Standard Missile-3
Published on Feb 27, 2008 at 6:33 pm.
3 Comments.
Filed under rockets.
Last Wednesday night, while a lot of us were watching the lunar eclipse, the guided missile cruiser USS Lake Erie (CG-70) fired a missile to shoot down the defunct USA-193 satellite. The missile’s kinetic warhead struck the satellite at an altitude of 133 nautical miles, shattering the satellite. A cloud of expanding gas from the shattered satellite indicated that the hydrazine tank had been ruptured, which was the stated goal of the exercise.
I think that concern over the hydrazine may have played a factor in the decision to shoot down the satellite. However, this also provided a ready made opportunity to test the nation’s anti-ballistic missile system. A lot of talk has gone into how this is a response to China’s destruction of one of their old satellites in a test of their anti-satellite weapon. But, if this is the US response, it is really not a very good one. The missile used, a slightly modified Standard Missile-3 is unable to hit a satellite that would be a likely target for any anti-satellite weapon system. The missile is designed to hit missiles or missile warheads just barely outside of the atmosphere. At that low altitude, satellite orbits are not stable (which is why USA-193 was spiraling back to Earth anyway). Some have commented on the cost of shooting down the satellite. However, the anti-missile defenses were going to be tested again, anyway. And, it is quite a bit less expensive to shoot down something that is already up there than it is to shoot something else up there just to shoot it down! And, of course, destroying the satellite prevents any sensitive technology from falling into the hands of those who may be unfriendly towards us. And, the upgrades to the system needed to hit the satellite could also be used against enemy missiles.
As I said above, the missile used to hit USA-193 was a modified Standard Missile-3 (SM-3) missile. This is basically a derivative of a standard high altitude ship based anti-aircraft missile, heavily modified for exoatmospheric use. The missile was developed as part of the nation’s anti-ballistic missile shield. USA-193 was spiraling down in a decaying orbit. Without intervention, it would have likely reentered sometime in the first half of March. The SM-3 is designed to hit ballistic missiles that are just outside of the atmosphere. The SM-3 probably could not have boosted its warhead much higher than that. So, the satellite could not be shot down until it had spiraled low enough to be sure of a good chance of an intercept.
The SM-3 was designed to intercept ballistic missiles. In one sense, satellites may be tougher to hit. Ballistic missiles do not put their warheads into orbit. Rather, the warheads are suborbital. They are designed to reenter and hit a target. So, they move much slower than an object in orbit. Thus, the ballistic missiles are much slower than the satellites. The guidance software for the SM-3 used to shoot down USA-193, therefore, had to be modified to account for the greater speed of the satellite.
The Standard Missile-3 is also designated Raytheon RIM-161. There has actually been a family of these missiles. They are designed to be launched from warships equipped with the advanced Aegis combat system. The Aegis system is an integrated system of weapons, detection, tracking, and computerized command and decision subsystems. Rather than a sailor pulling a lever or pressing a switch to fire the weapons, the computers determine the best time to fire the weapons and give the appropriate commands to the on board computers of the weapons themselves. These are all features that are also needed to track and fire an intercepting missile at an incoming missile (or satellite).
The first two stages of the SM-3 are the same as the RIM-156 Standard Missile. The first stage that shoots the missile away from the ship in a vertical takeoff is a solid fueled stage that uses vectored thrust instead of fins to steer. After the first stage burns out, the second stage, a solid rocket dual thrust motor, takes over. For the antiaircraft missile, a blast fragmentation warhead can sit atop the second stage. But, the SM-3 has a third stage that is also a solid rocket motor that boosts the missile outside of the atmosphere, if needed. When approaching the target the third stage separates, and the kinetic warhead acts as the fourth stage and payload. It has its own maneuvering system and forward looking radar. All through launch, though, the missile is being sent updates on the target position from the ship’s sensors.
The whole missile is 6.55 meters (21.5 feet) long, and 0.35 meter (13.5 inches) in diameter. It can reach speeds of about 9600 kilometers per hour (6000 mph) and can hit targets somewhat higher than 160 kilometers up. That is plenty sufficient for suborbital ballistic missiles, but is not really sufficiently high to take out a satellite unless that satellite were already falling back to Earth. That altitude has enough atmospheric drag to make an orbit unstable. USA-193’s orbit was decaying rapidly and the satellite would likely have reentered within about two to three weeks anyway. Note that some sources give a service ceiling of about 250 kilometers for the missile. That is a somewhat more respectable altitude, but it is still a bit low for stable satellite orbits. The SM-3 is an anti-missile system, not an anti-satellite system (despite having knocked USA-193 to bits).
Despite USA-193 being out of control and coming down on its own, hitting a satellite is tough. Satellites move much faster than missiles. So, the warhead wasn’t so much sent to slam into the satellite as much as it was put in the way of the satellite and the satellite slammed into the warhead. Either way, the satellite is destroyed. It is rather remarkable that the warhead designed to take out missiles could be modified to respond quickly enough to do the same with a much faster moving target. Such precision work always astounds me. If the timing had been off by only a tiny bit, the warhead would have missed the satellite. But, it didn’t.
-Astroprof
Image credit: Raytheon
Coleman on February 27, 2008 at 10:17 pm: 1
I think we shot that satellite down just to show how awesome we are compared to China.
BA Visitor on February 28, 2008 at 6:09 pm: 2
What’s a duel thrust motor? Perhaps you meant dual thrust?
Astroprof on February 28, 2008 at 7:45 pm: 3
Oops. Thanks BA.
I fixed the typo.
Sometimes, I get to typing too fast, and my spell checker picks the wrong word for me.