Why don't we have SSTO's already?

[Secuas]

I think everyone here knows about the Skylon project and I am pretty interested in it and see a potential. But if it is such a big of breakthrough in spacetravel why isn't it here already? or is it the funding?

Thanks in advance. ^^

[Camacha]

I think everyone here knows about the Skylon project and I am pretty interested in it and see a potential. But if it is such a big of breakthrough in spacetravel why isn't it here already? or is it the funding

The short answer is that it is pretty hard to do it in a useful manner in real life. A lot harder than it is in KSP.

Edited December 12, 2014 by Camacha

[Frozen_Heart]

The technology is only just becoming viable.

[Nibb31]

We don't have single-stage-to-orbit launchers because they suck at launching stuff. Dropping stages along the way makes your vehicle lighter and therefore much more efficient.

SSTO is easy as long as you don't care about the payload. The ancient Mercury-Atlas was nearly an SSTO (it only dropped its engines on the way up), and the Titan first stage or the Saturn 1B with nothing on top had enough dV to reach orbit in a single stage too, although it was never tested because they weren't throttlable or restartable and because it was pointless. With modern technology, it would be easy to make an expendable SSTO launcher with a tiny payload.

The thing is, designing an SSTO launcher with a sucky payload is pointless when you can launch a multi-stage rocket with a significant payload.

PS. And yes, I know you weren't really asking about "SSTO" but about "reusable spaceplanes". So please use the term "reusable spaceplane" when talking about reusable spaceplanes and not "SSTO", which is a flight profile, not a type of vehicle. It's quite possible to envision SSTO rockets, reusable rockets, multi-stage spaceplanes, and all sorts of other combinations... To summarize:

- The reason we don't have SSTOs is because MSTOs are much more efficient.

- The reason we don't have spaceplanes is because they are much heavier than rockets because you need to carry all that plane stuff in addition to all your rocket stuff. Therefore you can't carry any payload unless you invent some new breakthrough material and breakthrough engines.

- The reason we don't have reusable launchers is because the demand for orbital launches doesn't allow launch rates frequent enough to make them economically viable.

Edited December 12, 2014 by Nibb31

[Bill Phil]

Why not? Well, the question really is why don't we have a space truck. Or better yet, why is nothing being mass-produced?

[John FX]

I only use an SSTO when I want to reuse the spent stage as a fuel depot (as part of a station) or have the whole craft be refuelled in space to act as an interplanetary shuttle.

That is not usually a requirement for real life missions.

[Secuas]

But aren't ssto's more cost-efficient?

[Nibb31]

But aren't ssto's more cost-efficient?

Again, SSTOs or spaceplanes ?

I just explained to you that SSTO is less efficient than MSTO because it has a smaller payload fraction.

A conventional MSTO rocket typically has a payload fraction of 0.5%. This means that you need a 1000 ton rocket to launch a 5 ton payload (0.5% if the total weight of the rocket).

An SSTO rocket typically has a much lower payload fraction because it is less efficient. It is less efficient because it has to carry all that heavy tankage and those high-thrust first stage engines to orbit. The payload fraction is eaten up by that extra weight and ends up either negative (it can't reach orbit) or so insignificant that it's useless (0.005% of your 1000-ton rocket)

So how is a rocket that can't put a payload into orbit more cost-efficient than a rocket that can ?

Edited December 12, 2014 by Nibb31

[Frozen_Heart]

But aren't ssto's more cost-efficient?

Reusable launchers are more efficient. I assume that's what you're talking about. Either SSTO or not.

However they need a larger market to be economically viable. To have a larger market requires lower cost of launching which can only be achieved with reusable launchers. (Which need a larger market)

Its a vicious cycle we're stuck in.

[Starman4308]

But aren't ssto's more cost-efficient?

Only if they're recovered. If they're not recovered, MSTOs are more cost-efficient, because they use less mass to get to orbit.

