First thanks for the article on the TRITON! Enjoyed it quite a bit. So I have to ask.
I have always played with the thought experiment of attempting to design a SSTO with nuclear technology (note not just for propulsion).
The Liberty ship was cool, but it was missing some items - but this TRITON makes we wonder about if a nuclear SSTO could be built.
Now - I would love to think I knew alot. But after reviewing many threads here - I'm just at the beginning in understanding how to engineer with this stuff.
I use to write real-time os for flying aircraft - that and a good juggling act might get you a job. Anyway, I always love working with systems and modeling such.
So the questions I throw out is could a nuclear SSTO be built? What would a it look like with a TRITON system?
The great thing about thought experiments and models - the paper work is alot less.
Some other items I would like to see in a nuclear SSTO. Very gross functions but what the hec.
1. Be able to fly to the moon or geo-sync and back - hey lets start simple.. 2. Be able to crew 4-6 people 3. Handle a Solar storm - could we use the TRITON to power some type of shielding 4. Power/Life-Support/other 5. Cargo hold 7. Ship Repair 8. [add others]
So - I'm heading to look up some reference books to figure out a first guess.
The big problem with any launch system is that any launcher has to have a thrust/mass ratio substabtially greater than 1 otherwise it will just sit on the launch pad looking like an overgrown blowtorch!
During launch, a rockets engines have to support its dead weight against gravity as well as build up speed
once in space this is not a problem! once in orbit DeltaV is DeltaV and low thrust electric engines such as ion engines can deliver it just as effectivly (and with much better propellent efeciency ie ISP) as high thrust chemical engines
NTR's fall between chemical and electric engines, they have better ISP but tend to be relativly heavy and power limitations mean that the thrust to mass ratio tends to be poor, Too poor for a launch. Liberty Ship was an exiting thought experiment but ISTR that elsewhere (cant find it now) I concluded that a "practical" GCNR engine would have a performance limit of only around 20,000Lb thrust at an ISP of arround 2000 Verry usefull in space but completly useless for launches.
I think that with current technology a two stage chemical booster would be used to put an NTR powererd ship into LEO from which point it would begin the next leg of its journey.
The only high thrust, High ISP nuclear launch system even potentially available to us for the time being is Orion!
Of course the other problem with any nuclear fission engines, including TRITON, is radiation shielding.
In space, this can be done with little mass penalty, because "shadow shielding" works very well in a vacuum environment.
On the ground or in a dense atmosphere shadow shielding DOESN'T work, because of reflection of radiation from the surroundings. This is called "sky shine." It means that your crew needs to be protected from radiation on all sides of their compartment, with great mass penalty. If the number of crew and their compartment size are small (tolerable for short-duration trips, such as SSTO), then the shielding may perhaps be of tolerable mass as well. But its not practical for passenger service. Anyhow, I seriously doubt that nuke-powered SSTOs would be allowed to fly on this planet. Maybe on Mars -- like Zubrin's global hopper.
A possible nuclear SSTO would be one which leaves the heavy reactor and shielding on the ground and gets its energy from it by way of a large laser beam, heating a hydrogen heat exchanger on its belly as it flies overhead on its way to LEO. The initial launch could be by maglev (also powered by the ground-based nuke plant), at a high-altitude mountain installation. This could certainly be done, without any international legalistic controversy.
quote: This could certainly be done, without any international legalistic controversy."
Appart from the 500Gw laser and hypersonic rail gun that is!!!
on a more serious note, last night I was thinking about the Maglev/rocket sled approach to gaining speed from a ground based power sourse and I was wondering how long the track would have to be to gain orbital velocity assuming reasonable g forces (say 6 g max and 6 g would be a bit of a rough ride for the crew!)
the awnser I got (this ignores air resistance BTW) were, for 6g, to achieve 9Km/S would take nearly 700Km of track! the acceleration run would take 150 seconds.
Of course in practace it would be dificult to get much more than 500M/S from a ground based system low in the atmosphere because the air resistance would be too great, it would help, but only a little bit! It would also only take 2Km of track That IS do-able!
the awnser I got (this ignores air resistance BTW) were, for 6g, to achieve 9Km/S would take nearly 700Km of track! the acceleration run would take 150 seconds [...] It would also only take 2Km of track That IS do-able!
Right before you turn into a crispy critter, that is. You realize that you would accelerate the craft to > Mach 25 in the lower atmosphere? I don't think we have the technology to build somethat that could withstand that sort of stress.
