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Two Stage to Orbit

Posted by: campbelp2002 - Fri Nov 11, 2005 5:22 am
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Two Stage to Orbit 
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Post    Posted on: Wed Nov 23, 2005 8:53 pm
Max Lange wrote:
A flexible system, like Ariane 4, would be more advantageous.
To me the system is only flexible unless it has BOTH Ariane 4 and Ariane 5. And Delta and Falcon and lots of others. You need to be able to quickly get the right size rocket for the payload on the pad. Having only big rockets makes the small payload users wait for a ride with a bigger payload, kind of like flying standby, and having only small rockets requires breaking big payloads into smaller parts that can link up in space, like disassembling a tank, sending the parts to Kuwait on a bunch of 737s, and reassembling it there instead of putting the tank in a C5A.

By the way, by 2STO I mean fully reusable 2STO. Ariane and others don't qualify. Even Falcon doesn't, because fishing a booster out of the ocean after it parachutes down is not good enough. The parts have to fly back under control to well defined landing areas where they can be refueled and flown again.


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Post    Posted on: Wed Nov 23, 2005 9:20 pm
...and SpaceX has yet to indicate how it is that they presume to retrieve aluminum from orbital trajectories in a re-usable configuration. I have not heard anything about how they are going to accomplish that feat.

Falcon I is only re-usable in the lower stage; friday's spectacle will include the eventual (probably in some weeks, actually) incineration of the upper stage and it's Kestrel engine after the AF geeksat is placed into it's orbit.

Still ...I'll bet the Academy is stoked that thier kids' bird is flying on a booster named for thier own mascot.

Meanwhile, as I have said in the past, the real trouble with fully-reusable TSTO systems lies in the fact that the upper stage must achieve orbital insertion velocities, and that means all of the thermal and aerodynamic loading associated with re-entry from orbit. A big hollow aluminum cylinder ain't gonna make that trip.


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Post    Posted on: Wed Nov 23, 2005 9:36 pm
SawSS1Jun21 wrote:
the real trouble with fully-reusable TSTO systems lies in the fact that the upper stage must achieve orbital insertion velocities, and that means all of the thermal and aerodynamic loading associated with re-entry from orbit.
A reusable SSTO will have that problem in spades. With a 2STO vehicle only the smaller 2nd stage needs the full thermal protection system.


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Post    Posted on: Wed Nov 23, 2005 10:17 pm
All of which begs for better energy densities than can be found in chemical propellants; if you can make your fuel tanks smaller, then you have much less area to shield from re-entry heating.

That's where the hydrogen slush and similar ideas came from, but even slushy hydrogen is damnably diffuse. The ideal solution may be methane, what with each particle having 4 hydrogen atoms. If you used a FLOX oxidiser, you might get usable returns on tankage reduction using CH4 as your fuel. High-energy combustion chemistry gets complex, though, and the experts I know I unfortunately don't speak with often enough.

I read a NASA article about fuel cells which indicated the optimum density for both energy and hydrogen atoms was obtained in a LiH slurry with mineral oil. I don't know how well that stuff could burn (with FLOX70 anything burns, though!), but you should still get high impulse numbers (Li is even lighter than Carbon, and has more valance energy, I believe) considering the worst part of the fuel is still a hydrocarbon.

Just speculating, anyway...


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Post    Posted on: Wed Nov 23, 2005 10:27 pm
Actually I think less dense works better for reentry. A small mass with a large surface area has less heating per unit surface area than a small dense object. So the LH2/LOX SSTO (or 2STO) vehicle should have less reentry problems than the dense propellant vehicle, not more.


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Post    Posted on: Fri Nov 25, 2005 4:02 pm
I'm still thinking that the tanks will be primarily empty at re-entry in any case; meaning that the density of the vehicle would be approximately the same when empty than the similarly empty hydrogen-based ship.

The advantage is more significant at the other end, i.e. when you are forcing the thing through the stratosphere at mach 6+, and every square centimeter is an exponentially increasing drag factor due to the additional propellant required to counteract the drag, which adds more volume, adding more area, etc...

If your second stage (all dead weight during the air-breathing portion of your soujourn) can be made as small as possible, then it is a simpler ship to build and to launch. If your exoatmospheric vehicle were in the range of say, X-38 (the full-size CRV, actually) or X-37; something like 15-20 meters and ~8 or 10 tons, then the hurdles are all a lot lower, and many of them have been done. The re-entry characteristics are well studied for the two x-planes mentioned above, the thermal issues are known and within the capabilities of extant materials. The XB-70 Valkyre bomber was built 40 years ago and could haul a payload nearly in that class to an altitude over 80Kft and faster than mach 3.

