Why is not a SSTO useful today?

[Kaos]

31 minutes ago, ModZero said:

Skylon won't actually win all that much dV from the jet mode, just enough to be viable at all, really (I don't remember exact numbers, but they've shown up multiple times in NasaSpaceflight forums, with sources and knowledgable (and not-so-knowledgable) people reading it, if you want to dig for sources). It's not really a heavy lift launcher, and it will need an upper stage (hopefully reusable? They made some noises about it, but that's a lot of mass for questionable win) to get things into high orbits. 

They're hoping for a business model efficient in terms of profits, not really a lifter efficient in terms of mass-to-LEO. 

What is used is not a question of efficiency in terms of mass-to-LEO, but what is cheap and economic.

[Nibb31]

And a one-off vehicle without a market isn't going to be cheap and economic. Skylon is only viable if a huge market for daily flights of small payloads to LEO suddenly appears out of nowhere.

[ModZero]

Well, they're hoping that there's a large market currently suppressed by a lack of Skylons (that must pay the hefty r&d and unit cost somehow). I'm trying to not roll my eyes too hard because technologically it's kinda cool. 

Really, I'm just hoping the project doesn't suddenly metastasize into some sort of hypersonic bomber or something equally lame and boring. 

[fredinno]

1 hour ago, ModZero said:

Well, they're hoping that there's a large market currently suppressed by a lack of Skylons (that must pay the hefty r&d and unit cost somehow). I'm trying to not roll my eyes too hard because technologically it's kinda cool. 

Really, I'm just hoping the project doesn't suddenly metastasize into some sort of hypersonic bomber or something equally lame and boring. 

Hypersonic bombers are not boring!!

[wumpus]

Basically, Skylon shows plenty of issues of why SSTOs are hard (hard to the point of impossible, really).

The main point of Skylon is 343* m/s of cheap delta-v (i.e. zero to mach 6).  After that, all that "lightweight" heat exchanger is just dry weight to orbit.  Remember, we need 9000ish m/s to orbit on Earth, so it makes Skylon's "big point" look a little iffy.

* yes, you should add a bit for getting aero losses thrown in "for free" with my fast and loose numbers, but it still isn't that big a fraction of the launch costs.  You need way, way, more if you want to build a SSTO out of that.

[ModZero]

1 hour ago, fredinno said:

Hypersonic bombers are not boring!!

They're boring right until the moment one kills you. 

58 minutes ago, wumpus said:

so it makes Skylon's "big point" look a little iffy.

Well, (I'm playing cool pointless (geddit?) toys advocate here) they mostly try to spin the point as an opportunity to change business models — creating an airline-like split between manufacturers, operators and end customers. They also spin the fact that Skylon would have enormous cross-range capability, able to take a load in UK, fly half the world to a proper space center, drop the loadrefuel and go back to the military basego to orbit from an equatorial location. I'm, uh, sceptical, it all feels like there's too many moving parts in the idea, but hey, they have the pre cooler working and they've been trying to get that hypersonic bomber, sorry, suborbital transport, sorry again, light cross-range capable reusable LEO lifter thing going for what, three decades now? Let's cut them some slack for enthusiasm ;-)

343* m/s of cheap delta-v

Isn't that really close to what a typical Soyuz has once in orbit? It would give a nice idea about what that dV actually means (no, you're not going to get to orbit on that, but you might get a rendezvous with ISS, if the ISS is still around, that is).

Edited January 12, 2016 by ModZero

[Kaos]

1 hour ago, wumpus said:

Basically, Skylon shows plenty of issues of why SSTOs are hard (hard to the point of impossible, really).

The main point of Skylon is 343* m/s of cheap delta-v (i.e. zero to mach 6).  After that, all that "lightweight" heat exchanger is just dry weight to orbit.  Remember, we need 9000ish m/s to orbit on Earth, so it makes Skylon's "big point" look a little iffy.

* yes, you should add a bit for getting aero losses thrown in "for free" with my fast and loose numbers, but it still isn't that big a fraction of the launch costs.  You need way, way, more if you want to build a SSTO out of that.

Where are these 343 m/s from? http://www.reactionengines.co.uk/space_skylon_tech.html states Mach 5.5, which is closer to 2 km/s. Which is still a lot till orbital velocity, but not neglectable.

[ModZero]

9 minutes ago, Kaos said:

Where are these 343 m/s from?

AFAIR that's how much extra dV they get thanks to using air-breathing engines instead of rockets. Which is nothing to sniff at, but not something to go to the Mun on either.

[wumpus]

2 hours ago, Kaos said:

Where are these 343 m/s from? http://www.reactionengines.co.uk/space_skylon_tech.html states Mach 5.5, which is closer to 2 km/s. Which is still a lot till orbital velocity, but not neglectable.

