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I’ve read your papers with great interest.
They demonstrate exactly the kind of really innovative thinking that is needed… but is apparently rarely found in established civil servant discussions.
I say “established” in recognition of NASA’s virgin work on Apollo.
Without Von Braun and many others, NASA has fallen on typical government intellectual hard times.
Responding to your specific proposals … you clearly recognize that SPS is hung up on the launch cost barrier.
My friend Phil Chapman has suggested that Brayton Cycle devices can be more efficient in power to mass than solar panels, and perhaps more durable.
It seems worth examining, and Phil is a very technically sound person.
Your suggestion of equatorial launch seems driven by the use of the space elevator elements, but geosynch SPS location also drives to using equatorial launches.
As you’ve seen in other e-mails, I’ve advocated equatorial launch sites from high mountain peaks to gain perhaps 10,000 ft. or higher starts … seems sort of obvious!
(I’ve not inventoried Earth’s equatorial or near equatorial mountains yet!)
The other factor that I think will help SSTO is large size … larger than Saturn V.
That’s because many elements do not grow when the rocket does, or at least not proportionately.
Because modern materials will allow such a monster without exceeding Saturn V mass, the existing crawler design, that was, I believe inherited from S-V can continue to be used … perhaps the only old hardware or design that I advise using.
It also helps assure a greater return from the larger cargo capacity that comes with the larger rocket.
Another possibility is to allow the SSTO to deliver its cargo to LEO with almost expended propellants … and refuel it from Lunar water, with sufficient delta V capability to return for a vertical landing, having used the ample propellants to penetrate the atmosphere during return.
There were some studies done decades ago in using the flame sheath of a tail first penetration to reduce re-entry heating, as the combustion products are cooler than the penetrated atmosphere.
All this can free the SSTO from the tyranny of the .92 mass fraction dictated by available engine Isp.
And finally, I think the metallurgical challenges posed by an adjustable throat diameter to optimize expansion ratio for altitude compensation can be overcome.
Combining these ideas should allow creation of the SPS SSTO delivery vehicle that will break the cost barrier… and not just for SPS, but for Luna and Mars colonization … especially with the addition of even a partial Space Elevator!
Bill
PS: I’ve not thought this through … but your rockets will not really fly straight up in inertial space.
They must enter a less than the required orbital velocity arc to dock at the elevator lower terminus.
That, of course, is an advantage of the elevator … and most of the necessary orbit rate is imposed on the rocket by the Earth before lift off.
December 18th, 2009 at 10:03 am
As always, James Martin has gone directly to a reasoned solution.
However, it seems to me there are some missing links in this case.
It seems that Jim’s rocket flies up to the lower end of the space elevator, slows to a stop and attaches to the end of the elevator and is then hoisted on into space by the function of the elevator.
One missing element would seem to be that the angular momentum increase of the rocket mass being hoisted radially outwards on the elevator must be provided by some means and which means is probably beyond the strength (power) of the elevator unless perhaps you climb out really slow.
As you know, I still think the SSTO is ill-advised in that the payload fraction is so low, and the required performance is so high.
It is analogous to using a Corvette to haul freight.
It is just not that hard to devise easy to operate TSTO systems that have long service life and can reasonably put up payload fractions in the 2.5% – 3% range.
They can also bring back significant downloads when desired.
The structural additions to an SSTO to bring back downloads would further diminish their limited payload fractions.
I also like the idea of being able to leave a working stage in orbit for a few weeks while the rest of the rocket returns to the launch site to boost another payload stage.
What we know is that first cost of a vehicle that can fly 200+ missions is not critical to the eventual cost of putting payloads into space.
Therefore, it seems reasonable to build several vehicles so that frequent flights are possible with a given size ground crew, without the need for fast turnaround, which is about all the SSTO would seem to offer.
Having several vehicles being prepped along the flight line also assures that if a flight is scrubbed for vehicle problems, another vehicle is close behind to substitute for the other.
Furthermore, we can build really good TSTO vehicles today.
We are bound to find a need for new technologies to build an SSTO.
NASP was a far out example of such thinking.
December 18th, 2009 at 3:38 pm
Understood … the angular momentum usually imparted as a side load on a space elevator that is tied to a ground station will be taken out as added tension during ascent.
Not being so tethered, there must be another source of the required orbital motion.
Your arguments supporting staged systems are, in my mind, potentially trumped by the advantage of full re-usability offered by the SSTO system.
