Project Longshot to Alpha Centauri - part 6

Now done 58 pages out of 74 of Project Longshot, a proposed mission to Alpha Centauri, the closest star system to our own. One more page and I'll be in the appendix where all the formulae are. The table on page 55 is in remarkably bad condition so I redid the table here.



3.5 Data Processing

One of the significant "enabling technologies" required to perform the interstellar probe mission is that of advanced processing. When the spacecraft reaches its target, the probe will be 4.3 light-years away from command and control facilities on Earth, and will thus have to be completely autonomous and self-repairing. The processing system will ideally have low power consumption to reduce heat dissipation requirements. It will be multiply redundant with advanced shielding and survivability features, and also be able to control all facets of probe operations, including high level decision-making. As shown in the sectional appendix, the probe must evaluate given mission objectives to control the scientific instruments in order to explore the Alpha Centauri system most effectively. If the system can integrate high-accuracy attitude determination and scientific data instantaneously, the attitude control requirement can be relaxed to a level easily maintained by such a large structure. Finally, the data must be taken out of processor's memory and sent to earth via laser. The communications lasers must be pointed with an accuracy of .067 arc-seconds, and a hard file of the position of the earth relative to Sol must be retained in memory to govern the pointing of the laser. Once the target system is reached, the processing unit must be able to achieve and maintain an acceptable orbit, and maneuver to investigate high priority phenomena, such as evidence of intelligent life. For a block diagram of the data processing system, see appendix.

3.6 Guidance

A system of star trackers will be used for both navigation and attitude determination. This system has been chosen for its high accuracy (4 arc sec) and adaptability, and for its low weight (7 kg) and power requirements (18 watts). Star scanners were not chosen since the spacecraft is not rotating, and they are less accurate. The trackers will be coupled to a computer system which will have a star catalogue of 200-300 stars' locations. "Adaptability" refers to this catalogue, because a "best guess" of star locations that the probe will "see" on the trip and in orbit in the Centauri system can be programmed into the computer before launch, thus increasing the accuracy of position/attitude determination. This best guess could even be updated enroute or in Centauri-orbit by the astrometry calculations that the computer will make.

Initially, during the transit between Earth and the point where the probe-head separates from the propulsion system, trackers located on the last fuel tanks will be used. These trackers will be oriented in different directions in order to gain a nearly complete field of view. 18 trackers will be used, 3 in each of the 6 axis directions to get the greatest field of view and triple redundancy. The power for these trackers will come from generators drawing energy from the propulsion system waste heat. In the final mission phase, 9 trackers located on the instrument booms will be used, 3 on each boom instrument head. (This will require a very accurate position determination of the boom rotation angles.)

Three 3-axis rate-gyro assemblies will determine the rate of change of any two pointing angles and the spacecraft roll rate. This data will supplement the trackers' information and increase the attitude determination accuracy.

Star tracker parameters:

Solid state (vice photomultiplier tube)
4 arcsecond accuracy (future improvement is expected)
Magnitude range -1 to +6
Field of view 6x6 degrees
6 seconds to search field of view
1.2 seconds to search field in 1 x 1 degree search mode
Has a track mode during which it follows a specific star
Total weight - 189 kg
Total power in transit - 324 watts
Total power after probe separation - 162 watts

A summary of the attitude control systems available to choose from are listed in Fig. 3.6a. The probe's attitude control will be accomplished using two sets of flywheels arranged on 4 axes (described in section 3.3 Power System), and an auxiliary system of hydrazine thrusters. These flywheels will serve as momentum wheels, controlled by the computer using the attitude/rate information, providing torque to maintain spacecraft stability. The 4-axis configuration will enable the reaction wheels to absorb external torques from any direction. The magnitude of this reaction torque is easily modulated by electronic control of the reaction wheel motor current. One disadvantage of this system is the need to control wheel speeds in order to limit vibrational effects. "Unloading" the energy of the wheels is done by transferring momentum to the second set of wheels, discharging energy through the power system, and using the hydrazine thrusters. The hydrazine tanks are located within the main truss between the flywheels, encircled by the main fuel tanks (so they may be cooled by the same refrigeration units). The nozzles are located on the circumference of the spacecraft pointed in each axis direction. The flywheels will be used for attitude control in the solar system phase of the mission, and in the Centauri system. The hydrazine system is primarily used as a backup.



Text:

ATTITUDE CONTROL TECHNIQUE COMPARISON

Type	Cost	Accuracy	Missions	Comments
Y-axis active control (1) reaction wheels	very high	0.007 arc-seconds - 1.0 deg.	astronomical, weather	Vibrations at high rpm's may reduce accuracy
(2) hydrazine thrusters	very high	.1 - 1.0 deg.	deep space	Fuel limited
Spin stabilization	low to moderate	.1 - 2.0 deg.	Earth orb. Interplanetary	Spin rate and direction require control
Dual-spin stabilization (1) half spin, half stabilized	high	.01 - .1 deg.		Requires complex technology for elec. and mech. connections
(2) internal momentum wheels	high	.01 - .1 deg.	Geosynch. comms. satellites	High technology requirements
magnetic stabilization	very low	10-20 deg.	low alt., scientific	Requires a magnetic field
gravity-gradient stabilization	low	1.0 - 10 deg.	circular, low alt.	Requires a gravity field & large moment of inertia for satellites

3.7 Thermal Control

There are two significant problems to consider in the thermal design:

There is a huge amount of waste heat from the propulsion system and the nuclear reactor (both fission and fusion reactions);
The fuel will be stored in large tanks at near-absolute temperatures, and must be shielded from the waste heat of the nuclear reactions.

The spacecraft will require highly efficient radiators to dissipate the thermal energy released by the interstellar drive, namely the inductors, particle beams, and fusion reaction. The radiation from this dissipation process will be reflected away from the rest of the spacecraft by a mirror specifically engineered to reflect infra-red energy. Additionally, conduction will be buffered by special ceramic materials between the power and propulsion units.

The nuclear reactor will dump its waste heat to the same radiators used by the propulsion unit. Ceramic buffers will also be located between the power unit and the fuel tanks.

The fuel tanks will contain pelletized helium and deuterium which must be shielded from conductive and radiative heat energy. During th einitial phase of the mission the tanks will be shielded from the sun by a shroud which will be blow off at a sufficient distance from the sun where the solar radiation becomes negligible. The mirrors and ceramic buffers will keep the spacecraft's waste heat away from the tanks, while refrigeration units will keep the fuel vapor pressure low enough to remain in pelletized form for the duration of the mission. The tanks will be painted black to emit as much radiation as possible in deep space.

The probe head will be protected from the radiation from Beta in the Centauri system by a thermal blanket. Heat from the computer, instruments, and lasers will be convected through heat pipes to the radiators at the rear of the spacecraft.