This thread is weird. SpaceX knows exactly how the separation mechanism is designed, so why all the guesswork? Someone just drop them a note and ask and save us from the Heath Robinson/Rube Goldberg/Gyro Gearloose guesses.

Jim, taking cryptic comments to the next level.

Quote from: Lars-J on 07/05/2017 09:32 PM Jim, taking cryptic comments to the next level.Not everything that's undecipherable is cryptic.

Quote from: meekGee on 07/06/2017 05:04 AMQuote from: Lars-J on 07/05/2017 09:32 PM Jim, taking cryptic comments to the next level.Not everything that's undecipherable is cryptic.We've finally driven Jim mad...

Quote from: laszlo on 07/05/2017 10:59 PMThis thread is weird. SpaceX knows exactly how the separation mechanism is designed, so why all the guesswork? Someone just drop them a note and ask and save us from the Heath Robinson/Rube Goldberg/Gyro Gearloose guesses.You can ask, but based on experience you won't get an answer, because: (a) It's proprietary, (b) It could help someone else design a missile, so it's covered by ITAR and they can't legally say, and (c) They've got better things to do with their time than explain to curious strangers the details of their engineering.This applies to almost all aspects of rocket engineering, not just this thread. So this forum is filled with guesses based on experience, guesses based on physics, guesses based on intuition, and wild speculation based on nothing whatsoever. Caveat lector.

Quote from: Semmel on 07/05/2017 07:40 PMQuote from: LouScheffer on 07/05/2017 07:18 PMQuote from: Jim on 07/04/2017 10:32 PMThousands of pounds?Well, we can guess the thrust of the nitrogen thrusters. From the NROL-76 mission, we see they fire for about 3 seconds to start the first stage rotating. The rotation reaches 90 degrees, more or less, in 7 seconds. So one revolution every 28 seconds, or 0.224 radians/sec. To acquire this rate in 3 seconds means an angular acceleration of 0.075 radians/sec^2Let's make the crude assumption that the booster rotates around the engines, since that's where most of the mass is located (engines + remaining fuel). We know the empty stages masses about 27t. 9 engines mass about 7t, so let's assume the rest is a 20t cylinder, and that the moment of inertia of the cylinder dominates (the rest of the mass, engines and fuel, is close to the axis of rotation). Rotating a cylinder around its end has a moment of inertial of mL^2/3. Using a length of 47 meters and a mass of 20t, this gives a moment of inertia of 14,800,00 kg x m^2.The torque to accelerate this at 0.075 radians/sec^2 is about 1.1M N x m. Assuming a lever arm of 47m, that's a thrust of 23400 N, or 2400 kg-force, or 5250 lb-force. At a typical ISP of 73 for cold gas nitrogen thrusters, that's a flow of 32 kg/second.So the cold gas thrusters can generate thousands of pounds of force. On the other hand, separation rocket motors can generate even more force. The shuttle boosters had 8 motors per booster, each generating 20,000 lb-f for 1.2 seconds. Each motor massed 80 kg.Thank you Lou, I really like the use of Math and Physics instead of hand waving and authority. More of that please!Meaning numbers. Does not factor in time