The background to my thinking is that all of history tells me that we have certainly not yet discovered the ultimate means for accessing a stable Earth (or any other) orbit or deep space traverse.
In fact, if we take any other transportation system in history, there is always a design progression from the primitive (dug out canoe?) to the “modern” (with the latter being whatever is the latest current model). space transport, to Earth orbit primarily, but between planets and moons eventually, is as certain as the sun rise.
But our initial, primitive efforts, as exemplified by the throw-away rocket designs we came up with half a century ago, can not and will not be the vehicles we will use to build the immediate future space industry.
The (to me, at least) self evident most cost reducing development will be full reusability, and the SSTO/VTOL rocket offers that capability.
Of course there are technical and operational challenges, but the SSTO exemplifies the investment principle that far greater return is achieved by achieving reduction in recurring costs, than by trying to skimp on non-recurring costs.
In short, it is time to move to the next stage and that is to eliminate staging .
I think you will agree that any staged, expendable system will be made immediately obsolete by the arrival of an SSTO/VTOL system if we can build it?
Then let us do that now, and begin by carefully analyzing and measuring the remaining short falls in our ability to build that system, inventorying the known means for circumventing those short falls and then building the system that, when we build it, will enjoy overwhelming cost advantage over its predecessors.
I submit that the technology that now exists, and did not in the Saturn V generation time-frame, should at the very least be inventoried to see whether we have arrived at the SSTO tech level or not.
We know exactly what we need.
I’ve listed them before, but it costs nothing to repeat them.
Mass fraction looms highest.
Two current and two potential tech/operating breakthroughs will overcome that barrier:
1. Large size
As I’ve said, a large rocket ipso facto reduces the mass fraction that will be devoted to such things as all electronics, human interfacing systems, cargo handling and docking hardware and systems.
Their actual mass will increase but will be reduced as a fraction of the total inert mass of the rocket, thus helping achieve the needed higher propellant mass fraction!
Large size will also increase the payload in absolute terms, even if it is less as a fraction of lift off mass than that in an expendable.
Large size will increase the payload (I think we should begin to replace “payload” with “cargo” to reflect a new posture) per flight operation, thus reducing the per unit cargo mass cost of the flight and ground crew and other otherwise fixed operating costs.
2. Exploiting natural advantages
Foremost among these is launch from higher elevations.
I’m going to inventory available natural elevations for suitability and report back, though I’m hoping someone else will beat me to it!
I know one who tried to; over twenty years ago Deke Slayton told me of efforts he had made in Hawaii to examine the feasibility of constructing a commercial rocket launch system on, I believe Mauna Kea.
The locals immediately launched a massive campaign to “save our volcano”.
I’ve not run the calculations, but a 14,000 ft head start will surely make SSTO more achievable.
Locating launches as near to the equator as possible will also aid in making SSTO easier… for equatorial space destinations and to exploit the greater Earth surface momentum for eastward launches.
3. Expansion Ratio Modulation
I have repeatedly cited Pg. 165 in Griffin & French “Space Vehicle Design” that says: “However, the pressure term is by no means negligible when accurate results are desired, as later examples will show.
Even rough calculations can sometimes require its inclusion.
For example, the Space Shuttle main engine suffers about a 20% loss of thrust at sea level compared to vacuum conditions because of pressure effects.”
Now I ask you, is that an opportunity crying out for exploitation or isn’t it?
What would a conventional turbojet engine manufacturer invest to achieve a 20% increase in operating efficiency?
I’ve sketched one such concept that will allow tuning the rocket exhaust expansion ratio to compensate for ambient pressure variation with altitude.
It is not simple, and the operating environment is fierce, but the payoff can be terrific!
4.Full reusability
Need I even discuss this?
I am not using the Space Shuttle system as an example, except perhaps as a prime example of how to give “reusability” a bad name.
The “reusability” that I aim for is exemplified by modern airline systems, with twenty year life for airline air frames common, and 10,000 hour TBOs on their turbine power plants.
Summarizing:
Achieving such performance numbers for our space access systems is inevitable.
Now, WHEN we will achieve them is driven primarily by how many of us recognize their inevitability and set about making them happen.
And WHO is another vital question. I sincerely hope that it is American engineers who will recognize it first.
And if a partial Space Elevator is also available, so much the better.
5. And finally, there’s the apparently now verified availability of water from Luna.
That will allow topping off, ne fully fueling the Earth-to-LEO rocket for its return flight, with every erg of propulsion energy usable to reduce thermal loading during re-entry.
What are we waiting for?