161 posts in this topic

 

17 hours ago, sevenperforce said:

Here's an idea. If Elon wants 100% stage reuse, why not offer a tandem launch with crossfeed? Call it Falcon TT.

Take a standard Falcon 9 FT first stage and replace one opposing pair of its outer engines with Merlin 1D Vacuum engines:

Falcon_TT.png

Strap it to a standard Falcon 9 first stage capped booster and add your payload on top.

Falcon_TT_2.png

On launch, fire all nine Merlin 1Ds on the strap-on booster and fire the seven SL-optimized Merlin 1Ds on the payload booster, keeping the pair of Merlin 1D Vacuum engines turned off. Crossfeed fuel from the strap-on booster to the payload booster up past Max-Q, then throttle down the engines on the payload booster while keeping the strap-on booster at full throttle.

Within moments, the vehicle will be high enough to ignite the vacuum-optimized engines at full throttle, downthrottling the other seven or even cutting off several of them entirely. Continue until the strap-on booster is down to its boostback and landing reserves, then separate. Allow the two Merlin 1D Vacuum engines to carry the remaining stage and payload all the way into orbit.

With the two vacuum-optimized engines, the stage will have a much higher remaining delta-v in orbit and can deliver its payload with enough remaining dV for an extended re-entry burn. It can RTLS without a boostback burn because it is already in orbit. It can touch down on the SL-optimized engine.

I think that would work, anyway.

Problems:

1. Crossfeed, which is untested.

2. Assymetric, meaning the balance will change over time, increasing complexity.

3. Probably less efficient due to the extra mass the sustainer must carry to orbit.

4. The sustainer core would not be able to be reused without substantial modifications to add a proper heat shield to the bottom or top.

 

Overall, I think it's just better to have normal 2nd stage reuse. It has a better mass fraction (which is already a horrible mass fraction).

14 hours ago, sevenperforce said:

I have yet to see any advantages in such a design over using something like the Falcon 9R as the first stage.

If the VTVL concept fails, this is the next best thing. :P It was also the only way you could do reuse effectively back in the day, so...

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On 4/4/2016 at 3:34 PM, fredinno said:

 

Wait, not even X-33 and DC-X?

Oh wait:

 Here we go:

http://www.sciencedirect.com/science/article/pii/S0094576597001422

 Thanks for that link. From the abstract:

Quote

 

In case of a commercial development and operation it is estimated that an SSTO vehicle with 400 Mg propellant mass can be flown for some 9 Million $ per mission (94/95) with 14 Mg payload to LEO, 7 Mg to the Space Station Orbit, or 2 Mg to a Full-size image (<1 K) km polar orbit. This means specific transportation cost of 650 $/kg (300 $/lb), resp.3.2 MYr/Mg, to LEO which is 6 −10% of present expendable launch vehicles.

 

 A Mg is a million grams, a thousand kilos, 1 metric ton. So this SSTO is about the size of a Falcon 9 first stage. The author claims it could carry 14 metric tons to orbit. Can't tell from the abstract if it is to use altitude compensation such as an aerospike.

 But the $9 million per flight estimate seems low unless he means reusable.

  Bob Clark

 

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I say that SSTOs are actually necessary to commonplace spaceflight. What we need is to take the risk of building one, testing it, and learning from it. SSTOs (good ones) pay for themselves

 

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3 minutes ago, Emperor of the Titan Squid said:

I say that SSTOs are actually necessary to commonplace spaceflight. What we need is to take the risk of building one, testing it, and learning from it. SSTOs (good ones) pay for themselves

 

Try installing RSS and seeing how long you believe that.  Or just play with the rocket equation.  Building a SSTO with Kerolox or even hydrolox makes no sense.  We already know that anything a kerolox or hydrolox SSTO can do, a TSTO do the same with a bigger payload to orbit and vastly less re-entry stress to 90% of the spacecraft (i.e. the first stage).  These are basic laws of physics and are true as long as you use rockets.

Now build something that isn't necessarily a "rocket" like Escape Dynamic (who, unfortunately closed): http://spacenews.com/startup-makes-progress-in-beamed-propulsion-for-reusable-launch-vehicles/ and you could make an efficient SSTO.  But that uses just hydrogen and not hydrolox (so it can have an ISP of ~800).