And that's the rub: SSTO rockets are very impractical to recover (any heat shielding would basically eliminate their remaining payload), and Skylon is the first SSTO spaceplane that might work with something approaching an economic payload.

[RainDreamer]

I wonder, what is the advantage of SSTO? It is, after all, possible to have a multiple stage craft where each stage is reusable/controllable and return where it is launched.

[Red Iron Crown]

It hasn't even been demonstrated yet that reusable launchers are more cost effective in reality. The only somewhat reusable real life spacecraft was the Space Shuttle, and it was no stellar performer in cost efficiency.

SpaceX might do better with the Falcon 9R, but we'll have to wait and see if the numbers play out like they think they will.

[Iskierka]

I wonder, what is the advantage of SSTO? It is, after all, possible to have a multiple stage craft where each stage is reusable/controllable and return where it is launched.

The issue with this is that mating/reassembly is still expensive, and the flight path constraints on making each part returnable significantly reduces payload capability - plus, far more engines to maintain. SSTO is massively beneficial if it can provide significant payload as it cuts out many maintenance and reassembly steps allowing faster and cheaper turnaround, removes a point of failure, and gives greater operational simplicity overall.

The shuttle is kind-of useless as a cost reference point as there were so many bad design and operational considerations, drastically increasing its cost with things that either weren't or shouldn't have been necessary. Reusability, in any number of stages, won't look like the shuttle ever again.

[Starman4308]

It hasn't even been demonstrated yet that reusable launchers are more cost effective in reality. The only somewhat reusable real life spacecraft was the Space Shuttle, and it was no stellar performer in cost efficiency.

There were confounding factors for the Space Shuttle, like the extraordinarily poor decision to make a manned vehicle with no launch escape system, thus requiring incredibly expensive inspections prior to each launch.

[Red Iron Crown]

There were confounding factors for the Space Shuttle, like the extraordinarily poor decision to make a manned vehicle with no launch escape system, thus requiring incredibly expensive inspections prior to each launch.

Oh definitely. My point was that it is not a proven fact that reusability will be more cost effective, we won't know for sure until someone is doing it regularly and we can see the true costs. IME just about every proposal for full reusability (SSTO or otherwise) is optimistic about costs, sometimes wildly so.

[magnemoe]

The short answer is that it is pretty hard to do it in a useful manner in real life. A lot harder than it is in KSP.

Yes, dW to reach orbit in KSP is 4.5km/s on earth is 9km/s who is more like Eve.

Fast planes are far harder in real life than in KSP where its no issue getting anything up to 1.5 km/s, this is 2/3 of orbital speed of kerbin and 1.5 times the max speed of an SR71.

Edited December 12, 2014 by magnemoe

[jrphilps]

I think everyone here knows about the Skylon project and I am pretty interested in it and see a potential. But if it is such a big of breakthrough in spacetravel why isn't it here already? or is it the funding?

Thanks in advance. ^^

Skylon is still mostly a concept at this stage. Unless we are willing to put alot of effort into something like the x-program of the 60s, its unlikely that we will achieve the propulsive breakthroughs for SSTO anytime soon. However, we still have the option of using sky ramps to assist space planes into orbit: http://www.g2mil.com/skyramp.htm

[NERVAfan]

Also, developing an orbital launch vehicle is really expensive, so there aren't that many organizations that can do it.

If you could develop an orbital LV cheaply, IMO somebody would have made a non-spaceplane SSTO, because even if the payload fraction is much less there are some potential advantages (theoretically you could have higher reliability since there are fewer parts and no staging events).

But since developing an orbital LV is super expensive you need a really good justification to develop a new one.

---

You might be able to get an interesting (expendable) SSTO by using the Mercury Atlas design with SpaceX's Merlin 1D engines. The Mercury Atlas only dropped engines, not a whole stage, and is probably the closest thing to an SSTO that's ever actually been used. But SpaceX's Merlin 1Ds have much better TWR so you might not need to drop the engines (they seem to have better Isp too).

