While there are several instruments that still await testing for our future Extremis probes (and other Kerbin orbital missions) one of them, the hemispherical ion trap, is small enough to fit on the payload truss of a Mk6. Originally designed to study charged particles from the kerbolar wind, similar charged particles (also from the wind) are trapped inside the radiation belts that surround our planet. With ample experience in exploring the inner radiation belt, the Progenitor team was able to put together a mission that launched last week which would allow for the instrument to be exposed directly to space and gather data that could be used to not only further study the environment inside the belt but better tweak the instrument for use during deep space exploration.
The Flight
No delays led to an on-time launch, followed by a nominal ascent. The rocket’s center of mass was shifted ever so slightly to the rear since the truss carrying the extra batteries and the truss carrying the payload were switched from their normal positions. This did not have any adverse affect on the rocket’s flight. All stages were separated without issue and during the final burn, after passing 45km, the top fairing halves were jettisoned to expose the test instrument – this is the first time we have detached a payload fairing while under powered flight and it did not quite go as planned.
For some reason the rocket began to pick up a relatively large wobble as it finished its ascent through the upper atmosphere, likely from excess drag due in some way to the payload fairings, or lack thereof. It exited the atmosphere one second after MECO on its way up to an apokee of 541km. Launched during the middle of the day rather than right after sunrise, the trajectory would give it plenty of time within the inner radiation belt to collect more than enough data.
Coming back down, the rocket was rotating around at nearly a 45° angle rather than its more straight up and down orientation which was cause for some concern since the missing upper payload fairings would also expose the instruments and battery to the heat of re-entry. Being near the nose cone and as far as possible away from the supersonic shockwave was counted on to save them from damage but entering slightly broadside moved the shockwave closer to the upper parts of the rocket.
The rocket ended up surviving re-entry and upon recovery 363.531km downrange the instruments were found slightly singed but intact.
Flight Analysis
The only thing non-routine about this flight was the detachment of the payload fairings, in this case the upper ones as we swapped the normal location of the instruments and extra batteries. While we have detached fairings in the past, it was only on atmospheric flights in the early days of the Progenitor program, and usually after chute deployment. The decision to detach at 45km was made based on previous ascent data showing it would have minimal affect on the rocket if its innards were exposed since the atmosphere has largely thinned out by that point.
Separation did not appear to go smoothly however. We still don’t know exactly what happened because by this point the rocket was too small on our ground tracking camera to make out details. The fairing halves use a small explosive charge to destroy their bolts and allow them to be flung away by the spinning force of the rocket. It is possible there was still enough outside pressure to partially hold the fairings to the rocket for a time or that one or both of them got hung up on the truss instead of being flung away.
What we do know is that upon recovery the fairing halves were not found, which means they did at some point successfully detach from the rocket. We suspect it happened during ascent because re-entry data does not show the existence of any unplanned drag.
Future Plans
Scientists will take some time to study the data that was returned and decide if they would like a follow-up mission to the outer radiation belt by hitching a ride on a Block II. If that is the case, we will need to slightly redesign the fairings to not just frag their bolts but also provide a stronger push away from the rocket, or just let them go after the rocket reaches space since the loss of their minimal mass earlier would not largely affect the rocket’s apokee.