Quick google, likely wrong.  Still, 2km/s out of 9km/s isn't much (I really should have recognised the number from KSP: 343 is close to mach 1).

Anybody know how far you can get a suborbital to go on 2km/s?  I'm guessing nowhere close to transatlantic, and that London-China is going to take over 8km/s, but the last time I went googling wasn't too good.  But from watching the trajectory while circularizing seems to be short hop, short hop, short hop, almost there, half way around the planet and done!

[Kaos]

LEO is more like 7 km/s. You need 9-10 km/s to come there with a typical rocket, that is right. But you loose a lot due to gravity and air resistance. But when you have 2 km/s in this height, you already had most of this losses.

[KerikBalm]

if 343 is an actualy number that makes sense... maybe that's the advantage gained after factoring in the increased dry mass of the engines and wings?

Keep in mind, Sabre's won't be getting Isps in the thousands like in KSP... those sabre's even in airbreathing mode, will be guzzling fuel.... at the point it gets to high altitude flight at mach 5.5, it will have consumed a lot of fuel, and suffered far more losses due to drag than a normal rocket.... still, even 800 Isp would be great and could provide a lot of savings.

[Temstar]

They have a separate engine design for sub-orbital transport called Scimitar. It's basically a SABRE minus the closed cycle mode, so hydrogen burning, precooled jet engine.

Seems to me like one way around this SSTO payload fraction problem is to launch a rocket stage (or a rocket spaceplane if you really want reusable) on top of a Scimitar powered aircraft. So basically one of those many two stage spaceplane design from early shuttle proposal days, only with a more advanced air breathing stage.

Edited January 12, 2016 by Temstar

[fredinno]

5 hours ago, Temstar said:

They have a separate engine design for sub-orbital transport called Scimitar. It's basically a SABRE minus the closed cycle mode, so hydrogen burning, precooled jet engine.

Seems to me like one way around this SSTO payload fraction problem is to launch a rocket stage (or a rocket spaceplane if you really want reusable) on top of a Scimitar powered aircraft. So basically one of those many two stage spaceplane design from early shuttle proposal days, only with a more advanced air breathing stage.

Unfortunately hypersonic deployment in the lower atmosphere is very dangerous. It would need to be a SABRE powered 1st stage which can go through the atmosphere, while the 2nd stage, which deploys in the high upper atmosphere, does the circularization.

[magnemoe]

9 hours ago, Kaos said:

Where are these 343 m/s from? http://www.reactionengines.co.uk/space_skylon_tech.html states Mach 5.5, which is closer to 2 km/s. Which is still a lot till orbital velocity, but not neglectable.

Mach 5.5 is fast, much faster than Falcon 9 first stage for one. 
However much of it is lost by having to drag the entire airframe up into orbit.

Again its an reason why an first version of Skylon should be suborbital. Far less stress then having to reenter atmosphere is an major one, Smaller air frame as you don't need so much fuel is another major benefit.
As you will release the payload in vacuum and zero g this is no problem unlike release it in atmosphere and the upper stage need no fairing and no powerful engine. 

An amusing idea had been an reuseable upper stage here. again you don't need an fairing around the cargo, you will need an mounting bracket on front of upper stage for the satellite and this is dropped before reentry. This is most practical for smaller satellites 

[Kaos]

I have seen a reusable upper stage planing in a Skylon video:

It is brought with Skylon to orbit. Then it transfers the payload to GTO. It then returns to LEO without the payload and returns with the next Skylon available to earth.

As with most of the Skylon plans for me: I am not sure that it works, is economical or is technical feasible. But it sounds cool.

This plan again is no SSTO anymore. But it shows a bit why SSTO are not used: Even if there is a comparable cheap way to go a good part of the way to orbit, not carrying so much mass around is better, so no SSTO design is used -- even on the proposed SSTO designs not for all operation modes.

[wumpus]

6 hours ago, Kaos said:

It is brought with Skylon to orbit. Then it transfers the payload to GTO. It then returns to LEO without the payload and returns with the next Skylon available to earth.

Taken from the xkcd "orbiting horsepower" comic, the shuttle itself weighed three times its maximum payload.  Part of the reason was to return "whatever", presumably keyhole satellites, to Earth.  One of the biggest issues with SSTO is that the designs imply that you take the kitchen sink to orbit every time you want to fly.  This is one of the *many* reasons the shuttle never met most of its [official] design goals, especially those concerning cost.

https://xkcd.com/1461/

And apropos to the xkcd alt-text, I can't help but wonder if the technology to make the Skylon cost effective isn't all that close to a space elevator (I suspect the SSTO method is harder).  Although I have to say that if the Skylon could claw its way out of the atmosphere (presumably in rocket mode) with roughly 4km/s under its belt (2+km/s from velocity [the plus would presumably be done in rocket mode], "2km/s" from aero losses) it would be well on its way to at least making sense (but still requiring a huge number of flights to explain why to develop this instead of SpaceX's working tech).