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4 hours ago, Emperor of the Titan Squid said:

I say that SSTOs are actually necessary to commonplace spaceflight. What we need is to take the risk of building one, testing it, and learning from it. SSTOs (good ones) pay for themselves

 

 Hear, hear! Elon says the F9 first stage can be a SSTO, though with reduced payload, Let's do the experimental flight!

 

  Bob Clark

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by reduced payload, he means basically no payload.

And by SSTO, he means it gets to orbit, but doesn't have a chance in h311 of surviving reentry, and won't have any fuel for a powered landing... so its still a single use rocket, except now you only lift like 1/10th the payload... what a great idea Bob Clark...

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17 hours ago, Emperor of the Titan Squid said:

I say that SSTOs are actually necessary to commonplace spaceflight. What we need is to take the risk of building one, testing it, and learning from it. SSTOs (good ones) pay for themselves

Every time we've tried that it has done the exact opposite of paying for itself. X-33, NASP, Roton, HOTOL, all just soaked up dev funds and coughed up diddly squat. The margins are so thin with SSTO that the first big engineering issue is likely to just kill the project stone dead.

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There was a company that was dropping a little rocket from a modified 737 a while back, not sure if they're still around.  I would still consider an air launched spaceplane that was fully reusable to be a viable SSTO although it would not be strictly a "SSTO".  Getting your fully fueled spaceplane to 45,000 ft elevation is nothing to snort at delta V wise and we already have big honking cargo planes figured out.  Even if the SSTO in question is only hauling people to orbit that is still a win, the more we do in space, the more peeps we need up there.

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6 hours ago, Kryten said:

Every time we've tried that it has done the exact opposite of paying for itself. X-33, NASP, Roton, HOTOL, all just soaked up dev funds and coughed up diddly squat. The margins are so thin with SSTO that the first big engineering issue is likely to just kill the project stone dead.

Thats mainly because those didn't actually happen, and no one will try again

 

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25 minutes ago, Emperor of the Titan Squid said:

Thats mainly because those didn't actually happen, and no one will try again

There was years of work put into all of these projects, and a good bit of hardware development for many. You can also add DC-X and HTV-R to that list, and they both flew. SpaceX and their F9 architecture have moved far closer to practical reliability than all the straight-to-SSTO programmes put together, and with a lot less time and money. The most that's ever come out of an SSTO team are some of the DC-X team members going to Blue and working on New Shepard, and even then only by ditching their previous straight-to-SSTO mentality.

EDIT: Heck, even Jess Sponable, one of the guys heading DC-X and responsible for their straight-to-SSTO stance, is now running a flyback/boostback TSTO booster program, XS-1.

Edited by Kryten
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19 minutes ago, Thor Wotansen said:

Even if the SSTO in question is only hauling people to orbit that is still a win, the more we do in space, the more peeps we need up there.

As much as I wish that were true, it really isn't. There is very little space stuff that a robot can't do for cheaper. Most of human usefulness in space has been figuring out human presence in space...which is great, and which I totally support, but it doesn't pay for itself. 

A small-as-possible SSTO crew ferry would be fantastic, don't get me wrong, but I can't figure out a way to make it economically viable. 

6 hours ago, Kryten said:

Every time we've tried that it has done the exact opposite of paying for itself. X-33, NASP, Roton, HOTOL, all just soaked up dev funds and coughed up diddly squat. The margins are so thin with SSTO that the first big engineering issue is likely to just kill the project stone dead.

All you need is an engine with a really great TWR and a really low thrust specific fuel consumption. 

I'll wait. 

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54 minutes ago, sevenperforce said:

As much as I wish that were true, it really isn't. There is very little space stuff that a robot can't do for cheaper. Most of human usefulness in space has been figuring out human presence in space...which is great, and which I totally support, but it doesn't pay for itself. 

A small-as-possible SSTO crew ferry would be fantastic, don't get me wrong, but I can't figure out a way to make it economically viable. 

I refuse to believe that the future of space travel will be robotic.  As soon as we have any sort expanded operations like asteroid mining going on we will need decisions to happen quicker than the usual hour + of latency that communication from Earth could provide.  Also, as soon as we get large scale asteroid mining we can build all our ships in space and people and supplies will be the primary cargo of most launches.

