Saturn V had to make sure it could get an Apollo command-service module into the right orbit before putting the spacecraft on its way to the Moon. And that meant the mighty rocket had to go through its complex and precise staging perfectly.
But how did this happen? The digital signals from the Saturn V’s Launch Vehicle Digital Computer were routed to boxes on each stage, near its associated pyrotechnic, also called “EBW (explosive bridgewire) firing units.”
Those units generated electrical pulses with enough energy to detonate the explosives. Read on to find out when and how this was possible.
Stage separation Saturn V
The rocket’s first stage’s flight was short; engine cutoff came at about 135.5 seconds after liftoff, the five F-1 engines having burned through the 203,000 gallons of Kerosene and 331,000 gallons of liquid oxygen. At that point, the Saturn V’s Instrument Unit, the “brains” of the Saturn V launch vehicle, sent a shutdown command to the center engine. The four outboard F-1 engines kept firing until liquid oxygen, or fuel depletion, was sensed, and 600 milliseconds later, the outboard F-1 engines cut off.
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The cutoff triggered two things. First, the onboard computer sent an electric signal that activated a linear charge of exploding bridgewire that physically severs the two stages. This event physically separated the two stages at approximately 205,000 feet. At the same time, the eight retrorockets fired, providing 88,600 pounds of thrust for just 0.666 seconds. This event slowed the now spent stage, giving the S-II stage room for its engines to ignite. The S-IC, meanwhile, was carried to approximately 366,000 feet before it lost its upward momentum and began falling to the ocean, landing about 350 miles downrange from the launch site.
To help add more distance between stages were eight ullage motors on the aft end of the second stage. The ullage motors fired for approximately 4 seconds. This added enough forward momentum to the rocket to force the propellants — liquid hydrogen and cryogenic liquid oxygen — into the feed lines to ensure the engines burned cleanly.
Around 30 seconds after stage separation, the aft interstage on the S-II was built by North American Aviation and separated by explosive charges. The ring fell away, a precise maneuver since it had just three feet of clearance from hitting the engines. Then the stage’s five powerful J-2 engines fired and burned for about 6 minutes, boosting the payload to about 605,000 feet.
About 10 seconds before the powerful S-II J-2 engines ran out of fuel, an electric signal initiated the separation system that separated the second from the third stage, the S-IVB. Severable tension strap, MDF (mild detonating fuses), and EBW (exploding bridgewire) physically separated the stages. And again, simultaneous with separation, four retros on the now spent S-II stage slowed its ascent before it fell down into the ocean. Four ullage motors on the S-IVB fired to put distance between the two pieces and ensured fuel reached the feed lines going into the S-IVB’s J-2 engine.
The S-IVB cut off after approximately 115 miles of flight, at which point the Apollo spacecraft was in orbit around the Earth. However, its job wasn’t over. The S-IVB had the unique ability to relight. Not only was it the orbit insertion stage, but it also fired a second time for the translunar injection burn, the burn that sent the spacecraft out of Earth orbit on a path to the Moon.
Summary
Saturn V Launch Vehicle Pyrotechnics Components for Stage separation
- Retrorockets
- Ullage rockets
- Detonator blocks
- Firing Units Firing Units
The ordnance systems on the mighty Saturn V are as follows (in the order that they are ignited):
- Eight retrorockets on the S-IC, ignited by a redundant pair of EBW (explosive bridgewire) firing units”, to pull the first stage away from the rest of the rocket during separation.
- A redundant pair of EBW firing units on the interstage ring detonate a linear-shaped charge to cut the S-IC first stage from the interstage ring.
- Four ullage rockets on the S-II second stage, ignited by a redundant pair of EBW (explosive bridgewire) firing units, to settle the propellants in their tanks and to help with the separation of the interstage ring.
- Another redundant pair of EBW (explosive bridgewire) firing units on the interstage ring detonate a linear-shaped charge to cut the interstage ring from the S-II second stage. Please note that this separation disconnects the signal that caused it (although it does not matter).
- Four retrorockets on the S-II, ignited by a redundant pair of EBW (explosive bridgewire) firing units, to pull the second stage away from the rest of the rocket during separation.
- A pair of EBW (explosive bridgewire) firing units on the interstage attached atop the Saturn V’s second stage, which servers the tension straps between the Saturn V’s S-II and S-IVB stages. This is another separation that disconnects the signal that causes it.
- Two ullage rockets on the S-IVB third stage are ignited by a redundant pair of EBW units.
- A redundant pair of EBW on the S-IVB detonate four frangible nuts to jettison the ullage motor assembly to save weight.
Moreover, each stage of the rocket has a redundant pair of self-destruct systems. Those are controlled by a radio signal from the range safety officer rather than the Launch Vehicle Digital Computer.
Thank you for taking the time to read this, and please feel free to browse my website for more exciting facts about the mighty Saturn V. How about this article? Why Was The Saturn V Rocket Painted Black And White?