Alternative Rocketry
 

[ Talk most probably given in 2003 ]

 

   Conventional rocketry, as we are all aware, is the let-go balloon, and  the firework rocket, and a toy  compressed air / water rocket that my brother showed off last weekend.   For those who have not seen one this toy rocket is very simple and effective. It consists of a strong plastic ‘pop’ bottle that you fill a quarter full with water, then pump in air through a tube and nozzle with a bicycle pump. Suddenly, when the pressure builds, the valve opens and we have a very fast lift-off.  Flights of 200 feet up are achievable. No heat, no chemicals. Then, of course, we have the familiar liquid fuelled space rocket.

     All rely upon Newton’s 3rd Law - Action gives an equal and opposite reaction.

     The basic difficulty comes down to the amount of energy that can be packed into any rocket. This is the specific impulse of a fuel. Mathematically , the specific impulse (in seconds) equals the exhaust velocity divided by the gravity acceleration. The laws of physics dictate that if this factor is low then the fuel consumption is very high.

     The laws of economics dictate that a reliable engine burning huge amounts of low energy fuel to give decent thrust will be extremely expensive to run. Finally the laws of politics dictate that any extremely expensive project will be dominated by political and military concerns rather than scientific ones.   

    Chemical rockets are now an established technology. Conventional rockets can be ‘given a lift’ by various substitutes for a primary stage ; launching from a platform held aloft by a balloon, or by using an aircraft to get it up the first (and hardest) few miles, like the X15 of the 1950’s. There is also the possibility of launching a rocket from a superfast ‘train’ or magnetic levitation to give it an energy boost without using fuel until it leaves the launch vehicle.                         

     For decades the US military have experimented with rocket sleds on rails to test all sorts of hardware, and have constantly improved upon the sled, which never leaves the ground. The latest has achieved an acceleration of 0 to 6,500 mph in 6 seconds, covering three miles in that time, then effectively decelerated even faster to zero by means of 30 feet of concrete. Only dummies take this ride ! Air resistance at this speed would melt steel so this sled runs in a thin plastic bag tunnel of helium, which presents far less resistance and less heating. By using a tunnel filled with helium going up a high mountain a normal rocket on a sled could get a great boost until it took over under its own power at the top where the air is thinner.

     Less exciting, but already established, launches near the equator take advantage of the Earth's rotation and centrifugal force, reducing the escape energy by about 0.3 %.  It all helps

     A while ago I did a talk on the Orion Project, an American top secret vehicle, originally conceived for peaceful purposes was shaped like an enormous military shell, which could carry cargo into space and which would be propelled by small atomic bomb blasts. To continue with funding the project was taken over by the military to whom the primary purpose of the vast spacecraft was to ferry troops and military supplies. Starting in the late 1950’s only small conventionally powered prototypes were ever built. The whole thing was abandoned in 1963, partly because of radiation hazards, but mainly because the Nuclear Test Ban Treaty came into force.  

   The essential advantage would have been the huge amount of energy available using nuclear as opposed to chemical power, though controlling it would have been challenging.

 

     Taking the nuclear powered rocket a stage further is Zubrin’s Nuclear Salt Water Rocket (NSWR). Like Orion, its impact on the environment means that it would not be practical for a ground launch but it could be appropriate in deep space missions. The salt is not common salt but a water soluble salt derived from 20% enriched uranium (U Br4), with 3 to 5 % dissolved in water. Fissionable isotopes in such concentration can produce great heat from fission reactions, or even a nuclear explosion. An uninterrupted mass of a few dozen kilograms of this liquid would reach critical mass and explode.

     In Zubrin’s design a huge storage tank for this liquid would be made from long tubes of boron carbonate, which is not only a strong material but readily absorbs neutrons, damping down a chain reaction. To power the rocket this liquid is pumped from the tank to an absorber free ‘reaction chamber’ where the neutron flux build-up reaches a point where continuous nuclear fusion can occur, the critical mass having been reached.

