Rocket thrust

If polywell fusion is developed, in what ways will the world change for better or worse? Discuss.

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KitemanSA
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Postby KitemanSA » Thu Dec 03, 2009 8:29 pm

93143 wrote:This is why I want to go airbreathing for most of the ascent. Maybe carry LOX for orbital insertion...
Use a hypersonic sky-hook for orbital insertion. A HSK could use an electro-dynamic tether powered by a polywell to provide the momentum change needed by the SSTO to get from ~M14 to orbit. No reaction mass needed at all (unless you count the Earth!! :o )

93143
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Postby 93143 » Fri Dec 04, 2009 2:30 am

Weak sauce. Why are we talking about fusion-powered spaceships that can't make orbit on their own? That's as bad as dropping pieces...

I want flexibility. That's why I want a full shield rather than a shadow shield. That's why I'm going for the zero-mass-overboard approach that allows for indefinite supersonic loiter (if one can be said to be loitering at 1500 mph...). A tether system means you can only go to a particular set of orbits at a particular set of spots, and your timing has to be exquisite.

And you can forget about taking off from Mars...

KitemanSA
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Postby KitemanSA » Fri Dec 04, 2009 3:06 am

93143 wrote:And you can forget about taking off from Mars...
Why?? Mars is easy. You can do a space elevator there now, almost.

Betruger
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Postby Betruger » Fri Dec 04, 2009 5:19 am

Wouldn't building that Mars SE require infrastructure, itself requiring lots of payload deliveries, themselves favoring a workhorse like 93143 is thinking of?

And why pass up on an opportunity to build huge stonking fusion powered self-orbiting spaceships?

kunkmiester
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Postby kunkmiester » Fri Dec 04, 2009 7:24 pm

You have to consider the economies of scale here. When you're traveling a couple million miles, a bigger ship will work better than a smaller one. I don't see a three or four man Mars mission. I see a ship with at least a few dozen, with half a dozen or more heading down at some point or another. Infrastructure wouldn't be too terribly difficult--drop a few robots to clear, pack, and mark a runway suitable for your machine. If you really need to, you can bring them back up and drop them somewhere else to make a new strip.

It would be somewhat like Star Wars. You'd have smaller ships like the Millennium Falcon that can not only travel between planets/objects, but also land on them. Most effective ones though will be like the Star Destroyers--big massive machines carrying massive amounts of stuff, and taking it down on smaller shuttles.

Skylon or something would take people/stuff up to either build a ship or meet one. Transfer over, then go to Mars. Switch again to a "Skylon" made for Martian atmosphere and conditions, and head down.

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DeltaV
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Postby DeltaV » Fri Dec 04, 2009 8:10 pm

93143 wrote:Weak sauce. Why are we talking about fusion-powered spaceships that can't make orbit on their own? That's as bad as dropping pieces...

I want flexibility. That's why I want a full shield rather than a shadow shield. That's why I'm going for the zero-mass-overboard approach that allows for indefinite supersonic loiter (if one can be said to be loitering at 1500 mph...). A tether system means you can only go to a particular set of orbits at a particular set of spots, and your timing has to be exquisite.

Agreed. Why bother with fusion if you can't go where you want, when you want? As long as you're licensed and follow reasonable regulations, like general aviation pilots do, the government should have no problem with this. US transcontinental railways were built after the pioneers roamed all the way to the west coast on horses (some pulling wagons, but still free to go wherever physics allowed). Tethers, space elevators, launch loops, beam-powered propulsion, etc. can be considered later, but it would be unAmerican to not develop independent vehicles first. Serious hobbyists/enthusiasts (e.g., Wright brothers) played a huge role in airplane development, and I hope the same will be true at least for reusable earth-to-orbit vehicles (incidentally, Rutan/Scaled Composites has roots in the "hobby" realm).

kunkmiester
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Postby kunkmiester » Fri Dec 04, 2009 8:20 pm

big problem is that the Wright flyer needed a massively smaller amount of capital to build.

Considering the shield, do you really need a 360 * 360 shield? Take a VTOL. No one's going to be standing underneath, so do you really need to shield the ground? Block it downward, and shadow shield the crew. Would that be any lighter than a full shield?

HTOL is a bit easier provided the glide path is clear--just a cylinder to shield the craft and the surroundings, using distance above and below. Reasonable?
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93143
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Postby 93143 » Fri Dec 04, 2009 9:51 pm

Yeah, it's probably reasonable.

Two things, though:

First, I'm not really comfortable with having parts of the vehicle that you can't point at inhabited areas/other vehicles. Fly one of these under a cloud and you could give a Cessna pilot who wasn't paying attention to his radio a severe case of radiation sickness without even knowing it. Several seconds of exposure at 500 m from an unshielded 6 GW core could exceed the yearly maximum dose for a civilian (mostly due to X-rays)...

Second, you don't really save all that much mass until you do go to straight-up shadow-shielding, which makes the no-step area quite large indeed. If the mass saved by not shielding above and below the reactor is important enough to make or break the design, then IMO the design is insufficiently robust.

Note that I've been fiddling around with conceptual variables and radiator pumping schemes rather than running complete sets of design numbers, so take the above opinion with a grain of salt...



Regarding minimal-mass shadow-shielding of the crew compartment, or the even lower-mass chairs-and-computers-only approach, it's still a problem because it instantly means everything else on the ship needs to be rad-hard, payload included...

Stoney3K
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Postby Stoney3K » Sat Dec 05, 2009 2:25 am

kunkmiester wrote:It would be somewhat like Star Wars. You'd have smaller ships like the Millennium Falcon that can not only travel between planets/objects, but also land on them. Most effective ones though will be like the Star Destroyers--big massive machines carrying massive amounts of stuff, and taking it down on smaller shuttles.


Or Battlestar Galactica. Or Firefly. Or Freelancer. Or any other decently thought of space-ship piece of fantasy that actually bothers to adhere to Newtonian physics.

Consider the Freelancer case, for example, someone's been doing their homework on that game. Space-ship engines have basic components that require Superconductors, and the main fuel is Boron and Hydrogen. Smaller craft will land or ascend to orbit through an orbital elevator system, and capital ships cannot descend to orbit.

Incidentally, they threw in FTL travel by using a system called 'Trade Lanes', which propel the ships in-system and a Stargate-like system to travel between star systems. Ships themselves aren't FTL capable.

Firefly is an interesting one too, as it borrows quite a bit from current aerospace technology. An interplanetary craft powered by a small Polywell could look quite like Serenity, which uses 'conventional' turbofan engines for atmospheric flight and only uses the big engine for interplanetary travel.
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