Developing controllable-thrust rocket motors may enable the U.S. military services
to condense multiple families of tactical missiles into a single all-purpose
A crucial first step was taken last year with the first flight in history of
a solid-fueled, controllable-thrust motor, according to Aerojet, a Sacramento-based
manufacturer of rocket engines.
The engine burn was 50 seconds for a flight that lasted two minutes at the
White Sands Missile Range. The test vehicle was a demonstration Precision Attack
Missile, part of the Army Non-Line-of-Sight Launch System (NLOS-LS), also known
as Netfires. Designed to be part of the Future Combat Systems, NLOS-LS will
fire containerized missiles capable of loitering and precision attack. The program
is currently in a risk reduction phase.
While NLOS-LS missiles can use normal rocket engines, a variable-thrust motor
has several advantages, according to Bob Keenan, business development manager
for advanced tactical propulsion. “Suppose that you have soldiers who
are surprised by the enemy,” he said. “They can command the missile
to go to the target at the highest possible speed. … Hypothetically, it
can travel a kilometer in two seconds.”
But if the motor is commanded to travel at normal speed, the missile’s
range can be extended to seven kilometers. Slow it further to extend range,
and it might reach targets 40 kilometers away. The Army could accomplish the
mission with one missile, instead of fielding three missiles for each of those
range bands, Keenan said.
These missiles also could serve as bunker busters, by conserving fuel until
they reach the target, and then accelerating to maximum velocity. Another role
being examined is air defense, said Keenan. A controllable-thrust motor could
vary its speed and thus its turning radius when chasing aircraft. The missile’s
guidance system can adjust thrust based on real-time parameters, or via preprogrammed
or radio-controlled instructions.
Controllable-thrust rocket motors using liquid fuel have been around since
the Apollo era. But controlling a solid-fuel motor is a much trickier beast,
akin to attempting to control how quickly a Roman candle burns once the fuse
is lit. The motor’s burn rate varies strictly according to the shape of
the propellant grain. A bigger propellant surface area creates more thrust.
In the White Sands test, Aerojet used a pintle—essentially a plug—that
moves in and out of the throat of the engine nozzle. An actuator inserts the
pintle to raise pressure and increase the burn rate, which can then be throttled
down by removing the pintle.
“When the missile says to the motor to go faster, the pintle goes in,”
noted Keenan. “When the missile wants to go slower, it drops the pintle
to balance drag. We also found out that if you pull the pintle out fast enough,
you can shut down the motor.”
Aerojet has been examining controllable thrust since 1959, said Keenan. The
problem has been the prohibitive weight of a guidance system sufficiently sophisticated
to precisely control the motor. But computers have now become light enough to
make this possible.
Controllable-thrust motors could be retrofitted to older missiles, Keenan said.
While not giving specific numbers, he added that the control system is estimated
to be no more than 2 percent of a missile’s total cost.