    Of course, no conceivable structure or material for the reaction chamber is currently possible. It would have to contain the sustained nuclear blast and heat, but erosion could be reduced without sacrificing efficiency.

    The concentration of fission induced neutrons in the chamber depends crucially upon the speed of the fuel as it goes through the reaction chamber. This means that once the engine is started the hottest areas will be in the middle and just outside the exit of the chamber, in both cases away from the structure. The structure could be cooled by pure water and also by the constant arrival of ‘cold’ new fuel into the sides of the reaction chamber. Not only the nuclear fission products but the water the salt is dissolved in will flash to very high temperature steam / plasma.

     The exhaust speed of this engine has been given as 66 kms per second ( compared to the 4.5 kms / second for the best chemical rockets ) and could deliver 3 million pounds of thrust. A one way trip to Saturn at 4g acceleration would take but a matter of days.

     Zubrin envisioned a modified NSWR one way trip to Alpha Centauri using a 300 ton spacecraft with a 3000 tank of  90% enriched U Br4 fuel dissolved in water. A high efficiency engine ( if ever built ) would have an exhaust velocity of 4,700 kms per second, building up to a vehicle speed of 3.6%  the speed of light. Mission time, 120 years.

     The potential for contaminating space will no doubt draw opposition, but Zubrin considers the speed of the exhaust is so high it will be far in excess of the escape velocity of the Sun or any planet, so it will quickly dissipate and leave the solar system altogether. Also, since the NSWR would not be a weapon or a ‘bomb’, its testing and use, according to Zubrin, would not violate The Test Ban Treaty. However anti-nuclear groups still oppose any kind of nuclear power source in space.

 

     Ion Drive. Solar electric propulsion. Solar electric is from the usual solar cell array, but this electricity drives the Ion Propulsion System  ( I.P.S ).

     In contrast to Orion and NSWR, the IPS is non nuclear, of very low power but long endurance, and is already in use. Deep Space 1, launched by a chemical rocket in October 1998, passed an asteroid in 1999, and did a fly-by of comet Borelli in September 2001, revealing a hot dry tarry surface typical of an extinct comet.  

  Yet Deep Space 1 had another purpose, to prove the viability of IPS with solar electric power. In essence the ion system uses a hollow cathode to produce electrons which collide with atoms of xenon gas, ionizing it. .In Deep Space 1 this ionized gas was then accelerated through a potential of 1280 volts (in Deep Space 1) and  emitted through the thruster via a pair of molybdenum grids. The solar arrays supplied 2.5 kw., of which 2.3 kw. was available by way of the power processor which powered the drive, giving a thrust of 92mN.

    Now, 92 milli Newtons is not much. A Newton is that force which, when applied to a mass of 1 kilogram, will accelerate it to 1 metre / second squared. Note the word “mass” as opposed to “weight”.

    Imagine a 1 kilogram weight, hanging at rest by a very light thread from a very high ceiling. If you now push this weight sideways, so that in 1 second it has moved 1 metre, that force is 1 Newton.  The same weight, held in the hand against gravity, needs a push of 10 Newtons to keep it up. By the same calculation 92 milli Newtons exerts as much force on this spacecraft as (approximately) 9.2 grams, or one third of an ounce. This is the ion drive thrust, and it keeps going so long as there is solar power. This weakens with distance from the Sun, but so, also, does the Sun’s gravity.

    The actual acceleration of Deep Space 1 also depends upon its mass (not quoted) but the report said that 1880 hours (75 days) of this thrust raised the speed by 700 metres per second. Also tested were the solar concentration arrays,, autonomous navigation, a miniature camera, and low power electronics. Of course, the craft had to be cleared of Earth’s gravity by a conventional chemical rocket.

     Progress may improve the performance in two ways. Firstly, to the ion drive itself. Secondly, increasing the size and efficiency of the solar array while keeping its mass down. Great strength is not needed in space if accelerations are gentle.

