SSTO challenge

[lkm]

Well, I did say largely Skylon assembled. If you watch the excellent project Troy video it's hard to see how the Mars landers could have been brought up by Skylon just by the size of their heat shields, they look like good contenders for an SLS with an extra large payload fairing. And of course if somebody were to add an NTR and a giant expandable habitat I'm sure the crew would be pleased, but that still leaves the rest of the mass to be put up by Skylon.
I do like the reusable EDS though and the idea of sending 18 people at a time on three separate craft to explore three separate areas of Mars. It all feels like a proper sustainable plan of exploration.
My question about Skylon is not whether it can work, but where the design can develop to. If it's built it could be the 707 of the space age but improvement in performance through greater hypersonic air breathing would need a radically different airframe and effectively a whole new development process. Incremental evolutionary progress would seem more likely so I wonder were that progress could be found. Is there any benefit from a tripropellant Sabre cycle using liquid propane?
Replacing the rocket chamber with a small NTR would greatly improve performance but could it ever be accepted?

[93143]

Yeah, I read about the SUS.

Reusability seems to be key.  If you use an extra stage, and you have to throw it away because it can't be retrieved and refueled, you pay the full cost of the stage and your costs don't come down nearly as much as they could have, although small kick stages aren't as bad as full-on second stages (and SUS is pretty darn small).  Even if you can retrieve the stage, using it increases the complexity of ground operations, so it's best to avoid it if you don't need it.  If you do a TSTO design, the upper stage is large, mandatory, and unretrievable, and the sophistication of the booster stage is wasted.

If, on the other hand, the first stage can make it to orbit on its own, you can retrieve the upper stage under certain circumstances.  Or you can use an orbitally-based reusable transfer stage like Fluyt.

Suborbital staging is nice as an option if you have to put up a load that Skylon can't loft in one shot, but you don't want to lock yourself into requiring it.  They do note that it increases cost per kg significantly, despite the increase in gross payload per launch...

There's nothing wrong with using a reusable SEP tug for orbital transfer, but it's more expensive both to develop and to build, and takes longer to deliver its payload.  Also, the main advantage of SEP is high Isp, which shouldn't be as big of an issue within cislunar space if Skylon is providing loads of cheap prop in LEO...

I'm not a fan of using solids in space.  They may be cheap, but they're heavy and inefficient, and only nominally 'reusable' at best - a dumb kick stage as described would be a complete throwaway.  Not to mention explosion hazards and toxicity issues with respect to ground handling - remember, this system is supposed to operate like an airliner...

[tnphysics]

Yeah, I read about the SUS.

Reusability seems to be key.  If you use an extra stage, and you have to throw it away because it can't be retrieved and refueled, you pay the full cost of the stage and your costs don't come down nearly as much as they could have, although small kick stages aren't as bad as full-on second stages (and SUS is pretty darn small).  Even if you can retrieve the stage, just having it increases the complexity of ground operations, so it's best to avoid it if you don't need it.  If you do a TSTO design, the upper stage is large, mandatory, and unretrievable, and the sophistication of the booster stage is wasted.

If, on the other hand, the first stage can make it to orbit on its own, you can retrieve the upper stage under certain circumstances.  Or you can use an orbitally-based reusable transfer stage like Fluyt.

Suborbital staging is nice as an option if you have to put up a load that Skylon can't loft in one shot, but you don't want to lock yourself into requiring it.  They do note that it increases cost per kg significantly, despite the increase in gross payload per launch...

There's nothing wrong with using a reusable SEP tug for orbital transfer, but it's more expensive both to develop and to build, and takes longer to deliver its payload.  Also, the main advantage of SEP is high Isp, which shouldn't be as big of an issue within cislunar space if Skylon is providing loads of cheap prop in LEO...

I'm not a fan of using solids in space.  They may be cheap, but they're heavy and inefficient, and only nominally 'reusable' at best - a dumb kick stage as described would be a complete throwaway.  Not to mention explosion hazards and toxicity issues with respect to ground handling - remember, this system is supposed to operate like an airliner...

Ooops! (as far as the solid)

What about using a later Skylon (carrying another payload) to retrieve the US?

You are correct that you do not want to be "locked-in" to having a US.

One other option: The payload must have an OMS (for station-keeping at the very least). What about having the payload have bigger tanks (cheap) and use its own engines (which it must have anyway) to make the burn?

[93143]

What about using a later Skylon (carrying another payload) to retrieve the US?

That could work.  Obviously it would only work if the Skylon itself were capable of achieving orbit...

One other option: The payload must have an OMS (for station-keeping at the very least). What about having the payload have bigger tanks (cheap) and use its own engines (which it must have anyway) to make the burn?

You're talking about ~4 km/s of delta-V to get from LEO to GEO.  The spacecraft can certainly do the circ burn at GEO, but the GTO insertion is getting to the point that you're blurring the line between an OMS and an upper stage; 3.9 km/s (starting from equatorial) is a mass ratio of almost 3.6 using MMH/N2O4, neglecting gravity losses from using the dinky little maneuvering engines...