Quote from: LouScheffer on 07/05/2017 07:18 PMQuote from: Jim on 07/04/2017 10:32 PMThousands of pounds?Well, we can guess the thrust of the nitrogen thrusters. From the NROL-76 mission, we see they fire for about 3 seconds to start the first stage rotating. The rotation reaches 90 degrees, more or less, in 7 seconds. So one revolution every 28 seconds, or 0.224 radians/sec. To acquire this rate in 3 seconds means an angular acceleration of 0.075 radians/sec^2Let's make the crude assumption that the booster rotates around the engines, since that's where most of the mass is located (engines + remaining fuel). We know the empty stages masses about 27t. 9 engines mass about 7t, so let's assume the rest is a 20t cylinder, and that the moment of inertia of the cylinder dominates (the rest of the mass, engines and fuel, is close to the axis of rotation). Rotating a cylinder around its end has a moment of inertial of mL^2/3. Using a length of 47 meters and a mass of 20t, this gives a moment of inertia of 14,800,00 kg x m^2.The torque to accelerate this at 0.075 radians/sec^2 is about 1.1M N x m. Assuming a lever arm of 47m, that's a thrust of 23400 N, or 2400 kg-force, or 5250 lb-force. At a typical ISP of 73 for cold gas nitrogen thrusters, that's a flow of 32 kg/second.So the cold gas thrusters can generate thousands of pounds of force. On the other hand, separation rocket motors can generate even more force. The shuttle boosters had 8 motors per booster, each generating 20,000 lb-f for 1.2 seconds. Each motor massed 80 kg.Thank you Lou, I really like the use of Math and Physics instead of hand waving and authority. More of that please!

Quote from: Jim on 07/04/2017 10:32 PMThousands of pounds?Well, we can guess the thrust of the nitrogen thrusters. From the NROL-76 mission, we see they fire for about 3 seconds to start the first stage rotating. The rotation reaches 90 degrees, more or less, in 7 seconds. So one revolution every 28 seconds, or 0.224 radians/sec. To acquire this rate in 3 seconds means an angular acceleration of 0.075 radians/sec^2Let's make the crude assumption that the booster rotates around the engines, since that's where most of the mass is located (engines + remaining fuel). We know the empty stages masses about 27t. 9 engines mass about 7t, so let's assume the rest is a 20t cylinder, and that the moment of inertia of the cylinder dominates (the rest of the mass, engines and fuel, is close to the axis of rotation). Rotating a cylinder around its end has a moment of inertial of mL^2/3. Using a length of 47 meters and a mass of 20t, this gives a moment of inertia of 14,800,00 kg x m^2.The torque to accelerate this at 0.075 radians/sec^2 is about 1.1M N x m. Assuming a lever arm of 47m, that's a thrust of 23400 N, or 2400 kg-force, or 5250 lb-force. At a typical ISP of 73 for cold gas nitrogen thrusters, that's a flow of 32 kg/second.So the cold gas thrusters can generate thousands of pounds of force. On the other hand, separation rocket motors can generate even more force. The shuttle boosters had 8 motors per booster, each generating 20,000 lb-f for 1.2 seconds. Each motor massed 80 kg.

Thousands of pounds?

Quote from: Jim on 07/05/2017 09:29 PMQuote from: Semmel on 07/05/2017 07:40 PMQuote from: LouScheffer on 07/05/2017 07:18 PMQuote from: Jim on 07/04/2017 10:32 PMThousands of pounds?Well, we can guess the thrust of the nitrogen thrusters. From the NROL-76 mission, we see they fire for about 3 seconds to start the first stage rotating. The rotation reaches 90 degrees, more or less, in 7 seconds. So one revolution every 28 seconds, or 0.224 radians/sec. To acquire this rate in 3 seconds means an angular acceleration of 0.075 radians/sec^2Let's make the crude assumption that the booster rotates around the engines, since that's where most of the mass is located (engines + remaining fuel). We know the empty stages masses about 27t. 9 engines mass about 7t, so let's assume the rest is a 20t cylinder, and that the moment of inertia of the cylinder dominates (the rest of the mass, engines and fuel, is close to the axis of rotation). Rotating a cylinder around its end has a moment of inertial of mL^2/3. Using a length of 47 meters and a mass of 20t, this gives a moment of inertia of 14,800,00 kg x m^2.The torque to accelerate this at 0.075 radians/sec^2 is about 1.1M N x m. Assuming a lever arm of 47m, that's a thrust of 23400 N, or 2400 kg-force, or 5250 lb-force. At a typical ISP of 73 for cold gas nitrogen thrusters, that's a flow of 32 kg/second.So the cold gas thrusters can generate thousands of pounds of force. On the other hand, separation rocket motors can generate even more force. The shuttle boosters had 8 motors per booster, each generating 20,000 lb-f for 1.2 seconds. Each motor massed 80 kg.Thank you Lou, I really like the use of Math and Physics instead of hand waving and authority. More of that please!Meaning numbers. Does not factor in timeDoes also not factor in the mass of the steel casings of Shuttle boosters (90-100t each?) or their top-mount, puller design. Aerodynamics of the STS stack is also vastly different than the FH.