To the original poster, the Triton may be useful for SSTO if it has a sufficient Thrust to Weight ratio. For a comparison of nuclear engines to chemical engines, the NERVA NRX had a 1:1 thrust to weight ratio, while the Space Shuttle Main Engines have a 57:1 thrust to weight ratio. Yet to carry all that extra weight into space (~100 metric tons!), the space shuttle needs two solid rocket boosters for getting off the ground and into the air.
Basically, a low thrust to weight rocket may be useful for something light like human passengers and small satellites, but would require additional stages for massive loadouts like the space shuttle.
One other thing: A SSTO to the moon is not yet possible without a technology like Orion or NSWR (both of which pollute heavily). The Isp of the Triton is ~900. That makes it twice as fuel efficient as the SSMEs (~450). Twice the fuel efficiency is good, but would be offset by the extra fuel weight.
Dusty is quite right about 0.5 km/sec being doable (although its NOT hypersonic) -- and it certainly WON'T turn you into a crispy critter. It will get you off the ground though, and at a decent angle for subsequent acceleration to LEO. The ~900sec Isp cited for TRITON is pretty much what one can also expect from an optimised laser-heated heat exchanger driving a hydrogen engine (after all, a solid-core nuke reactor is nothing more than a glorified heat exchanger - albeit one with its own internal source of energy....). As for power-to-weight ratio, I doubt you can beat a laser-heated HX -- without any heavy reactors or chemical-fueled heat source, shielding, pumps, etc., its about as light as you can get. I don't know what the required laser power would be (that depends to a large extent on the size of the SSTO), but I doubt it would be anywhere near as high as the 500 GW cited by Dusty. There is probably some optimum mix of maglev launch velocity and free-flight powered acceleration -- so the 0.5 km/sec number may not be the right one. But as Dusty says, you would not want to have much more than 2 km of track.
In the more distant future, it may be possible to launch people at far higher accelerations than 6g -- using the technique of replacing gaseous respiration with liquid, thus allowing complete submergence of the body in liquid and avoiding harmful compression effects. At present the fluid-breathing technique is only used in medical procedures for saving premature infants, but it may eventually be made practical for adult humans as well, for such applications as high-acceleration launch or extreme-depth ocean diving....
Dusty is quite right about 0.5 km/sec being doable (although its NOT hypersonic)
Quite right. My bad. I saw his numbers for attaining 9 km/s and completely missed his 0.5 km/s correction. I was wondering why he suggested something he knew would result in a crunchy style platter.
In the more distant future, it may be possible to launch people at far higher accelerations than 6g -- using the technique of replacing gaseous respiration with liquid, thus allowing complete submergence of the body in liquid and avoiding harmful compression effects.
I'm not sure that would actually help the situation. Yes, you want to get up to speed as fast as possible, but it seems that more powerful engines are the most logical solution. After all, existing launch solutions can only generate so much thrust and can only withstand so much atmoshperic stress. By the time we get to the space stage, our engines are barely even pulling one G.
At the very least, we should be looking into making better use of the chemical infrastructure we have now. (Here's hoping for a successful Falcon V!)
It is time to go brush up some of the basics. But at 57:1 ratio for the shuttle engine started the memories sparking.
Given the political climate of nuclear - I don't foresee alot of money pored into research to build equilvalent nuclear engines.
The TRITON article has me hopeful, especially in light of the SpaceShipOne.
So the answer on nuclear spacecraft today is assembly them in orbit.
How about Antimatter type of system - not Anti-Hydrogen but one base on Positrons?
I once read an article by Dr. Robert Forward that it was possible and so I did a search on google - 'positron propulsion'
Was lost for the last 20 minutes in various threads. Short is that its still being considered.
Here is a link for a SSRV using a positron propulsion system. It's from a Dr. Gerald Smith - he's been a big proponent of antimatter type of engines from my readings.
ps... I grabbed some text below describing the specifics of the system.
