This week we're planning on opening doors on all of our four single-instrument spacecraft: the Narrow Field Imager (NFI) and one of the three Wide Field Imagers (WFIs) on Monday, and the other two later in the week. That's a big deal scientifically, because we'll get our "first light" images and begin to commission the cameras to do the science we came here for. We're all very excited to see the starfields and corona for the first time through our newest eyes in space!
But it's also an engineering big deal.
There are a lot of reasons to have a door on each instrument. In particular, the doors protected the instruments from dust and from tools and things falling on the sensitive baffles during development, and that's important since even one 10-micron dust grain in the wrong place could produce a noticeable stray light pattern. But there are some surprising constraints on them also. Something like $20M of investment hinges (literally) on each door opening in space, so they are engineered to absolutely, positively, ultra-reliably open.
But there are a lot of challenges to door design.
Metals that have chafed (for example by having the living daylights shaken out of them by a rocket) can cold weld in the vacuum of space, turning two separate pieces of metal into one unit (oops). So you need dissimilar metals and some sort of lubricant interface on the jamb interface and the hinges. But dissimilar metals expand and contract differently in thermal shifts (that's how bimetallic thermostats work), which can cause them to seize; and most lubricants are also pretty volatile (light oils) or give off dust (graphite) so those have to be carefully selected. So the hinges in particular are pretty sophisticated and carefully designed.
The doors have to seal reasonably tightly to keep dust from getting in – but they also have to vent air on ascent on board the rocket. They have to be strong, and lightweight, and precisely built, and did I mention ultra reliable? The WFI doors, for example, don't just sit on a jamb – there are complicated ball-and-cup interfaces to hold the edges in place, and precisely machined edges to almost (but not quite) seal the door when it's shut. NFI works similarly, though the geometry is slightly different.
All the doors are spring-loaded pretty strongly to swing open reliably in zero-gee – but not too strongly, which might break the spacecraft structure when they wham into their stopper. They are held shut with a pin embedded in heavy wax. We open them by melting the wax, which allows the pin to escape very, very reliably: the plug of wax is wide, so there's no way for the pin to seize on the side of the latch mechanism. But it'll take a few minutes to melt the wax, so we'll send the command at the start of a ground pass and hope to see the actuation before the end.
The doors have mass, and swing pretty energetically, so opening them will be an interesting challenge for the attitude determination and control system (ADCS). We expect the (relatively tiny) onboard reaction wheels to react strongly, and for each spacecraft to take a few moments to react and re-establish pointing as the door settles down. That can take a few seconds, because there's no air to slow the door down -- it has to bounce against a little elastomer stop, dissipating energy each time it bounces. We had to check to make sure the ADCS wouldn't get into a situation where it pumped energy into the door on each bounce, by pushing on the spacecraft at the bouncing resonant frequency of the door itself.
Once the doors are open, the ADCS will have another challenge: the mass distribution of the spacecraft will be noticeably different, especially on WFI which has a large door. So the spacecraft will react slightly differently to the reaction wheels in the ADCS. That's a big deal because the ADCS is not only used to point the instrument where we need it to see; it's also what keeps the solar arrays pointed at the Sun, to supply power to everything on board. We expect that everything will be fine, but we have to be prepared for it not to be. So everyone will be nervous until we see the spacecraft settle back down into fine pointing mode.
So wish us luck for first light on Monday 14-April! It's a big deal for PUNCH scientists, for the engineers who poured months of effort into making amazing works of mechanical art, and also for the spacecraft team who are monitoring and maintaining the health of the spacecraft as they fly.
