New Mars Forums

Good for you! 

Ok, this is an interesting idea..... 

Now, as far as I can recall, the largest flight system using ablative nozzles these days are the Space Shuttle SRB's.   (I don't think that has changed, although they tinker so much on that thing who knows what flight hardware is these days.)

You have to use ablative nozzles with most solid rockets, actually, since there is no liquid fuel to pump for regenerative cooling.

So, your idea seems like a good one, on the surface.  However, in the SRB the only part of the nozzle that is truly ablative is the throat.  The throat erodes quite a bit in each flight, but the mouth of the nozzle hardly erodes at all, if any.

Since the expansion ratio of a nozzle is mouth area over throat area, in practice the SRB is actually LOWERING its expansion during flight.  You see, the throat gets larger while the mouth does not.

An example:  The throat is 10 square feet in area at launch, while the mouth is 300 square feet in area.  At launch, the expansion ratio is 30 to 1.  At the end of the flight, the throat has eroded out to 12 square feet, while the mouth is still 300 square feet, an expansion ratio of 25 to 1.

To make your idea work, you'd have to coat the inside of the nozzle with some carefully weakened ablative material, so it would erode away smoothly and perfectly during flight.  Plus, you'd have to ensure that the mouth, where stresses are least, erodes more than the throat, where stresses are greatest.  This could be challenging, for only a minor gain in efficiency.

The way most rocket designers address this issue is by staging the vehicle.  Build nozzles optimized for low altitudes, run them for a couple of minutes tops, then dump them off and light up engines optimized for high altitudes.  With chemical fuels you have to stage anyway, might as well kill two birds with one stone. 

Plus, aerospike nozzles are automatically pressure correcting, without having to fool with fancy ablative coatings.  That's why the X-33 was going to use that kind of nozzle.

So, to sum up, yes it's a good idea, unfortunately there are simpler and easier ways to get the same performance. 

Hope that answers the question! 

Josh Cryer:

Wellll....  and topsoil.   Care to speculate on the amount of topsoil we'd lose trying to provide the energy needs of the world with crops of hemp and bamboo?

Personally, I'd rather use that topsoil for more important things, like FOOD. 

Call me crazy. 

Oh, but I suppose we could just raise biomass hydroponically, right?  How big would that greenhouse have to be....

Josh Cryer:  The shrieks of the greenies, combined with idiotic "counter culture" rebelliousness and the "anything for ratings" ruthlessness of the liberal media in the 70's combined to stop nuclear power dead in it's tracks.

Fortunately, California just got a nice taste of the fruits of those labors, and the simple facts of nuclear power's superiority for our needs are still there as the hysteria fades.

Yes, the current crop of nuclear reactors are horrible dinosaurs, and they are STILL the safest, cleanest, and most secure form of power we have, due to the 4 orders of magnitude energy density advantage nuclear has over fossil fuels.

We could build new plants hundreds of times safer than the current crop. such as the Westinghouse AP-600 and 1000.   

http://www.ap600.westinghouse.com/

Show that info to your "green" friends and see what they say.

While a space elevator is a wonderful idea, consider this:

Even if it becomes feasible to build one in the next few years, what are we going to do, build it from the bottom up?

A heavy lift rocket is needed to build a space elevator in the first place.   

Just a quick comment or three:

The current crop of nuclear power reactors are dinosaurs. They were designed using slide rules and chalk boards, for pete's sake.  We can build MUCH MUCH better plants today, if we'd just get over our irrational fears.

For example, Westinghouse has a fully licensed and ready to build modular PWR using extensive passive safety systems called the http://www.ap600.westinghouse.com/] AP-600  ready to go right now.

They have a larger and more efficient version of the same design called the AP-1000 which should be ready any time now.

These are designs which start their design life 100 times safer than the current crop of plants which have had 40 years of impovements added to them.

And these PWR designs are STILL fairly primitive compared to far more advanced designs which are on the horizon.

I like the lead-bismuth cooled "Energy Amplifier" Accelerator Driven system fueled with Thorium.   Able to safely generate vast amounts of power with huge burnup figures while simultaneously disposing of high level wastes.  Hard to argue with it, actually.

Thorium is three times as common as Uranium, and Thorium out of the ground has 100 times the available energy of Uranium out of the ground, when it is burned in an EA. 

