Project Longshot to Alpha Centauri - part 4

Monday, June 16, 2008

Image:Motion-of-Alpha-Cen.jpg
Where Alpha Centauri will move in the night sky over the next 4000 years (and a great deal closer at the same time)

Now at page 43 out of 74. Barring any weirdness I should be finished transcribing the whole project by Thursday, I think.





3.2 Propulsion

3.2.1 In Solar System

Since it will not be practical (or safe!) to ignite the fusion drive while in earth orbit, another propulsion system will need to be used for the plane change and to escape the Earth and the solar system (see Orbits). Advanced solid rockets will be employed to accomplish these velocity changes. It is assumed that they will be similar in size and fuel to the Space Shuttle's SRB's with twice the specific impulse (near 600 seconds). Based on this assumption, the following parameters for the various upper stages were calculated:

Booster
Fuel/total Length (m)
Diameter (m)
Plane change
352/417 163.7
Escape Earth
605/715 253.7
Escape Sun
252/298
11
3.7

Total mass: 1209/1430

(see appendix for calculations)

Mass (kg)

(page 33)



3.2.2 Interstellar Transit

Developing a Propulsion system capable of meeting the 100-year interstellar travel time is the most difficult part of the mission design. 4.3 light years is an easily misinterpreted distance. It is equivalent to 41,000 terra meters (41E15 km) which would take the space shuttle just over 190,000 years (assuming it had escaped the solar system at the speed of Low Earth Orbit). Although 100 years is a long time, this requirement expects a three-order-of-magnitude leap over current propulsion technology.

3.2.2.1 Choosing the System

After the initial inspection of potential Interstellar Drive candidates, it was decided that chemical fuels would not be able to produce a three order af magnitude over current systems in the near future. Five alternate technologies were compared for their potential as Interstellar Drive candidates: Pulsed Fusion Microexplosions, Laser-pumped Light Sails, Ion Drive, High Temperature Thermal Expansion of Gas, and Matter Anti-matter Annihilation (see Fig. 3.2a for a summary table). After a thorough inspection of each of the five candidates it was decided that only the Pulsed Fusion Microexplosion was adequately capable of carrying out the mission requirements.


Text:
INTERSTELLAR DRIVE TRADE-OFF STUDY
SYSTEM - /sp (1000s) - FEASABILITY
FUSION MICRO-EXPLOSION - 1020 - MEDIUM
LASER-PUMPED LIGHT SAIL - N/A* - LOW
ION DRIVE - 3.5 - 10 - HIGH
THERMAL EXPANSION OF GAS - 39 - HIGH - V.LOW
MATTER/ANTI-MATTER DRIVE - 100 - EXTREMELY LOW
*EXTERNAL DRIVE SOURCE--3.75 TERRAWATT LASER

The Matter Anti-matter Annihilation could potentially be capable of producing the necessary specific impulse of a million seconds, but it was not considered to be feasible to creat a system adequate for storing the anti-matter for 100 years under the limited power constraints of a spacecraft.

The ideal rocket equation was used to determine the potential specific impulse of an extremely high temperature expansion of gas through a nozzle. (See calculations and assumptions in appendix.) Using the critical temperature for sustained Deuterium fusion (3.9E8 degrees Kelvin) a specific impulse of 39000 seconds was calculated (1/300th the specific impulse desired). Since this specific impulse is insufficient for the mission requirements (using an extremely optimistic temperature under ideal conditions), this candidate was dropped.

Advances in technology for an accelerated ion drive (using a magnitec/electric field to fire charged particles out of a nozzle) have brought the specific impulse to 3500 seconds. Although this is a current technology that could be implemented now at relatively low cost, it is felt that the two remaining orders of magnitude will remain out of reach in the near future. Therefore, this candidate was also discarded.

In determining the feasability of a Laser-pumped Light Sail, another method besides comparing specific impulse becomes necessary, since the drive is external. The single impulse required to reach the designated system in 100 years was determined to be 13,500 km/sec. The size of a laser with continuous output, to accelerate the payload to 13,500 km/sec in a year, is 3.75 Terra Watts. Since a micropulsed 1 Terra Watt laser has been developed, it is conceivable (although extremely unlikely) that the necessary laser could be invented within the next 20 years. The low feasability, coupled with the lack of a system for decceleration into the Centauri System, led to the cancellation of this system's candidacy) see appendix for calculations).