You can ask, but based on experience you won't get an answer, because: (a) It's proprietary, (b) It could help someone else design a missile, so it's covered by ITAR and they can't legally say

That doesn't mean that SpaceX has to release all their notes for how they determined the best trade-offs, but this isn't something that can be hidden from North Korea.

Quote from: Norm38 on 07/06/2017 06:55 PMThat doesn't mean that SpaceX has to release all their notes for how they determined the best trade-offs, but this isn't something that can be hidden from North Korea.To be fair, your your ICBM needs to be three-boosters strapped together, you probably need to work on making the payload smaller before you worry about aeroacoustic loads on cross-beams.

Is it totally crazy to separate a booster using only rocket power, and to pivot on an attachment point while under power? It meets the simplicity requirement SpaceX seems to prefer, but they are as limited by physics as everyone else. It seems like the stress put on that bottom attachment joint would be incredibly high and it would have to pivot as well.

Quote from: old_sellsword on 07/04/2017 02:02 PMQuote from: GWH on 07/04/2017 02:01 PMQuote from: old_sellsword on 07/04/2017 01:52 PMThis is why they have two pusher mechanisms for each side booster octaweb. They detach and pivot the forward ends away using the N2 ACS, then they detach and push away the octawebs with the two outside octaweb connections (see Lars-J's helpful drawing).Yeah those two struts at the bottom of the wind tunnel model could definitely have a pusher component to them, Would simplify the whole arrangement a lot. Do you know this for fact or are you speculating?I know it for a fact.Sounds to me like this basically answers my original question. Pushers at the bottom and N2 at the top. With the attachment method being at the bottom of the booster they can pivot a bit before the pushers fire. Together with engine gimballing and throttling it sounds like clearing the center core is sorted. Suggestions of using the grid fins seem ridiculous considering they are designed to be used while flying backward.Anyone know if the side boosters will be firing at separation. I'd assume they will be firing 1-3 engines to match acceleration and then for boost back.

Quote from: GWH on 07/04/2017 02:01 PMQuote from: old_sellsword on 07/04/2017 01:52 PMThis is why they have two pusher mechanisms for each side booster octaweb. They detach and pivot the forward ends away using the N2 ACS, then they detach and push away the octawebs with the two outside octaweb connections (see Lars-J's helpful drawing).Yeah those two struts at the bottom of the wind tunnel model could definitely have a pusher component to them, Would simplify the whole arrangement a lot. Do you know this for fact or are you speculating?I know it for a fact.

Quote from: old_sellsword on 07/04/2017 01:52 PMThis is why they have two pusher mechanisms for each side booster octaweb. They detach and pivot the forward ends away using the N2 ACS, then they detach and push away the octawebs with the two outside octaweb connections (see Lars-J's helpful drawing).Yeah those two struts at the bottom of the wind tunnel model could definitely have a pusher component to them, Would simplify the whole arrangement a lot. Do you know this for fact or are you speculating?

This is why they have two pusher mechanisms for each side booster octaweb. They detach and pivot the forward ends away using the N2 ACS, then they detach and push away the octawebs with the two outside octaweb connections (see Lars-J's helpful drawing).

...FWIW - For a flying 1/72 shuttle model...

Well, the Russian R7 and descendants have been sort of doing that since 1957. Although the outer boosters do not pivot in that way, but they sep without any use of rockets or thrusters.