Plus, an EA can be easily run as high temperatures due to the lead/bismuth coolant.  Those high temperatures allow the EA to provide the heat needed to run a sulfur/iodine hydrogen cracking cycle.  For all you folks advocating hydrogen fuel cells, this is a non-hydrocarbon source for vast quantities of the stuff.  Plus, the hydrogen making process runs at a hot enough temperature that the "waste heat" from making the hydrogen is actually about the right temperature to run a conventional steam turbine to make electricity.

For a really complete utilization cycle, you can then use the waste heat from the steam stubines to boil sea water.  This makes fresh water.  You use a portion of the fresh water for feedstock to make hydrogen out of, and you use the rest for irrication or as drinking water.

So, by using new nuclear technology, we can:

A) Switch to a much larger supply of fissile fuel, in Thorium.

B) Have a much safer power source in the Energy Amplifier.

C) Provide high temperatures to create hydrogen, so we stop burning gasoline in our SUV's.

D) Make huge amounts of electricity, which the world is already hungry for.

E) Make huge amounts of freash water for crops and drinking.

F) Do all this while getting rid of 99 percent of our current nuclear waste.

Sounds too good to be true, doesn't it? 

Well, it's all based on solid science and all of these projects are being worked on my teams of scientists right now.  If you don't believe me, I will be happy to post links. 

Nuclear is Good! 

I will second that request for info on photon-generation of antimatter. 

If we could use a more energy efficient process to produce anti-protons, that would make ACMF and other anti-matter using designs much closer to reality.

Oh, and even positrons in bulk would be a very useful thing, I have seen designs for positron-fueled SSTO spaceplanes powered by positron-generated gamma rays in tungsten ramjets and using air as reaction mass. 

Anything that makes volume and inexpensive antimatter is a positive development for space travel. 

Why, thank you Preston!  That saved me the trouble of digging that stuff out. 

This is correct!  Using a 50/50 ratio, He3-D has one fortieth the high energy (14Mev) neutron flux of DD.  Most proposals I have seen for running He3-D aneutronic has consisted of running the plasma very helium rich, so the aneutronic reaction has the best chance of happening.  If there are 100 heliums for every deuterium, it stands to reason the DD reaction will be suppressed.  I will see if I can't find the article I read that stated the 1000 fold drop....

This reaction is the fly in the ointment for both DD and D-He3.  That tritium snaps up a D very easily and BANG, you have one of those hot neutrons to deal with.

Actually, I think the best material to shield neutrons, hands down, is liquid hydrogen, of all things.  The fast neutron Mean Free Path in liquid hydrogen is only about 60 centimeters, and every collision saps an average of 63 percent of the neutrons energy. 

LH2 is very light, so carrying a few meters of it as a shield is not a huge hardship.  Put a healthy dose of boron in it to mop up the thermal neutrons after they're moderated and voila, you have a very effective and pretty lightweight neutron shield.  You would place it as far from your reactor as you can get, and let the majority of the neutrons escape into free space, so that the thermal load on the LH2 doesn't make it boil too fast.  In other words, the side of your crew section that faces the reactor has a ten meter thick borated liquid hydrogen tank on it, and the rest of the ship just lets the neutrons zip away into space.

The escaped neutrons decay pretty fast into a proton and an electron, and render themselves harmless, so shielding against them is not needed. 

Soph: Fusion as currently being researched is not any better (cleaner) than a modern PWR design, and is actually WORSE than an Accelerator Driven system like the Energy Amplifier designed at CERN.

See, the easiest fusion reaction to ignite is Deuterium - Tritium.  It is also VERY energetic.  Unfortunately, the huge majority of that energy is represented by neutrons emitted at the blazing high energy of 14Mev.  (To compare, the energy of neutrons emitted by Uranium fission have energies of 2 Mev.)

So, fusion as it is currently being developed gives off most of its energy in super hot neutrons.

The problem is that converting those neutrons to heat is TOUGH, and neutron bombardment is going to be very, very high.  Huge absorbers will be needed to soak up those neutrons, and inevitably, they will become riddled, embrittled, and activated.  Nasty.

Add in the fact that tritium doesn't exist in nature and has to be bred from lithium jackets, AND that tritium is very radioactive AND is very easily absorbed by humans, and you have a very unpleasant combination.

Tritium is VERY nasty stuff, folks.  Think of it as gaseous plutonium.  Sad, but true.