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1 hour ago, Thor Wotansen said:

I refuse to believe that the future of space travel will be robotic.  As soon as we have any sort expanded operations like asteroid mining going on we will need decisions to happen quicker than the usual hour + of latency that communication from Earth could provide.  Also, as soon as we get large scale asteroid mining we can build all our ships in space and people and supplies will be the primary cargo of most launches.

I mean, I don't WANT to believe that the future of space travel is robotic. I just can't think of any near-term reason why squishy, fragile humans would be more economical than disposable robots. And if robot operation is standard when asteroid mining becomes lucrative, it will likely be cheaper to make it happen with robots. 

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On 2016-05-17 at 10:56 AM, Exoscientist said:

 Thanks for that link. From the abstract:

 A Mg is a million grams, a thousand kilos, 1 metric ton. So this SSTO is about the size of a Falcon 9 first stage. The author claims it could carry 14 metric tons to orbit. Can't tell from the abstract if it is to use altitude compensation such as an aerospike.

 But the $9 million per flight estimate seems low unless he means reusable.

  Bob Clark

 

Have you ever heard of a expendable SSTO?

On 2016-05-17 at 2:28 PM, wumpus said:

Try installing RSS and seeing how long you believe that.  Or just play with the rocket equation.  Building a SSTO with Kerolox or even hydrolox makes no sense.  We already know that anything a kerolox or hydrolox SSTO can do, a TSTO do the same with a bigger payload to orbit and vastly less re-entry stress to 90% of the spacecraft (i.e. the first stage).  These are basic laws of physics and are true as long as you use rockets.

Now build something that isn't necessarily a "rocket" like Escape Dynamic (who, unfortunately closed): http://spacenews.com/startup-makes-progress-in-beamed-propulsion-for-reusable-launch-vehicles/ and you could make an efficient SSTO.  But that uses just hydrogen and not hydrolox (so it can have an ISP of ~800).

How about air-augmentation or airbreathing SSTOs?

On 2016-05-18 at 6:35 PM, Exoscientist said:

 Hear, hear! Elon says the F9 first stage can be a SSTO, though with reduced payload, Let's do the experimental flight!

 

  Bob Clark

Doubt it would have the fuel to re-land, without air augmentation. In which case, you may as well start from scratch.

5 hours ago, Thor Wotansen said:

There was a company that was dropping a little rocket from a modified 737 a while back, not sure if they're still around.  I would still consider an air launched spaceplane that was fully reusable to be a viable SSTO although it would not be strictly a "SSTO".  Getting your fully fueled spaceplane to 45,000 ft elevation is nothing to snort at delta V wise and we already have big honking cargo planes figured out.  Even if the SSTO in question is only hauling people to orbit that is still a win, the more we do in space, the more peeps we need up there.

" There was a company that was dropping a little rocket from a modified 737 a while back, not sure if they're still around. "

Orbital Sciences? They're still around.

Good luck being able to fit a SSTO to a plane. Air launch comes with strict size limits- You probably need something the size of stratolaunch, and even then, you're lucky if you get a payload off that.

 

Unless you use something like Skylon to "air launch" from high altitudes (low altitudes would be dangerous due to air drag). But then you have a TSTO.

5 hours ago, Kryten said:

There was years of work put into all of these projects, and a good bit of hardware development for many. You can also add DC-X and HTV-R to that list, and they both flew. SpaceX and their F9 architecture have moved far closer to practical reliability than all the straight-to-SSTO programmes put together, and with a lot less time and money. The most that's ever come out of an SSTO team are some of the DC-X team members going to Blue and working on New Shepard, and even then only by ditching their previous straight-to-SSTO mentality.

EDIT: Heck, even Jess Sponable, one of the guys heading DC-X and responsible for their straight-to-SSTO stance, is now running a flyback/boostback TSTO booster program, XS-1.

 

4 hours ago, Thor Wotansen said:

I refuse to believe that the future of space travel will be robotic.  As soon as we have any sort expanded operations like asteroid mining going on we will need decisions to happen quicker than the usual hour + of latency that communication from Earth could provide.  Also, as soon as we get large scale asteroid mining we can build all our ships in space and people and supplies will be the primary cargo of most launches.

But what if the robots were fully automated? Then a checkout of the systems would be all that is needed.