    Unfortunately, according to “Sky and Telescope”, July, 2003, NASA’s next comet mission using ion drive is in trouble. Named Deep Impact, and due to launch in January,2004, it has been postponed until December, 2004. The ion propulsion system is plagued by contamination.

    It is intended to go to comet 9 p, Temple 1, and on arrival to fire a 375 kg copper bullet at 10 km per second into the comet and to analyse the crater. So far it has made a deep impact into somebody’s wallet and been close to cancellation.

 

    Another challenge is the “Lightsail”, in which a huge area of space is covered with a lightweight but rigid fabric or material. In its ideal form this does not involve solar cells - it just uses the very weak but consistent push of light from the Sun for movement. There might even be a possibility of steering it.

    At 1 AU (93 million miles) the solar light pressure exerts a force of 9 Newtons ( ie. the equivalent of 0.9 kg or 2 lbs. held in the hand ) per square kilometre.

    Beside light there is solar wind, but this is weaker and more restricted, and in any case would be restricted to outward journeys within the solar system..

 

    Laser Propulsion. One version is similar to the Lightsail, but the space craft has a smaller ‘sail’, and instead of relying upon sunlight an enormous Earth based laser is aimed at the sail of the craft, which would be millions of miles away. Thus it rides a laser beam. The beam would have to be very narrow, even by laser standards, and the aim absolutely ‘spot on’. I do not see how such a vehicle could return to Earth.

    Another type of laser propulsion again calls for a massive Earthbound laser, output 10 megawatts to 1 gigawatt. This is again directed at the spacecraft which, instead of a sail, has a solid, thick base plate, rather like that in the Orion project. But this one is a non nuclear ‘fuel block’. As the laser beam hits the plate a thin layer of the plate vapourises, ‘plasmaises’, and the resulting blast drives the spacecraft forward. Once again,  one cannot see how it could return, but we are talking about far into the future.

 

    Equally far ahead is project ‘Daedalus’, the name chosen by the British Interplanetary Society’s starship study. It met in 1973 to establish interest in designs for an inter-stellar mission to Centauri, or Bernard’s Star. A simple stellar ‘fly-by’, after 40 years of constant, gentle acceleration, would take about 70 hours, during which all measurements would have to be made. The undecelerated speed of the probe would by then be about 15% of light speed.

    There were several ideas for engines. ‘Controlled fusion’, nuclear supplied electric for ion drive, and NERVA (Nuclear Engine for Rocket Vehicle Application), which heated water to high exhaust steam, were all too heavy. A ‘proton rocket not only had to give 3 million kilowatts for every kilogram of the vehicle, but once produced the protons had to be reflected away from the ship using super efficient mirrors, absorbing under a millionth of the incident energy. Only an ‘electron gas’ mirror might do that, and this raised many eyebrows, even at the I.P.S. Eventually they settled on a ‘pulse’ (Orion type nuclear bomb) propulsion, with refinements like laser ignition, and a cusp shaped magnetic explosion area. The launch was to be in the late 1990’s.

 

   In  January 2004 one could obtain from (http://www.) “Space Daily” the concept of a ‘space elevator’ - utterly demanding, it is nevertheless just about technically feasible. A large satellite is installed in precise geostationary orbit some 41,000 kms. above a specific spot on Earth. On this chosen equatorial position a huge tower is built, largely solid except for a shaft and service access, and with today’s methods a tower 20km high is possible.

   The top of the tower is then equipped with a base station, from which a cable stretches all the 41,000 kms to the satellite, an immense challenge ! Although the cable is weightless where it joins the satellite in geocentric orbit, the nearer it is to Earth gravity will exert a pull on it. this is because centrifugal force created by its once a day orbit over the same spot will not match gravity at any level below the satellite. So the cable will need enormous tensile strength if it is to be only a few centimetres thick over the whole distance. Increasing the cable thickness as it ascends is not an answer. From  a minimal 1mm at the bottom a steel cable would have to increase to millions of kms in thickness if it were to support its own weight. Steel is not practical. Kevlar, stronger and lighter would still need to widen to 16 metres at the top, and it would require 2000 million tons of the stuff. Therefore, even with kevlar a space elevator remains science fiction.