Yeah, it wouldn't be able to loft anything heavy, but a small crew or a critical piece of spacecraft hardware that might be needed in an emergency would be well within the capacity of such a craft. The RASC guys might do better if they looked a little more at near-term capabilities; of course, that's not thier job :)


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Post    Posted on: Fri Nov 25, 2005 4:33 pm
SawSS1Jun21 wrote:
The XB-70 Valkyre bomber was built 40 years ago and could haul a payload nearly in that class to an altitude over 80Kft and faster than mach 3.
Oh yeah, I forgot all about the XB-70. You would think that after 40 years that could be improved upon. We could be closer to that HTHL 2STO than I thought.


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Post    Posted on: Fri Nov 25, 2005 6:39 pm
campbelp2002 wrote:
To me the system is only flexible unless it has BOTH Ariane 4 and Ariane 5. And Delta and Falcon and lots of others. (...)

By the way, by 2STO I mean fully reusable 2STO. Ariane and others don't qualify. Even Falcon doesn't, because fishing a booster out of the ocean after it parachutes down is not good enough. The parts have to fly back under control to well defined landing areas where they can be refueled and flown again.


It depends on what you call a "system". For me, Ariane 5 is a system. That's what I meant. Many launch vehicles are a market. And there, of course, I totally agree with you that all sizes are needed.

Considering winged 2STO, there are two points to be made. Firstly: A winged second stage only has benefits if the mass and complexity of the heat shield and wings is more than offset by re-use. That's actually not the case for the Shuttle (pity!) and I doubt it'll be achievable in the next development cycle (starting a phase A this decade). For the moment, better to reuse a first stage and have a simple, junkable second stage. Notwithstanding that, I agree that ultimately upper stages will be reusable. Then again, really ultimately upper stages might as well operate on electric propulsion and never actually re-enter. We'll see.
One word about fuels: Current wisdom says high-density fuel keeps the vehicle light because the envelope is small. Nothing stops you adding big wings to have a good aerodynamic behaviour. Low density means high volume, bigger mass, bigger wings as a consequence, bigger lower stage, more waste, more cost. Unless of course the higher ISP offsets the higher volume. At the moment, for chemical propulsion the match is still open, and again everything else is very hard to predict as yet.

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Post    Posted on: Fri Nov 25, 2005 6:50 pm
SawSS1Jun21 wrote:
(...) If your exoatmospheric vehicle were in the range of say, X-38 (the full-size CRV, actually) or X-37; something like 15-20 meters and ~8 or 10 tons, then the hurdles are all a lot lower, and many of them have been done. The re-entry characteristics are well studied for the two x-planes mentioned above, the thermal issues are known and within the capabilities of extant materials. The XB-70 Valkyre bomber was built 40 years ago and could haul a payload nearly in that class to an altitude over 80Kft and faster than mach 3.(...)


Taking us full circle to John Carmack's reply at the beginning of this discussion, I'd say.
I'm a big XB-70 fan, but unless an existing operational design can be used, you'll be saddled with developing a big Mach3+ plane. Remember the Valkyrie was, in real terms, the most costly aircraft development ever (at least before the F-22 :roll: ).

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Post    Posted on: Fri Nov 25, 2005 9:33 pm
There is little doubt that the technology Pete wishes to explore in this thread will be expensive to develop.... my point in mentioning B-70 was simply to illustrate the fact that much is already known about a significant portion of the envelope, and that the RASC paper is only one version of the solution (and a very optimistic one at that).

John is absolutely correct in stating that it will always be far cheaper to BUILD a hundred hydrocarbon/LOX boosters than a large-capacity hypersonic high-alititude waverider, but if the price of aluminum fabrication and kerosene were the governing factors, we'd already be living in publiusr's utopia of orbital factories and thousand-ton missions to extrasolar destinations.

The practical utility of flexible basing, gas-and-go operation, any-orbit-anytime accessibility which should be possible with a TSTO RLV would presumably make such a feat valuable in some aspect of the commercial launch industry.

P.S. I read that more than 80% of X-15's lift was produced by the fuselage at hypersonic speeds... I don't really think you want any thing more than tiny stubs on your exoatmospheric stage.


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Post    Posted on: Fri Nov 25, 2005 11:21 pm
SawSS1Jun21 wrote:
There is little doubt (...) The practical utility of flexible basing, gas-and-go operation, any-orbit-anytime accessibility which should be possible with a TSTO RLV would presumably make such a feat valuable in some aspect of the commercial launch industry.

P.S. I read that more than 80% of X-15's lift was produced by the fuselage at hypersonic speeds... I don't really think you want any thing more than tiny stubs on your exoatmospheric stage.


I absolutely agree!