[fredinno]

56 minutes ago, wumpus said:

Taken from the xkcd "orbiting horsepower" comic, the shuttle itself weighed three times its maximum payload.  Part of the reason was to return "whatever", presumably keyhole satellites, to Earth.  One of the biggest issues with SSTO is that the designs imply that you take the kitchen sink to orbit every time you want to fly.  This is one of the *many* reasons the shuttle never met most of its [official] design goals, especially those concerning cost.

https://xkcd.com/1461/

And apropos to the xkcd alt-text, I can't help but wonder if the technology to make the Skylon cost effective isn't all that close to a space elevator (I suspect the SSTO method is harder).  Although I have to say that if the Skylon could claw its way out of the atmosphere (presumably in rocket mode) with roughly 4km/s under its belt (2+km/s from velocity [the plus would presumably be done in rocket mode], "2km/s" from aero losses) it would be well on its way to at least making sense (but still requiring a huge number of flights to explain why to develop this instead of SpaceX's working tech).

Space Elevavators will likely be more expensive and difficult.

 

7 hours ago, Kaos said:

I have seen a reusable upper stage planing in a Skylon video:

It is brought with Skylon to orbit. Then it transfers the payload to GTO. It then returns to LEO without the payload and returns with the next Skylon available to earth.

As with most of the Skylon plans for me: I am not sure that it works, is economical or is technical feasible. But it sounds cool.

This plan again is no SSTO anymore. But it shows a bit why SSTO are not used: Even if there is a comparable cheap way to go a good part of the way to orbit, not carrying so much mass around is better, so no SSTO design is used -- even on the proposed SSTO designs not for all operation modes.

Like the Shuttle-Centaur plans with the Shuttle? Yeah, good luck caring flammable material inside an enclosed cargo bay...

[ModZero]

21 minutes ago, fredinno said:

Like the Shuttle-Centaur plans with the Shuttle? Yeah, good luck caring flammable material inside an enclosed cargo bay...

Well, Skylon is uncrewed (hear that sound? That's dreams being shattered), so it's just the super-expensive lifter you're going to lose.

[fredinno]

29 minutes ago, ModZero said:

Well, Skylon is uncrewed (hear that sound? That's dreams being shattered), so it's just the super-expensive lifter you're going to lose.

Yeah, and that lifter is super-expensive. Still not a good idea to carry volatile fuel inside. I think Propellant will always be carried in non-spaceplanes to Orbit- it's too dangerous to be in an enclosed space.

[ModZero]

18 minutes ago, fredinno said:

Yeah, and that lifter is super-expensive. Still not a good idea to carry volatile fuel inside.

Hah, you know, I might have been joking about Skylon metastasising, but I wouldn't be at all surprised if Alan Bond was actually motivated by hopes that military will step in and ask them to use Skylon for moving volatile "fuel" inside. Which won't happen, because the one military crazy and overfunded enough to go for that doesn't particularly like international suppliers, but hey, a founder can dream.

[Kaos]

Skylon should build an ion upper stage. Takes two years to GEO and one back, but Xenon does not tend to explode ;-p

[fredinno]

10 minutes ago, Kaos said:

Skylon should build an ion upper stage. Takes two years to GEO and one back, but Xenon does not tend to explode ;-p

Ion engines are expensive though, so it would need to be a reusable space tug of sorts. Io engines are suited since these things can run for years on end, like on DAWN.

[Kaos]

14 minutes ago, fredinno said:

Ion engines are expensive though, so it would need to be a reusable space tug of sorts. Io engines are suited since these things can run for years on end, like on DAWN.

That is what I meant with "and one back"

[Red Iron Crown]

I don't think many customers would want to wait 2 years after launch for their investment to reach its useful orbit. Better to pay more (possibly) for a conventional rocket and get that satellite earning that much sooner.

[ModZero]

Well, if you want to speculate about propellants for the upper stage, refer to the user manual (which somehow is a thing that exists), appendix B, which states engines with impulse of 4562N*s/kg (yay! A real unit! In KSP units, that's 1.6 uranium-241 half-lifes), which sounds reasonable, but not exactly an ion engine.

EDIT: oops, I used the old version of the manual. There is a more recent version here, which actually mentions electric propulsion. I guess they'd go for some of the more recent concepts, some of which sacrifice some of the efficiency for higher (but still not that high) thrusts.

EDIT2: the electric one proposes a 20kW thruster to do a relatively quick boost to 5900km circular, and then having the payload use its own thrusters to get to the target over 130-160 days. So not quite two years.

Edited January 13, 2016 by ModZero