OTOH, the old-style-Atlas balloon tanks probably wouldn't work well with reusability, so SpaceX wouldn't be interested.

[NERVAfan]

Again, SSTOs or spaceplanes ?

I just explained to you that SSTO is less efficient than MSTO because it has a smaller payload fraction.

A conventional MSTO rocket typically has a payload fraction of 0.5%. This means that you need a 1000 ton rocket to launch a 5 ton payload (0.5% if the total weight of the rocket).

Um, really? Falcon 9v1.1 has a starting mass of 505,846 kg with an LEO payload of 13,150 kg (according to the SpaceX website). That's 2.6%. (And IIRC that number includes some margin for reusability attempts, too).

Is SpaceX really doing 5x or more better than normal? That seems unlikely, as much as I like the company.

It's harder to tell with Delta IV since it has a bunch of different configurations, but looking at the numbers on Wikipedia it seems to be in the 4% range. (The Atlas V article lists a range of payload capacities but only one total mass, so I don't know what to do with that).

[merendel]

Um, really? Falcon 9v1.1 has a starting mass of 505,846 kg with an LEO payload of 13,150 kg (according to the SpaceX website). That's 2.6%. (And IIRC that number includes some margin for reusability attempts, too).

I'm prety sure that was a typo on Nibbs part and he ment a SSTO (not the MSTO as he stated) has a payload fraction down less than 1%. A conventional SSTO rocket has to be very well engineered or you get no payload at all so 0.5% does not sound unreasonably small. The falcon is a 2 stage rocket and would probably have a higher payload fraction if they were not reserving fuel for eventual reuseability.

[NERVAfan]

I'm prety sure that was a typo on Nibbs part and he ment a SSTO (not the MSTO as he stated) has a payload fraction down less than 1%.

Ah, that would make sense.

A conventional SSTO rocket has to be very well engineered or you get no payload at all

Well, I'm not sure it's quite THAT bad given that we were very close in 1961 and current engines (Merlin 1D anyway) are way better TWR and somewhat better Isp.

[merendel]

Well, I'm not sure it's quite THAT bad given that we were very close in 1961 and current engines (Merlin 1D anyway) are way better TWR and somewhat better Isp.

The improved TWR would kinda fall under the catagory of better engineered. Still the rocket equation is a cruel mistress. When you go for SSTO you prety much have to count the dry mass of the launcher agianst the potential payload. Lightening the tanks through better material science or improving the TWR of the engines shifts more of that weight to potential payload. Still no mater how light you manage to make the tanks and engiens they are going to be a non zero mass that your haveing to lug all the way to orbit. We can do better now than back in the 60s but its still more cost effective to strap an uperstage engien for the final push to orbit by a non trivial margin. Hence why there hasnt been a big push for SSTO rocket launch platforms.

[K^2]

Of course, but the crux is that the bulk of the launch cost isn't in fuel. If you make a larger, heavier ship, that needs way more fuel, but is 100% reusable, it's still cheaper to launch things. That's why SSTO concept exists, and why it received a lot of attention in the past couple of decades as it became clear that we can shift cost-efficiency balance that way.

On the other hand, SpaceX et alii push for re-using lower stages instead. That's also a viable option, since lower stage, certainly, is both the heaviest and most expensive part.

[NERVAfan]

On the other hand, SpaceX et alii push for re-using lower stages instead. That's also a viable option, since lower stage, certainly, is both the heaviest and most expensive part.

The lower stage thing is an intermediate step. Elon Musk has now said that reusing the 2nd stage of Falcon isn't practical since it goes to GTO and has poor specific impulse, but the next rocket (methane/LOX and much bigger) is supposed to be fully reusable.

[K^2]

I don't know if it makes economic sense to build second stage as reusable. It would make much more sense to integrate upper stage with the lander. Basically, an "SSTO" with insufficient delta-V to actually make orbit on its own rides on top of a reusable first stage that gives it a boost. Though, maybe that's what they meant.