2001 Advanced Space Propulsion Workshop Nuclear technology can be used for heat transfer in ramjets and SCRAMjets. In 1954, Project Pluto focused on developing a nuclear ramjet to power supersonic cruise missiles. At Mach 2.8, P = 513 MW and T = 35,000 lb. Project cancelled in 1964 due to environmental concerns (open-air nuclear system). Photos from Lawrence Livermore National Laboratory archives 2001 Advanced Space Propulsion Workshop Positrons--Advantages 0.511 MeV gamma rays are produced from the positronelectron annihilation equation: The gamma ray products have a range of 10 mm in tungsten, 20 cm in light-weight carbon, and 90 m in air. The energy is sub-threshold for all nuclear reactions (antiprotons are not). In other words, the by-products of a positron-based aircraft WILL NOT CONTAMINATE the atmosphere, ground, or the aircraft itself! SOLUTION: Develop an SSRV using positron-heating to replace fuel combustion in turbomachinery. g g + ฎ + - + e e 2001 Advanced Space Propulsion Workshop Mass Comparison *From NGC report to Kaiser-Marquardt for HTHL blended-body SSTO engine, Vision Vehicle Final Report, April 30, 1998. VEHICLE COMPONENT MASS Total Dry Mass 60,500 kg 15% Margin + Unus. Propellants 11,400 kg Payload 11,340 kg (25,000 lb) Burnout Mass 83,200 kg Total Propellant 176,000 kg GLOW 259,000 kg STI Positron-assisted TRJ, R configured SSRV *Assumes no bipropellants used during turbojet and ramjet phases of mission, and the dry mass is approximately the same after reducing LOX/LH tank mass and surrounding structure and increasing ramjet engine size. VEHICLE COMPONENT MASS Structure 25,700 kg Thermal Protection 12,300 kg Propulsion (4 engines) 14,900 kg Electronics 7,600 kg Total Dry Mass 60,500 kg 15% Margin + Unus. Propellants 11,400 kg Payload 11,340 kg (25,000 lb) Burnout Mass 83,200 kg Total Propellant 368,300 kg GLOW 451,600 kg (995,500 lb) How many positrons? Current positron production is 1 pg/sec (30 mg/yr). USAF target is for 1-10 mg/year. MISSION MODE MASS OF e+ Turbojet launch 2.5 mg Ramjet 76.5 mg 10% margin (for turbojet mode upon landing, inclination changes, etc) 8 mg TOTAL 87 mg 1 mg positrons can launch an SSRV with 1 MT dry mass to 300 km. In 10 years, cost of production of 1mg positrons can be $6M .* These costs are comparable to ~$6M/metric ton for existing unmanned launch systems.
What I was saying was that, in a suitable location, a 2-3 Km track could give arround a 500 M/s "boost" to a launch vehicle, (remember "When Worlds Collide") but due to air resistance, any more would probabally not be practical. Interestingly this is arround what was proposed for the Sanger "Amerika Bomber" which was to have been launched from a rocket sled powererd by 50 V2 rocket motors.
Perhaps I have an evil streak, Im kind of sorry that the germans never actually built any of their Amerika Bombers, not because I would have wished America to have been bombed, but because I am curious as to whether they could have actually made them work. I feel much the same about the Iraqi "Supergun" From a purly technical POV it would have been interesting to have seen them squeze off a round or two
(Of course, before anybody else comments, the supergun was an incredably vulnerable weapon, it was therefore a first strike weapon! had it become operational it would almost certainly have immediatly been used aginst israel and probabally iran as well. so it is probnabally just as well that it diddnt)
The 500Gw was taken from the "liberty ship" profile, of course smaller and lighter ships will take less but we would still be in the multiple Gw possibly 100 G pluss range for any sizable rocket with a usefull isp. (its NOT a weapon, its a launch system! Remember the Man Kzin wars?)
As for liquid breathing, I am not sure how much it would help. sitting in a liquid bath may stop your lungs from collapseing under high G and may also stop your body being squashed flat but I dont think it would stop your internal organs being torn free from their mountings and being crushed against your back bone (which is what often happens in motor accedents, people still die of internal injuries despite haveing no obvious external ones)
Yes, there are limits to acceleration loads that the human body could take safely even in a liquid submerged situation. But I recall reading somewhere that continuous acceleration on the order of 50g could be tolerated. That will get you up to speed in a hurry !
Incidentally, there have been quite a few studies of E-M and scram-accelerator orbital launchers for small, cargo-only vehicles, with muzzle velocities in the range of 9 - 11 km/sec (the higher velocity takes into account aerodynamic slowing in the atmospheric segment of the trajectory....). In the case of the E-M version, the launch tube would be completely evacuated, with a valve at the end to keep out the ambient atmosphere until the last possible moment. In the case of the scram-accelerator, the tube is of course filled with a mixture of combustible gases, with frangible diaphragms separating sections with different mixes, optimised for the particular speed range of the projectile along is track. As already alluded to in this thread previously, this sort of launch speed would cause serious heating of the projectile nose, even if the exit port is at a high altitude mountain location. But the studies I have read conclude that an adequate TPS would be feasible without compromising the payload fraction.
The nice thing about the laser-heated hydrogen engine scheme is that it lets you play with the tradeoffs between launcher muzzle velocity (lower being less technologically demanding) and payload fraction, which goes down as the launch velocity decreases and more hydrogen propellant needs to be carried aloft in order to provide the remaining boost needed to get to LEO.
