April 1, 1997
Boosted Dart
Not all two stage rocket vehicles use boosted sustainers, or second stages. An interesting alternative, which allows low cost, high altitude sounding, is the Boosted Dart. By using a high mass/density sustainer, the booster, or first stage, accelerates a small diameter Dart to very high velocities. Upon burnout, the Dart is released. It coasts unpowered until overcome by gravity and drag, following a ballistic trajectory into the sea or desert. Some designs carried built in timers that released a ballute (a high altitude, balloon shaped parachute) from the vehicle after apogee for safe recovery of the radiosonde instrument payloads.
To maximize coast time, the dart must be constructed of very heavy, high density materials. Tungsten steel is the usual choice. The dart must also be very small in diameter with very thin, minimum surface area fins. The Super Loki Dart (designed and built by Space Data Corporation), for example, is 1.58 inches in diameter and 45.7 inches in length. First commissioned for use in 1970, these rockets are still launched daily from Wallops Island as part of the long term weather monitoring program.
PERFORMANCE CONSIDERATIONS
With a powered rocket vehicle, maximum velocity and altitude are products of thrust, weight and drag. With an unpowered vehicle or dart, the influencing factors are drag, weight and momentum. A powered rocket must optimize its altitude through velocity and thrust management. An unpowered vehicle must maximize momentum and reduce drag to achieve optimum coast time.
As a rocket increases in velocity, it becomes increasingly difficult to overcome drag forces. Drag increases exponentially from Mach .85 to Mach 1.05 (Mach 1 = 1138 feet per second at sea level). Once the sound barrier is broken at M1 (Mach number), drags decreases and higher Mach numbers can be reached with less difficulty. Optimally, a boosted dart vehicle should remain below M .85 or punch through Mach and continue acceleration, reaching maximum velocity at burnout.
Boosted darts remain attached to the booster during the first phase of flight. The delay between burnout and separation must be as short as possible so that the dart is released at the highest velocity. The booster, having a much larger surface area, also has much higher drag. Once the dart is released, the drag is substantially reduced, allowing momentum to carry the dart till the apogee is reached and the instrument, or other sounding payload, is deployed.
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