3.2.2.2 Pulsed Fusion Microexplosion Drive

The Pulsed Fusion Microexplosion Drive is not a current, but rather an enabling technology. The system concept, modeled after the British Interplanetary Society's project DAEDALUS, is to fire high energy particle beams at small fusionable pellets that will implode and be magnetically channeled out the nozzle (see Figs. 3.2b and 3.2c). The expected specific impulse is 1.02E6 seconds. The specific mass breakdown for separate sections (including fusion chamber, particle beam igniter system, and magnetic nozzle/inductor system) is included in the structures section (2.3.2). Finally, the entire system is expected to gimble a full degree in two axes to enable navigational corrections in three dimensions.


Text:
PULSED FUSION CONCEPT
1. THE PELLET IS LOADED AND FIRED UPON BY SEVERAL HIGH POWER PARTICLE/LASER BEAMS.
2. THE FUSION EXPLOSION IS DIRECTED OUT OF THE EXIT VIA THE MAGNETIC "NOZZLE".
Text:
PULSED FUSION CONCEPT
3. THE HIGH VELOCITY PULSE (~10000 KM/S) INDUCES A CURRENT IN THE COILS THAT SURROUND THE EXIT PORT. THIS ENERGY IS USED TO RE-CHARGE THE PARTICLE/LASER BEAMS AND THE MAGNETIC FIELD.

The type of fuel used in the pellets is of critical importance. Due to the extremes of temperature ad duress inherent in fusion reactions, a magnetic field is required to supplant the casing around the fusion chamber. The problem involved with using such a field (besides the obvious requirement for immense quantities of energy) is that only charged particles will be channeled out the nozzle. Although the extreme temperatures will instantly ionize all of the atoms and molecules, any neutrons produced in thefusion reaction will not be affected by the magnetic field. Instead, they will irradiate the drive and the entire spacecraft over the 100 year transit, and reduce the drive efficiency. Since this is a highly undesirable result, a reaction which produces few to no neutrons is required (see appendix). He3 + H2 yields no neutrons (although realistically some of the deuterium will react with itself producing a limited number of neutrons in each implosion). The problem is not solved, however, since there is not enough He3 on our planet to fuel the spacecraft! Three methods of gaining the necessary He3 have been compared: mining the planet Jupiter; creating He3 through the bombardment of Lithium in nuclear accelerators; and capturing He3 from the Solar Wing. Another possibility is for a further technological breakthrough to enable using higher threshold-energy fusion reactions (higher than H,He) which use more abundant elements in a no-neutron reaction. None of the options seem very reasonable, and each should be explored and further developed to determine the best method for collecting the necessary fusionable material.

The pellet size, in order to obtain the proper mass flow through the nozzle, depends upon the pulse frequency. Smaller pellet size could potentially lower the coil mass as well as the igniter mass, although the higher frequency would complicate fuel injection in a system that must run for 100 years continuously, without repair. The appendix shows the spectrum available between the DAEDALUS pellet size and frequency (since DAEDALUS required a higher mass flow).

After the final upper stage separation, the nuclear reactor will be increased to full power in order to charge the interstellar drive capacitors for initial ignition. The Interstellar drive will then be used for both acceleration and deceleration. The system is to be turned off at the appropriate time (determined through an internal navigational calculation), rotated 180 degrees, and restarted, all while staying on course. The payload contains a 300 kw nuclear power reactor which must be also capable of starting and restarting. The nuclear power will have to be ignited, and rechanneled to repower the slowly draining capacitors of the Interstellar Driver igniter system, after the spacecraft has fully rotated and stabilized in the proper alignment.

3.2.2.3 Feasability

The entire Interstellar Drive is highly dependent upon enabling technology. Building an actual scale model that is capable of running continuously for 100 years will be a challenge by itself! Barring further significant technological breakthroughs, the collection of fuel will be the most difficult and time consuming portion of the building. Never the less, within 20 years, these projects should be possible with the proper funding. Current technology is already capable of creating singular microexplosions in the laboratory.

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