raptor SSTO because it's better thrust at SL, when the reason for that difference is that you are using a sub-scale raptor for your Raptor 40 and extrapolating the SL thrust in the wrong direction for the Raptor 80. It may be better, but that will be because it's vacuum characteristics, despite it's lower SL thrust.

I'm not trying to bash OneSpeed, I'm just trying to contribute to better simulations and learn in the process.

Online OneSpeed

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... I'm just trying to contribute to better simulations and learn in the process.

The figures I've posted above are those currently used in my simulations. I'm not asking anyone else to use them, they are purely FYI.

If you would like to try some alternatives, I'd suggest you download SpaceSim from https://github.com/zlynn1990/SpaceSim and plug in your own figures. That sounds more like fun to me.

Offline radozw

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I did a similar simulation recently of a half scale BFS with two Raptor 80 engines. I got 7mT to 400 x 400 kms, perhaps I was more conservative on mass fractions and ullage.

Anyway, we're starting to build up quite a range of ITS alternative configurations. Some of these figures are projections from 300 x 300km simulations.

NameConfigurationMass to 400 x 400 kmsVehicle massPayload
Micro BFS SSTO1 x Raptor 5025.5 mT9 mT6.5 mT
Mini BFS SSTO2 x Raptor 8043 mT18 mT7 mT
BFS SSTO9 x Raptor 50187 mT90 mT25 mT
BFS SSTO3 x Raptor 50 + 6 x Raptor 80193 mT92 mT29 mT
FH Booster / Mini BFS27 x Merlin 1D + 2 x Raptor 5068 mT73 + 18 mT36.5 mT
BFS SSTO12 x Raptor 80236 mT100 mT63 mT
Mini ITS7 x Raptor 50 + 2 x Raptor 80116 mT50 + 18 mT80 mT
ITS Crew42 x Raptor 40 + 3 x 50 + 6 Vac578 mT275 + 150 mT300 mT

Edit: added FH Booster / Mini BFS


I think there should be configuration as standard ITS + 2 side boosters F9.
That will solve T/W problem and reduce gravity loses.
It will complicate loading of fuel and launch pad configuration but it will enable testing of standard ITS and have usefull payload.
I can not do calculations

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