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6 hours ago, Kryten said:

There was years of work put into all of these projects, and a good bit of hardware development for many. You can also add DC-X and HTV-R to that list, and they both flew. SpaceX and their F9 architecture have moved far closer to practical reliability than all the straight-to-SSTO programmes put together, and with a lot less time and money. The most that's ever come out of an SSTO team are some of the DC-X team members going to Blue and working on New Shepard, and even then only by ditching their previous straight-to-SSTO mentality.

EDIT: Heck, even Jess Sponable, one of the guys heading DC-X and responsible for their straight-to-SSTO stance, is now running a flyback/boostback TSTO booster program, XS-1.

HTV-R? Looks like a capsule concept to me.

HTV-R1.png

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On 5/19/2016 at 10:53 PM, fredinno said:

How about air-augmentation or airbreathing SSTOs?

Certainly.  There was a reason I was pretty specific about kerolox and hyrdolox.  A atmosphere+hydrogen would likely work better than laser+hydrogen (likely a hotter temperature, thus a higher Ve.  Also the free mass of the oxygen has to help).  Unfortunately, the more I learn of the theory (on these endless threads) the less optimistic I become.  My main point was "build a rocket" is *not* the way to get a SSTO.

Note that even then, in terms of simple vehicle cost the TSTO will still likely be better.  But at least with something other than an ordinary rocket, you have a chance that logistics and other launch costs might make up the difference.  Assuming your spaceplane can pull up out of the atmosphere (and then stage) it should be possible to stage safely and efficiently (of course, if need to use your rocket engines before staging things get complex).

One last thing to remember: for all the Skylon threads we see, NASA has *flown* an airbreather plane to 3000m/s (vs. 2000m/s when Skylon needs rocket engines).  While I'm sure it didn't have Skylon's vaunted efficiency, the 'effective' ISP was over 1500s (and still over 2500s when Skylon would have to use its own oxidizer).  I try to imagine a orbit-capable spacecraft using this engine and strongly suspect that there is no reason to stage such a beast (other than the obvious reason that the previous "stage" is now dry weight.  But the logistics are much more likely to override that).

https://info.aiaa.org/Regions/Western/Orange_County/ASAT%20Conference%202013%20Presentations/The%20Specific%20Impulse%20Potential%20of%20Hydrogen%20Scramjet.pdf

As far as I can tell, NASA/DoD is turning the entire program over to the air force to make missiles (so far the main use of ramjets).  Seems like a tragic waste of the best chance for a SSTO I've ever seen.

Edited by wumpus
fuel/oxidizer error
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The biggest problem with scramjets to orbit is the airbreather's burden. When acceleration drops (which it MUST do with high Mach numbers for an airbreather, simply because the increasing speed of the airstream DIRECTLY sucks away net thrust), you end up having a longer acceleration stage, which rapidly destroys the advantage of specific impulse.

Specific impulse is proportional to thrust, not to acceleration. For a pure rocket, your thrust-specific acceleration increases with increasing speed, because your vehicle is getting lighter and lighter. For an airbreather, your thrust-specific acceleration decreases with increasing speed. And that's what kills you.

At lower Mach numbers, you gain an advantage by using the increasing airstream speed for ram compression to improve the efficiency of your reheat, but this goes away rapidly at higher Mach numbers and you are desperately trying to squeeze impulse out of a losing battle.

Rules of thumb:

  • When vvehicle <<< vexhaust, airbreathing is great.
  • When vvehicle < vexhaust, airbreathing is not so great.
  • When vvehicle = vexhaust, airbreathing is not worth it.
  • When vvehicle > vexhaust, airbreathing produces negative thrust.

The ideal SSTO/PSTO* solution, I think, is to use airbreathing up to 50-60% of your exhaust velocity, then go to pure rocket mode. IF you can repurpose your airbreathing hardware for re-entry landing (lift fans, movable vanes for re-entry control or heat shielding, etc.), then do so; otherwise drop them. Note that a tripropellant arrangement becomes a little more lucrative here, as a higher exhaust velocity from methalox or hydrolox gives you a longer airbreathing boost phase.

*PSTO stands for Parallel Stage To Orbit, which basically means you use the same engine from liftoff to orbit, but you drop a parallel assistance stage on the way.

