Kind of stumbled across this and found it to be pretty fascinating. I’ve read quite a about everything involved with the Apollo program and was always amazed at the number of time explosive devices were mentioned:
Ordnance cutting devices and initiators are highly compact and very reliable ways to seperate or initiate functions like pyrovalves or deploying mechanical assemblies that would be very complicated to do with a non-ordnance electromechanical system, and are easy to implement in a redundant design to increase reliability. The Single Bridgewire Apollo Standard Initiator (SBASI) has proven to be a highly reliable design that was subsequently used as the Hi-Shear NASA Standard Initiator (NSI) on virtually every crewed and uncrewed spacecraft operated by NASA, and NSI-type initiators are produced by every ordnance manufacturer and used extensively in the aerospace industry for critical ordnance initiation functions in aircraft, spacecraft, and space launch vehicles.
Thanks for the info. I was curious to see what SpaceX uses and they have gone with pneumatic’s for various reasons. I’m sure you’re aware of this. One guy mentioned that pyros will eventually phased out. Do you see that happening?
Also found out today that due to concerns for the Apollo ground crew during the filling of the hypergolic fuel in the LM, they had pyros installed on the SLA to help get the ground crew out if a leak occurred. Never thought about something like that and I’m wondering if there are other instances like this to ensure worker protection and survival?
My lab recently did some analysis on a device worn by some military pilots that automatically separates the parachute canopy if they land in seawater (link). It contains a small explosive (“squib”) to actuate the release mechanism. Clever design.
The drive to virtually eliminate pyrotechnic devices from separation and actuation devices on SpaceX launch vehicles comes from the (rational) desire for reuse and avoid handling ostensibly hazardous devices and (irrational) hatred Elon Musk has for pyrotechnics. (Musk has been on record numerous times protesting the necessity of flight termination systems by arguing that the Falcon 9 will be so reliable that they are not necessary to protect public safety.) There is a “six of one and half a dozen of another” argument here; electroexplosive devices (EED), ordnance transfer lines, shaped charges, et cetera can only be functioned once, so in order to assure reliability a large sample has to be made of every production lot; historically, half or more of a production lot of EEDs, transfer lines, and linear shaped charges were often tested to statistically assure reliability, which is prohibitively expensive for a commercial program. In response, AIAA S-113 “Criteria for Explosive Systems and Devices on Space and Launch Vehicles” was developed as a standard for commercial applications based on prior MIL and DOD standards as well as industry experience which required less testing. Nonetheless, testing is expensive and the materials which are used in EEDs all have some degree of environmental hazard as well as having to be transported and handled with protection against electrostatic discharge (ESD).
Pneumatic devices, on the other hand, do not have these hazards, and can be repeatedly tested. However, it should be acknowledged that any contact-based electromechanical or pnuemomechanical device will wear and its characteristics will change with use which requires considerable characterization testing to assess a functional lifetime. These devices also require plumbing of high pressure air lines, and in general have much lower energy release thresholds than ordnance devices, as well as being much more difficult to design in true redundancy, so the overall system tends to be heavier and potentially less reliable. SpaceX went through considerable design and development testing effort in the design of their stage and payload fairing release systems to get them to a point of high reliability; however, these systems have worked apparently flawlessly in operation.
I think a move away from ordnance initiation systems for staging and payload release in launch vehicles is a good move from a cost and handling standpoint, but there are certain types of applications for which ordnance devices are just ideal; among them being emergency separation/ejection systems, cutters for parachute separation and staged reefing, and initiatiors for solid propellant motors and gas generators. They will be mandatory for flight termination systems for the foreseeable future on the basis of proven reliability and functionality, and there are numerous other applications such as pyrovalves on spacecraft where they are just the only weight-budget acceptable solution, especially when required to function after spending years in space. So pyrotechnic devices will continue to be used both in space applications and other emergency devices.
Pyrotechnic devices are used to initate all vehicle airbags. They are used all over aircraft ejection systems to do everything from cutting an overhead canopy to the mortar launch of parachutes for zero-zero ejection seats, and are used in some very specialized applications for cutting and severing operations for emergency egress.
Thanks for the great explanations. I didn’t know about airbags using them but the ejection seats was something obvious. I remember reading Chuck Yeager’s book and the chapter were he got to do some flying in the Mig-15 that was handed over by the defecting North Korean pilot. Before one of the test flights Yeager was confronted by a high ranking officer about why he was enabling the pyros before his flight instead of the ground crew. Something along the lines about it was his ass being put at risk and he wanted to make sure it was done properly. I wonder if other pilots ever felt the need to do it themselves? I haven’t read a whole lot of books of military pilots in the ejection seat era so maybe it’s something more common then I think.
Thank you for posting this. It’s fascinating.
I pretty much assumed that pyrotechnics were used for things like the separation of stages and modules, and chute deployment. I never realized that they were used to open valves or interrupt electrical circuits, but I can see the reasons.
NP. A couple other things I didn’t know about until a few years ago specific to the Saturn V:
First off is the tapered pins and dies used to help “soft release” the Saturn V during its launch as mentioned in this thread:
Next is the LES regarding the ballast below the nose cone that had depleted uranium and lead in it to help with aerodynamic stability.