The compact Inertial Measurement Unit is 3.7" diameter and 1.9 pounds. Three boards: sensor card, digital card, and microcontroller card. The accelerometer/gyro unit contains 3 orthogonal accelerometers and laser ring gyros, and high-voltage laser power supply. 35/57
The accelerometers are Honeywell RBA-500 Vibrating Beam Force Transducer Accelerometers. Each uses a pair of vibrating quartz beams where the difference in oscillation frequencies depends on the applied force. 36/57
The guidance system uses a Kalman filter to combine IMU and GPS data. This clever algorithm from the 1960s combines multiple measurements while accounting for inaccuracy. Even your phone uses a Kalman filter to determine location. 37/57
The guidance system ensures that the missile knows where it is at all times. youtube.com/watch?v=bZe5J8… 38/57
The guidance system is powered by a lithium thermal battery. The electrolyte is solid so the battery won't deteriorate in storage. Just before launch, pyrotechnics inside the battery melt the electrolyte. The battery then provides electricity like a normal (but hot) battery.
I think that GPS guidance made HIMARS possible. The tracked, heavy MLRS system can rapidly fire unguided rockets. But the truck-based HIMARS rocks severely when launching, so unguided rockets would go way off course. 40/57
The Electronic Safe and Arm Device ensures that the warhead detonates at the target, but not before. One FPGA circuit verifies arming commands from the guidance system. An independent FPGA checks an accelerometer so even faulty guidance signals can't detonate the warhead. 41/57
The ESAD uses a Motorola MMA1201P Analog Accelerometer, measuring up to 40G of acceleration. It ensures that the missile accelerates for at least 5.7 seconds after the umbilical disconnects. Thus, the warhead can't detonate unless the rocket successfully launched. 42/57
To detonate, a high-voltage capacitor puts 1250 volts through a Low-Energy Exploding Foil Initiator, or "slapper detonator". This vaporizes a foil strip, driving a "slapper" into a small internal explosive. This triggers the fuze's explosive charge and then the warhead. 43/57
The ESAD was used with the GMLRS's DPICM cluster bomb warhead, packed with 404 tiny grenades, but now obsolete. The newer GMLRS unitary warhead uses a cylindrical Electronic Safety & Arming Fuse (ESAF). It has a control board and a high-voltage board. 44/57
To determine when the rocket is at the right height about the target, it uses a "Frequency-Modulating Continuous Wave—Directional Doppler Ranging height of burst sensor". This radar sends a signal with varying frequency and the reflected signal indicates the height and speed.
GMLRS is an expensive way to deliver a small explosive. Each rocket is ~$160,000 and contains just 51 lbs of explosive in the 200-lb warhead. In comparison, a B-52 can carry 70,000 pounds of weapons (payload in photo). If you have air superiority, planes seem much better. 46/57
HIMARS can also shoot ATACMS, a much larger missile with 300 km range and a 500 lb warhead. Similar launch pods, but only one missile per pod. ATACMS (1991) is much older than GMLRS (2005). Over 450 ATACMS were used in Operation Iraqi Freedom, mostly to destroy air defenses.
Ukraine has been requesting ATACMS missiles since the improved range would open up new targets, but hasn't received them. ATACMS is mostly obsolete now, so we might as well use them up. 48/57
The ATACMS missile uses a Honeywell H700 guidance set. This IMU uses three Honeywell 1328 ring laser gyros, 2.8" on a side, much larger and 100 times as accurate than the 0.8" gyros in the GMLRS rocket. It has three Q-Flex QA2000 accelerometers. 49/57
The QA2000 accelerometer is the most popular sensor in commercial and military aircraft. It's a 1" by 1" cylinder and weighs 71 grams. Inside, a flexing quartz strip detects acceleration, and its motion is picked up by a capacitive sensor. 50/57
ATACMS used dual Zilog Z8002B processors, then upgraded to Intel i960 RISC processors. One processor for the inertial sensors, the other for navigation, autopilot, guidance, and communications. Software was designed in Ada, implemented in Jovial or Zilog assembly. 51/57
The ATACMS guidance system consisted of seven printed-circuit boards plugged into an interconnect board, along with a complex power supply. Two CPU boards, a gyro ADC board, a gyro pulse accumulator board, and 3 gyro electronics boards. 52/57
The ATACMS cluster warhead (now obsolete) was fired by a microcontroller-controlled device that charged to 2500 volts and then discharged through a spark gap, setting off an Exploding Foil Initiator. Now, ATACMS uses a "unitary" explosive warhead. 53/57
ATACMS is being replaced by the Precision Strike Missile (PrSM). It is HIMARS-compatible with two PrSM rounds in a pod. PrSM's old range was 499 km since the INF Treaty prohibited ranges ≥500 miles. The US withdrew from INF in 2019; now the range is potentially 800-1000 km.
Disclaimer: This is all public information from many sources. I'm no HIMARS expert and haven't even seen one in person. Much of this information is obsolete. I'm not revealing anything even slightly helpful to the Russians. Photo credits and other information are in the alt text.
Disclaimer: I want to avoid militaristic technology cheerleading. Even if the underlying technology is interesting, the fact remains that it is designed to kill people and blow things up (or at best provide deterrence). It would be better if this technology wasn't needed. 56/57
Disclaimer: It's important not to fall into the trap of thinking that technological wizardry and a new super-weapon will make all the difference. Even though providing tools is important, success primarily rests on the skill and bravery of the Ukrainian soldiers. 57/57
Most of the technical info is from "Ring laser gyro applications for tactical missiles: the Army TACMS solution" (1990) and "The International GMLRS Development Program" (2002). I'm not revealing anything new or secret; this was openly published in detail decades ago.