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3 hours ago, sevenperforce said:

(all the things that go wrong with airbreathers)

I'll have to dig into my own link (all I was looking for were the ISP numbers) but a back of the envelop calculation tells me that the Thrust-to-drag ratio of the X-43 around 2000m/s (the delta-v provided by Skylon sabre engine and Falcon9 first stage) is higher than 9/4 (presumably after re-tuning from mach 9 to mach 6), so it would certainly be accelerating fast enough.  3000m/s might not be terribly realistic, but it at least shows that it can likely still be carrying a load at 2000m/s.  Between a thrust-to-drag ratio significantly higher than one and an ISP of ~2500s, I still think this makes a tremendous first stage (not necessarily an economically justifiable one, but still tremendous).  No idea if the dry weight is low enough to be an SSTO or not, lots of that require knowing way too many government regulations and the personnel required to certify a launch.

The other big takeaway was that the bird actually flew.  3000m/s wasn't just on a powerpoint side, it was measured on real "spaceplane" (if only an unmanned, single use probe).

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17 hours ago, fredinno said:

But what if the robots were fully automated? Then a checkout of the systems would be all that is needed.

Fully automated probes doing complex science would require full on AI in order to make the kinds of decisions I'm talking about.  Not having been to any large asteroids yet, we don't really know what they're like inside, there may be gas pockets and other geological features that make whatever form of mining they may be doing impossible for a non intelligent system.

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38 minutes ago, Thor Wotansen said:

Fully automated probes doing complex science would require full on AI in order to make the kinds of decisions I'm talking about.  Not having been to any large asteroids yet, we don't really know what they're like inside, there may be gas pockets and other geological features that make whatever form of mining they may be doing impossible for a non intelligent system.

While, again, I TOTALLY want major human presence in space, I just don't see it. Surely it is cheaper to allow for a command lag than to ship humans a light-hour from Earth.

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On 5/18/2016 at 10:32 AM, KerikBalm said:

by reduced payload, he means basically no payload.

And by SSTO, he means it gets to orbit, but doesn't have a chance in h311 of surviving reentry, and won't have any fuel for a powered landing... so its still a single use rocket, except now you only lift like 1/10th the payload... what a great idea Bob Clark...

 The latest version of the Falcon 9 with full thrust can get 23 metric tons to LEO. So if it is 1/10th this much as a SSTO, that would be 2.3 metric tons to orbit, hardly zero payload. In fact the Arianespace Vega rocket has a similar payload range and is charging €40 million, about $45 million.

 But SSTO's get their best performance as an SSTO using altitude compensation such as the aerospike. This would allow the vacuum Isp of the Merlins on the first stage to be increased from the current 311 s to reach that of the Merlin Vacuum at 342 s. Using this, the Falcon 9's first stage's payload as a SSTO is increased significantly.

 One start-up, RocketStar, LLC,  is working towards using the aerospike to achieve SSTO:

  

  Bob Clark

Edited by Exoscientist

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still won't survive re-entry... making it a disposable rocket with a terrible payload

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On 5/19/2016 at 8:31 AM, wumpus said:

Certainly.  There was a reason I was pretty specific about kerolox and hyrdolox.  A atmosphere+hydrogen would likely work better than laser+hydrogen (likely a hotter temperature, thus a higher Ve.  Also the free mass of the oxygen has to help).  Unfortunately, the more I learn of the theory (on these endless threads) the less optimistic I become.  My main point was "build a rocket" is *not* the way to get a SSTO.

Note that even then, in terms of simple vehicle cost the TSTO will still likely be better.  But at least with something other than an ordinary rocket, you have a chance that logistics and other launch costs might make up the difference.  Assuming your spaceplane can pull up out of the atmosphere (and then stage) it should be possible to stage safely and efficiently (of course, if need to use your rocket engines before staging things get complex).

One last thing to remember: for all the Skylon threads we see, NASA has *flown* an airbreather plane to 3000m/s (vs. 2000m/s when Skylon needs rocket engines).  While I'm sure it didn't have Skylon's vaunted efficiency, the 'effective' ISP was over 1500s (and still over 2500s when Skylon would have to use its own oxidizer).  I try to imagine a orbit-capable spacecraft using this engine and strongly suspect that there is no reason to stage such a beast (other than the obvious reason that the previous "stage" is now dry weight.  But the logistics are much more likely to override that).

https://info.aiaa.org/Regions/Western/Orange_County/ASAT%20Conference%202013%20Presentations/The%20Specific%20Impulse%20Potential%20of%20Hydrogen%20Scramjet.pdf

As far as I can tell, NASA/DoD is turning the entire program over to the air force to make missiles (so far the main use of ramjets).  Seems like a tragic waste of the best chance for a SSTO I've ever seen.