I remind you that I am not an authority on this.  I'm still a student (of rocket science, yes, and at Ph.D. level, but still a student), so take my remarks as argumentation rather than authoritative pronouncements.

[alexterrell]

If a sub orbital launch with second stage is used to double payload, this becomes economical if the upper stage cost is less than a launch stage.

A reusable upper stage is good, but doesn't work with sub-orbital launch. A SEP upper stage is even better, but SEP will have problems below about 400km.

If however a few Skylons get built, and together they're good for a launch per day, it would certainly be worth having a base at about 400km, complete with electrodynamic or VASIMR reboost, a Bigelow module, a fuel depot and an upper stage handling facility. Most of the skylon flights will be delivering fuel.

[lkm]

With regards Fluyt, just how restartable is, or will be, a Vinci engine? What sort of life expectancy would such a reusable stage have?
And if Fluyt and its orbital  support architecture is built, could a larger version be built for the SLS sharing some of the infrastructure?

[tnphysics]

Do electrodynamic tethers work at 400km?

If so, then one could be used to reach an altitude at which an ion engine could function.

[93143]

Do electrodynamic tethers work at 400km?

If so, then one could be used to reach an altitude at which an ion engine could function.

Don't overcomplicate things.  The more components you add to a transport system, the more expensive and hard-to-use it becomes.

EDIT:  silly anti-ion-tug paragraph removed.

...yes, I think ED tethers have been tested substantially below 400 km.

With regards Fluyt, just how restartable is, or will be, a Vinci engine?

Five restarts according to Wikipedia.

Of course, that's for a single mission.  Servicing might reset that number.  As for overall reusability, I don't have enough information to guess, but plainly REL thinks it's a plausible plan...

[mlorrey]

If a sub orbital launch with second stage is used to double payload, this becomes economical if the upper stage cost is less than a launch stage.

A reusable upper stage is good, but doesn't work with sub-orbital launch. A SEP upper stage is even better, but SEP will have problems below about 400km.

If however a few Skylons get built, and together they're good for a launch per day, it would certainly be worth having a base at about 400km, complete with electrodynamic or VASIMR reboost, a Bigelow module, a fuel depot and an upper stage handling facility. Most of the skylon flights will be delivering fuel.

Any reusable launch system that is built to dock with an orbital station is inherently incapable of one launch per day per vehicle. It takes a minimum of a day and a half to rendevous and dock with a space station, and a similar amount to undock and reenter.

Most of the time is spent on very slow approach and departure while in the vicinity of the station, as well as the docking/undocking maneuvers. So, you should expect that any reusable launch vehicle will never be able to launch more than twice per week, even if they have a ground turnaround time of less than 24 hours.

This time spent in orbit automatically reduces the potential sortie rates of such a reusable launch system by over 70% versus a reusable launch system that places its own payloads into orbit or releases payloads with their own upper stages to make a GTO burn and insertion.

So, even if you double the payload by doing so, you still wind up losing money due to the lower sortie rate. Having a sortie rate that is 3.5 times greater with half the payload means 1.75 times greater revenues per time period, and thus the capital costs of the vehicle are amortized to a much greater extent.

This is completely ignoring the other gorilla in the room: the need to place into orbit the needed fuel to fuel whatever reusable upper stages the Skylon would redezvous with at the orbital station. Since upper stage mass fractions are at least 80% fuel, you really are only saving the 20% of the upper stage that isn't fuel, which isn't worth reducing your sortie rate by 3.5 times.

[lkm]

Reaction Engines believes Skylon will have a two day turn around and a fleet of thirty for first generation operations. It is my understanding that the OBS is merely a staging post for the OTV and not something Skylon tries to dock with during GEO operations.
I would guess the OTV prepositions itself for rendezvous, how that affects orbital times I don't know.
It also should be noted that the Fluyt design is reused both in CIS-lunar operations and in project Troy to a certain extent.
  When it  comes to fuel RE states that launch cost for low value products, such as fuel, should be less than half that for a high value launch and that Fluyt has a mass ratio of 0.12.

[93143]

Having a sortie rate that is 3.5 times greater with half the payload means 1.75 times greater revenues per time period, and thus the capital costs of the vehicle are amortized to a much greater extent.

Three things.  First, a sortie twice a week means that the vehicle reaches its design lifetime of 200 flights in less than two years.  At the expected fleet sizes, spaceframe costs dominate over development costs, so there's no huge rush to launch an individual vehicle as often as possible.

Second, you can't make money if the payloads you want to launch are heavier than your vehicle can carry.  Skylon D1 is sized for an Alphabus+US; go bigger and you need Fluyt.  Suborbital staging with a larger upper stage would help, but then you're expending the upper stage and your costs start to balloon...

Third, your factor of improvement is based on a 24-hour turnaround time.  Doubling it to match REL's projection seriously harms your argument.

Since upper stage mass fractions are at least 80% fuel, you really are only saving the 20% of the upper stage that isn't fuel, which isn't worth reducing your sortie rate by 3.5 times.

I hope you aren't talking about an expendable upper stage.  The cost of expending a stage, even a small one, would be much higher than the cost of a single Skylon launch.