Basically, we should build new PWR's as fast as we can to phase out coal, then build Energy Amplifiers once we get them perfected.  We should not start using fusion until we mature it to the point that Deuterium - Helium3 is possible, then we should mine He3 off the Moon.

If we ever run the Moon out of He3, we just mine it from Uranus.  We should have enough energy to last us literally MILLIONS of years, at consumption levels FAR higher than we have now.

It's just a matter of getting over this irrational fear of the "Ebil Nooklure!" we have been brainwashed into. 

Nuclear is natural.

Oh, you are thinking about a sub-orbital system, like the "Orient Express" ideas that got kicked around in the late 80's.

<shrug>  While that is a worthwhile field of development, it does nothing at all to get us (as a race) access to the vast resources of the Solar System.

I like my standard of living, and I see no reason why a peasant in Bangladesh should not have a decent shot of attaining one just as good.  Scarcity of resources is the true root of many of our problems, getting off Earth is the solution.

It seems simple to me, I just don't understand why it gets so much resistance.

soph, preston:

I am curious as to what sort of a spaceplane you would want?  What cargo capacity, fuel, etc?

I feel a SSTO space plane is quite doable, IF you are willing to make it nuclear powered/assisted.  If you are not willing to use nuclear power, then it can't be done with any useful mass fraction.

For nuclear powered designs, I have seen types using a NERVA style core to simply power your way to orbit using air as reaction mass, switching over to water/hydrogen as you get above the air.

I have seen pure antimatter powered SSTO planes as well.  (These are FIERCE!)

I have seen a nuclear assisted design, where hydrogen is heated to 2500 C in a NERVA-style core and the exhaust is then combusted with external air at low heights, switching to pure rocket operations at high levels.

The real problem with planes is the runways and landing-gear design.  Plus, wings are mainly dead weight on the trip up to orbit, and lifting bodies tend to have a HIGH landing speed.

This means that spaceplanes don't scale in size very well. 

And size counts. 

tjohn: LOL, A picture is worth a thousand words, isn't it?  A fellow who goes by "nukepulse" at the nuclearspace forums made the pretty cgi's and they are awesome, indeed.

soph: In the giant thread over at the nuclearspace forums this same question was brought up, and I basically shot the idea down.  Sorry.

You see, the empty mass of this launcher is a whopping 800 tons, no runway on Earth could hold the thing up.  See, without wings, and with that heavy a mass, it would have to come in for a landing at a HIGH speed, to avoid stalling out.  High speed plus huge landing mass = wrecked runway. 

The Space Shuttle is tiny compared to this, and it has the highest landing speed of any vehicle on the planet.  (I think that's right, the old X-15 had a really high landing speed, too.)

Now, it is possible to try and build it as a flying boat, but again, the speed of touchdown is going to make that problematic.

Instead, I simply put on enough fuel to give the loaded Liberty Ship a total deltaV at takeoff of 15 km/sec.  The flight profile I have assumed is to use 10km/sec getting to orbit and dumping the ascent fuel into deep space or the Sun.  For the flight back down, 6 or 7 km/sec of velocity is scrubbed using a nice gentle powered aerobraking manuever, and the last 2 or 3 km/sec is supplied by the thrusters as it flies down, tail-first, to land on the launch ship.  Please note, that flight profile assumes it has 2,000,000 pounds of cargo going up, AND 2,000,000 pounds of cargo coming back!

If you are willing to run the safety margins during ascent a little thinner, the Liberty Ship could easily place 3,000,000 pounds or more into orbit, and simply fly back empty. 

"Impressive, young Skywalker!"

For much more info, go read the thread at the nuclearspace forums.  When I have  a little time, I plan to neaten up all this info and present it as an article over there, and even if they turn me down, I'll stick it on my ghetto web page. 

Thank you for the encouragement, Shaun, and I hope that folks with strong backgrounds in this sort of stuff do indeed go and take a look at it. 

I am far from brilliant, and my math skills frankly suck.  About the only thing I have going for me is a total lack of comprehension of how impossible this thing is, and a facility with finding oddball solutions to problems.

In other words, BF&MI will carry the day!   

So, if there are any scientists, physicists, smart math folks, etc who want to kick holes in my reasoning, please, hit the link above and start poking holes in the design. 

Also, a fellow over at www.nuclearspace.com was nice enough to make a cgi of the Liberty Ship, which I have posted at a ghetto webpage.  If you want to look at some pictures, try http://www.angelfire.com/space/nuclearmauk2/

Whee!