Why would a atmosphere+ H2 work better? Lazer+H2 has higher ISP, and you can pump lasers at it for far longer on its orbit.

 

Quote

As far as I can tell, NASA/DoD is turning the entire program over to the air force to make missiles (so far the main use of ramjets).  Seems like a tragic waste of the best chance for a SSTO I've ever seen.

Probably an admission that investing heavily in those kinds of RLVs (TSTO or SSTO) doesn't make much economic sense right now. SpaceX and Blue Origin only bothered with reuse because it was their dreams- a good thing, but not good for profits or economy.

On 5/19/2016 at 9:16 AM, sevenperforce said:

The ideal SSTO/PSTO* solution, I think, is to use airbreathing up to 50-60% of your exhaust velocity, then go to pure rocket mode. IF you can repurpose your airbreathing hardware for re-entry landing (lift fans, movable vanes for re-entry control or heat shielding, etc.), then do so; otherwise drop them. Note that a tripropellant arrangement becomes a little more lucrative here, as a higher exhaust velocity from methalox or hydrolox gives you a longer airbreathing boost phase.

*PSTO stands for Parallel Stage To Orbit, which basically means you use the same engine from liftoff to orbit, but you drop a parallel assistance stage on the way.

PSTO sounds like you might as well just use a TSTO. The main advantage of SSTO is easier integration, and PSTO removes that. And thing with dropping enignes/tanks.

And airbreathing up to 50% of needed velocity seems too high- is that possible?

And I honestly wonder why CH4 is being hyped so much. RP-1 has been shown to work about as well, with little soot on the engine insides, and Ch4 has a mere 10s max isp increase, for a 30% higher tank size.

It's not a "in-between" of H2 and RP-1, it's an overhyped fuel that 60s engineers didn't bother with for a good reason. It can't even offer automatic pressurisation, as claimed, because that would require pumping CH4 into O2 tanks to pressurise the O2 tanks- not a good combination for obvious reasons

3 hours ago, Exoscientist said:

 The latest version of the Falcon 9 with full thrust can get 23 metric tons to LEO. So if it is 1/10th this much as a SSTO, that would be 2.3 metric tons to orbit, hardly zero payload. In fact the Arianespace Vega rocket has a similar payload range and is charging €40 million, about $45 million.

 But SSTO's get their best performance as an SSTO using altitude compensation such as the aerospike. This would allow the vacuum Isp of the Merlins on the first stage to be increased from the current 311 s to reach that of the Merlin Vacuum at 342 s.  Using this, the Falcon 9's payload is increased significantly as an SSTO.

 One start-up, RocketStar, LLC,  is working towards using the aerospike to achieve SSTO:

  

  Bob Clark

Rocketstar? Good luck to them, the vast majority of NewSpace companies fail before even starting up on the actual vehicle (even big ones like Rocketplane).

And F9 might be able to carry 2-3T as a SSTO- but is is reusable? You need a LOT more mass if you want a reusable version, due to adding a heat shield, bigger batteries/solar panels for on-orbit operations, more fuel due to the higher speed landing from LEO, a dedicated Space tug module (preferably H2/O2- it's going to be expendable anyways, unless refueling and IVF infrastructure are built)... and that takes directly from payload. The F9 1st stage is also not very well designed for a SSTO- it's way too long, meaning it will not generate much drag on reentry (or need a LOT of heat shield area for a ong axis reentry)

A decent Vega-sized payload would probably mean a 5-4m diameter tank used instead.

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1 hour ago, KerikBalm said:

still won't survive re-entry... making it a disposable rocket with a terrible payload

  The Vega is also expendable and costs $45 million for a payload to LEO of only 2,500 kg, yet still has a market.

  But as I said there are many ways of accomplishing altitude compensation that would MAJORLY increase the payload to LEO. I have discussed how much more it could be but nobody believes it. Try the calculation yourself: use the rocket equation or one of the launch simulators to see how much is the payload when the vacuum Isp of the Merlins on the F9 first stage is increased from 311 s to 342 s.

    Bob Clark

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