    Enthusiasts have suggested exotic things such as crystalline hydrogen fibres, or antimatter, or diamond nanotubes -  tiny hollow cylinders made from sheets of hexagonally arranged carbon atoms. At present they are too expensive, at $500 per gram, and too short to be spun into long fibres. But if possible they might make a practical cable for nanotubes are 30 times as strong as kevlar.

 

    Just suppose we had our tower and cable, there is the matter of reaching the satellite. Due to the vast distance it has to be a system of very low maintenence with minimal wear and tear. Magnetic levitation is the key, with no friction, little air resistance, and a descent can be electromagnetically braked, thus putting power back into the system to be stored for the next ascent.

   There are still hazards like cosmic rays, solar radiation, meteorites, and space junk cutting the cable. If some 40,000 kms of cable crashing to the ground is a concern, extra cables and a sparsely populated area for the tower will help. And the ultimate prize ? It currently costs $22,000 to put 1 kilogram of payload into orbit. The space elevator would cut this to $ 1.50 per kilogram, because it is so much more efficient once built.

   

   Long distance interstellar / intergalactic travel is where the real challenges lie ; where science can flirt with science fiction and fantasy.

   Medium (solar system) and ‘long haul’ (beyond) spaceships can be given artificial gravity by a slow spin to create centrifugal force at the circumference to equal gravity. This would stop the human body deteriorating.

    Speeds approaching that of light, and for serious distances 90% of the speed of light would be woefully inadequate, bring a host of problems. Hitting a rock would vaporise most of a vehicle, and a grain of sand would burn its way through to the other side. Background radiation becomes more intense, and while time slows down for the crew, the mass increases, as does the power needed to accelerate it.  The effect only becomes important when the speed exceeds about 70 - 80 % of light speed, but tends to infinity when light speed is reached.

     Another approach might be to extend mission time by putting most or all the crew into hibernation. This would incur serious research, and brave volunteers, especially if a computer was responsible for keeping you alive and waking you at the end. If this was viable then traditional ion drives and gravity assists could do the rest.

    The Space Ark. A massive ship with an ecological environment and specially chosen crew / passengers who will have children, because the journey extends over several generations. The last generation presumably inherits the instruction manual on how to land the ship at the end of the trip ! With both hibernation and the Space Ark the vehicle will have to be slowed down on arrival by the rocket’s own power, gravity sling shots in reverse or dangerous atmospheric braking.

 

    Bussard’s Ramscoop. A deep space engine that collects interstellar hydrogen ( by a big magnetic field ? ) and uses this as fuel. Complete science fiction. Interstellar space contains approximately 1 hydrogen atom per cubic centimetre. So a scoop 1kilometre square would collect 1 kilogram of hydrogen while the ship travelled 600 million kilometres. For further fiction there are warp drives, where the ship rides on a wave in space time, tractor beams that pull, and transporter beams that move objects and people across space

  

   Finally, are there any “Star Trek” fans here? The “Astronomy Now” 2003 Yearbook has an article on Seth Shostak, who works on the project “Phoenix” radio search for extra-terrestrial intelligence - S.E.T.I. This involves Arecibo, Jodrell Bank, and soon the dedicated Allen telescope array of 350 X 6 metre radio dishes.

   When Seth was a graduate student “Star Trek” was on TV but not yet of cult status. He wrote to Gene Roddenbury ( also living in California ) saying he enjoyed it, but there were lots of elementary scientific errors. For the bus fare he could come over and correct scripts without interfering with the storyline. Roddenbury replied, saying thank you but we already have an expert from the Rand Corporation doing the correcting.

    Seth’s room mate saw this letter and, equally aware of the continuing mistakes, asked :- “Is this not the Rand Corporation that is entrusted to protect us all from a nuclear holocaust ? We’d better build bomb shelters in that case !”.