In fact, why not take it from here and speculate in a bit more detail about how such a hypothetic vehicle might look like? I'm thinking of a little theoretical trade-off, to see where a TSTO ship might be going. I'm thinking of something with, say, a 6t LEO capacity (way short of what Publius wants but comparable to existing rockets). Say you start with a second stage that uses the latest in ceramic composites for the structure (like Shafex, say) and so doesn't need an extra heat shield. Say it's a low L/D affair, only just good to land after reentry. Furthermore, let's assume for the moment we have a first stage that travels at 57000ft and 300m/s, that should give us around 900 m/s of delta-V, and the second stage might need to provide about 8,4 km/s of delta-V. Then we'd be looking at something like a 14% mass fraction for the structure and payload combined if we assume an ISP of 430s throughout. Furthermore, if we assume that's 10%, no more, for the structure, we end up with something that's 25 times the P/L mass, or about 150 tons, launched from our first stage. About what an A380 will carry. There you have a reference, from where one may change some numbers to see what comes out.

For example, if you go to Mach 3 with the first stage, you end up with about 1,8km/s for it, and say 7,5km/s for the second stage. With the same assumptions as above, that's a 17% mass fraction ship weighing about 83 tons at separation. Which sounds more likely to you?


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Post    Posted on: Mon Nov 28, 2005 8:34 pm
Well, before I answer, can we establish an approximate relationship between velocity and altitude in this equation? Just roughly, mind you, it would be useful to know which is better: Mach3 at 60 Kft or Mach1 at 120Kft? (sorry for the non-SI units)

You're the expert and I'm lazy, otherwise I'd dig it out myself :)

Just because it seems that in some portions of the flight envelope (and by that I am referring to aerodynamic flight, not rocket-propelled vertical flight), aquiring the altitude may be easier; the F-4 had an operational ceiling at 50 Kft, but an early prototype set a time-to-climb and absolute altitude record (for the time) of over 100 Kft. OK, it was near stall speed and not in level flight, and sure, I don't think that it scales in a linear fashion, as I am certain that a performance climb in a B-1A or XB-70 is going to be a very different excercise. Still, having another tweakable parameter in this thought experiment may prove to be worthwhile...

In the interim, here's some AFA-sponsored Pentagon material for your reading pleasure... http://www.milnet.com/pentagon/hyper/v3c12-1.htm

...Ooooh, look, there's my anwser on page 4 under "staging"

Quote:
Studies have shown that the altitude is not so much a factor as is the staging mach number. In order to reach a LEO, the required velocity is around 26,000 feet/s (30,000 feet/s considering losses due to losses from pressure, drag, etc.). Since the desired orbit is at least 100 nm, the effects on required velocity change of staging at 50,000 feet versus 100,000 feet versus 150,000 feet are nearly negligible versus staging mach number.


This is in a paragraph wherein their wish-mach-number is 12 (eeek!).

Hmm.... let me stew on this a bit.

P.S. I'm still lazy, dammit! You're not allowed to erase my hard-earned bad reputation simply because I found something accidentally.


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Post My 2 bits   Posted on: Tue Nov 29, 2005 8:54 am
If you want TSTO, why bother with wings at all?

Try an Armadillo style VTVL first stage with separation at 50 km and 1000 m/s.

The first stage would need about 2.5 km/s total dV for launch, thus for an ISP of 220 would have a mass ratio of 3.1, so a fuel fraction of about 69% of launch weight. Allowing 20% for the second stage, that leaves about 10% for just the empty vehicle & 1% for landing fuel. It would land about 200 to 300 km down range. (Partly re-fill the tanks and fly back?)

The second stage needs about 7 km/s dV to reach orbit, and with an ISP of 430 for LOX/H2, its fuel fraction would be 81%. 9% payload?

6t payload, 61t second stage, 268t first stage, total glw 335t?

Or use a lower ISP second stage and make the whole thing bigger.


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Post    Posted on: Tue Nov 29, 2005 1:45 pm
In the VTVL scenario there is no reason to stay with a low separation velocity. That constraint is to allow a reasonably cheap winged, air breathing launch aircraft. You can easily (well, more easily) get a higher velocity separation from a rocket powered first stage.


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Post Re: My 2 bits   Posted on: Tue Nov 29, 2005 4:47 pm
WannabeSpaceCadet wrote:
If you want TSTO, why bother with wings at all?
Try an Armadillo style VTVL first stage with separation at 50 km and 1000 m/s.
(...) 6t payload, 61t second stage, 268t first stage, total glw 335t?


The only reason that HTHL is better in the near term is basing and mission flexibility.

Thermodynamically it is cheaper (the 'ISP', such as it is, of air-breathing turbojets is well into the thousands), but due to the development and maintenance costs, it won't be economically cheaper until thousands of such systems have been built.

Still, if the governing costs of spaceflight were aluminum fabrication and kerosene, you would be able to buy a ticket to the moon from your travel agent right now. The trouble with VTVL rockets is not that you can't make them big enough, it's that you can't use them easily enough. Ask Musk. That problem may be social or political or economic or (more likely) some combination of the three, but the solution is not an engineering solution... unless you can build a ship that can fly from any class A runway and doesn't need range clearance to turn on the main power bus.


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