A portion of the Naro-1’s payload can be seen in the low right hand corner of this frame of the payload rocketcam. Half of the clam shell nose fairing remains in the left half of the image.
Today’s techno-wonk exploration of last year’s Naro-1’s failure is only made possible by the brilliant internet searching of Josh Pollack. (Thanks, Josh!)
Last year, South Korea failed in its attempt to orbit a satellite using a Russian liquid-propellant first stage and an indigenously designed and manufactured solid-propellant second stage. In preparation of the June 9th second attempt, the South Korean space agency released a video of selected rocketcams of that first launch. The ROK space agency said that the video had not been released earlier because of concerns about how it would effect relations with Russia (which supplied the first stage) and for security reasons. Its hard to see how either reason makes sense. Nevertheless, the video does reveal some interesting tidbits about the ROK’s guidance capability, the subject of this post.
The second stage separates from the first stage some time after the first stage has burnt out and considerably after the nose fairing failed to separate cleanly.
When the nose fairing failed to separate, at 216 seconds after launch, it left a slight mass imbalance between the “left” and the “right” halves of the rocket. However, the first stage continued to burn for several tens of seconds. The rocketcam showing the separation of the first and second stages clearly shows that the second stage was not tumbling at the time of separation. Thus, the Russian first stage was easily able to accommodate the relatively slight mass imbalance. (Because of the first stage’s large mass, the half clam shell that remained caused, proportionately, a considerably smaller fractional mass imbalance than it would for the lighter second stage.) The next portion of the video shows the ignition of the second stage and shows the start of a very interesting oscillation.
The solid-propellant second stage ignites and immediately induces an oscillation in the direction of the rocket. The Naro-1 was never able to recover and, in fact, the oscillations gained in amplitude until the rocket was tumbling out of control. (Note the slight shifts on the nozzle with respect to the rest of the motor. That’s TVC in action.)
As regular Wonk readers will know, I have a fascination with thrust vector control and this oscillation is directly related to the guidance system (as it determines the direction of flight and possibly the attitude of the rocket). The second stage of the Naro-1 uses a flexible nozzle where four hydraulic jacks pull the nozzle to one side or another as the guidance system determines which small offset in the thrust direction would counter any small perturbation.
The Naro-1’s second stage is shown here. The dark tube is the solid-propellant motor casing and the cone on the left is the motor’s nozzle. The white coverings attached to both the nozzle and the bottom of the second stage house the hydraulic jacks that maneuver the direction of the nozzle for thrust vector control.
In principle, it should be possible for a guidance and control system to damp out the oscillations (see the animation below that was taken from the South Korean video) induced by the mass offset and arrive at a new average direction for the nozzle. This offset would probably change with time as the propellant from the motor is used. However, the Naro-1 was not able to do this. Why this is so is a matter for speculation.
One possibility is that the guidance system is not sensitive enough to detect the drift in direction early enough to counter act the effect of a change in the mass axis (remember, we said that the direction through which the thrust must pass through to “balance” the rocket changes as the propellant is burned). Instead, it must—under this hypothesis—wait until the direction of travel has changed enough to be detected. At that point, the TVC system must compensate to a larger extent and might actually overshoot. This overshoot behavior is actually fairly common in guidance algorithms. But there is normally a mechanism built in to damp out the errors. Under this hypothesis, this damping is missing for this sort of malfunction.
I, of course, await the alternative ideas and comments of our wonk-readers.