Oh, hang on - if the upper stage isn't expended, either the Skylon has to wait around for it, or a subsequent flight has to rendezvous and pick it up.  So your argument is blatantly incorrect.

[KelvinZero]

Any reusable launch system that is built to dock with an orbital station is inherently incapable of one launch per day per vehicle. It takes a minimum of a day and a half to rendevous and dock with a space station, and a similar amount to undock and reenter.

Hey, random thought, a bit off topic:

A very reusable low power space tug could remove the need for the launch vehicle to have any docking requirement, allowing faster turn around and also allowing much lower regulation, since if docking goes wrong a lot more than the vehicle could be lost.

Besides, space tugs attached to the ISS would be cool

[tnphysics]

The reason for the tugs I proposed is manyfold.

First, they allow payloads to be lofted from LEO for minimal cost. This is crucial, as Skylon, while a major improvement, will still not be cheap, if for no other reason because LH2 costs something.

Second, they allow spacecraft to dispense with much of the "station-keeping" propellant. Rather, the tug visits every so often, kinda like ISS reboost.

Third, it would allow easy prop resupply of satellites. This would dramatically increase their operational lifetimes.

The last two benefits would be most pronounced for GEO sats, as they are hard to otherwise access yet have negligible delta-V to go from one to another. Thus, a tug could visit multiple ones in one run.

[hop]

It takes a minimum of a day and a half to rendevous and dock with a space station,

There's no inherent technical reason you couldn't dock in the first couple of revs, although it's hard to see where it would be worth the hassle.

and a similar amount to undock and reenter.

Current Soyuz undock to landing is about 3.5 hours, and you could probably cut quite a bit off that if you really wanted. Prep and leak checks add a few more hours.

[93143]

After giving it a bit more thought, I agree that there are advantages to an electric tug.  There are also disadvantages, such as the cost of the tug and the long travel time, as well as a fairly high minimum altitude.  Polywell fusion could remove those last two problems (likely at the expense of more of the first).  Either way, if the tug is highly reusable, cost can be amortized over many missions.  However, it would be quite expensive to develop, and Fluyt looks like a good interim solution at the very least.

From the user's manual, it looks like Skylon's payload only drops from 15 mT to 14 mT if it has to go up to 400 km.  It can reenter safely from as high as 800 km, at which altitude the payload is 11 mT or so.

[KelvinZero]

I guess the tug could be electric. I was actually thinking just small robustly built maneuver thrusters, perhaps hypergolic. I imagine the tug does not need much fuel to reach the cargo pod or return if aborted, and and perhaps the cargo pod itself could provide the necessary fuel to push the cargo back assuming docking is successful?

Sorry tnphysics. I missed your initial post on tugs. pm a link to me and I will go and have a look.

[A_M_Swallow]

The third option for a LEO tug is solar thermal.  Isp in the 800 to 1000 second range, so they use less hydrogen propellant that chemical tugs.  Solar thermal engines have a higher thrust that a similar sized solar electric tug.  They continue working in the Van Allen Belts because the optics are radiation hard.

[93143]

Well, I'm thinking that if you've got an SSTO space shuttle with an OMS (Skylon) and a reusable in-space HEO transfer vehicle (Fluyt), the role of a tug should probably fall to one or the other, rather than to a specialized third vehicle.

And I don't really see that sortie rates are going to need to be high enough that it couldn't be the job of the SSTO (this also has the advantage that the payload transfer takes place at a station).  But the transfer vehicle could theoretically go pick up a payload placed in orbit by the shuttle, and the transfer vehicle doesn't, strictly speaking, need to be chemical (though REL seems to think chemical is a good idea).

It's also a good idea to remember that orbit phasing and turnaround times apply to whatever is doing the rendezvous; to the tug and/or transfer vehicle fleet as much as to the SSTO fleet.  Actually, it occurs to me that the tug idea gets hit twice with this; once outbound and once inbound.  By contrast, if you use the SSTO to do the rendezvous it only has to do it once, and the same applies to the transfer vehicle (assuming it has enough prop to do the GTO burn after intercepting the cargo).  So a tug is actually bad for your schedule, and it adds an extra layer of costs.

EDIT:  Just realized that the transfer vehicle has to rendezvous again after it returns from GTO.  So it gets hit twice too.  Looks like sortie rate is best with the SSTO doing the rendezvous...

If a tug is just picking stuff up in LEO and taking it to a transfer vehicle depot also in LEO, it almost has to be chemical.  The amount of delta-V is trivial, it gets to refuel after each trip, the ability to fire the engines on the night side is useful, and if you must have a middleman, which I'm not remotely convinced of, keeping costs down is absolutely vital.

[Calorspace]

Having read about this project quite a bit and liking it's ideas. One thing puzzles me.. if this is such a great thing, why are other countries/companies not looking into taking the same approach?

[QuantumG]

Having read about this project quite a bit and liking it's ideas. One thing puzzles me.. if this is such a great thing, why are other countries/companies not looking into taking the same approach?

You mean Skylon?  Because they don't know the secret pixie dust.