AeroTech Open Thread

[Banzai88]

I have a question for any of you, what length thrust ring on a 75mm motor can be retained securely in an Aeropack motor retainer? I believe the thrust ring is currently 1/2” long. For example, could it be 1/8” longer or 5/8” long overall, and still have enough thread retention?

I just measured that last night. It is currently 1/2" but I'm certain that 5/8" would hold. However, it might be on the edge of what some would feel comfortable with(since we're enculturated to having the cap fully down on the retainer ring), and would probably rule out anyone using some of the Giant Leap slimlines.

Overall, I think that an extra 1/8" is NOT that big of a deal in this case, and is nothing at all when I compare it to my first gen Loki 38mm tubes where an aeropack cap only gets about 2/3rds of a full turn engagement.....cinch it down hand tight and I've never had one come loose.

 

[TBob]

I just measured that last night. It is currently 1/2" but I'm certain that 5/8" would hold. However, it might be on the edge of what some would feel comfortable with(since we're enculturated to having the cap fully down on the retainer ring), and would probably rule out anyone using some of the Giant Leap slimlines.

Overall, I think that an extra 1/8" is NOT that big of a deal in this case, and is nothing at all when I compare it to my first gen Loki 38mm tubes where an aeropack cap only gets about 2/3rds of a full turn engagement.....cinch it down hand tight and I've never had one come loose.

I'd tend to agree that an extra 1/8" isn't a big deal with the standard Aero Pack retainers.
Where it it may be a slightly bigger deal is on the tailcone retainers. Will they still hold the motor in? Yes. But there would be an 1/8" gap where the tailcone retainer cap meets the airframe. Easy enough to adjust those measurements when working thru a new build but a pain to change on an already completed build.

 

[jmasterj]

Sorry for the confusion. I just wanted, as someone advised, to put the question in this, Aerotech Open Thread, forum.

I think the idea was that if you wouldn't accept the evidence presented as sufficient, get an answer directly from Aerotech, rather than from another forum member presenting no evidence (no disrespect to @AndrewGil at all, he just answered the question).

But it seems like you've gotten that too and are ready to launch, so enjoy!

 

[Tractionengines]

I prefer 1 full turn (or close to that) on the retainer cap. Less than that could be a issue when ejection charge (from altimeter) fires pushing engine back. These are coarse threads and are not self locking due to pitch. So at only 2/3 turn it is possible to come off. (Maybe not at time of ejection charge, but if it loosened it slightly, the vibration and shaking of descent could open it.)

If it would be only 2/3 turn, I would back it up with a wrap if foil tape, safety wire, or something else to keep it from "working" loose, from thrashing about.

 

[Syclone]

Where it it may be a slightly bigger deal is on the tailcone retainers. Will they still hold the motor in? Yes. But there would be an 1/8" gap where the tailcone retainer cap meets the airframe.

The tailcone retainer needs a extra ring to fit CTI & AT 75mm motors. If AT were to match the end margin of the snap ring cases, it would fit perfectly without the need for the additional ring.

 

[loopy]

I just measured that last night. It is currently 1/2" but I'm certain that 5/8" would hold. However, it might be on the edge of what some would feel comfortable with(since we're enculturated to having the cap fully down on the retainer ring), and would probably rule out anyone using some of the Giant Leap slimlines.

Overall, I think that an extra 1/8" is NOT that big of a deal in this case, and is nothing at all when I compare it to my first gen Loki 38mm tubes where an aeropack cap only gets about 2/3rds of a full turn engagement.....cinch it down hand tight and I've never had one come loose.

I have a question for any of you, what length thrust ring on a 75mm motor can be retained securely in an Aeropack motor retainer? I believe the thrust ring is currently 1/2” long. For example, could it be 1/8” longer or 5/8” long overall, and still have enough thread retention?

Since it's 75mm, and it's literally just holding the weight of the motor and not having to handle an ejection charge, I would say it would be fine. It's not going to take much stress.

 

[AeroTech]

Just so you understand why I asked, the price of the 75mm fiberglass cases with the machined-in thrust ring became astronomical, so we are opting for straight cases with a 3D printed thrust ring bonded on like the smaller DMS motors (even those are significantly more expensive than before). I'm in the process of engineering them so that they have a decent safety margin, and lengthening them a bit would help.

 

[StreuB1]

Just so you understand why I asked, the price of the 75mm fiberglass cases with the machined-in thrust ring became astronomical, so we are opting for straight cases with a 3D printed thrust ring bonded on like the smaller DMS motors (even those are significantly more expensive than before). I'm in the process of engineering them so that they have a decent safety margin, and lengthening them a bit would help.

Will a 3D printed ring bonded in place be sufficient for a large 75mm load?

Would a simple machined ring with course ID grooving (a rough pass of 10 TPI) be better?

 

[AeroTech]

Will a 3D printed ring bonded in place be sufficient for a large 75mm load?

Would a simple machined ring with course ID grooving (a rough pass of 10 TPI) be better?

We are trying to determine that. We have been running strength tests using our hydraulic press and we are beefing up the design to demonstrate a minimum 100% safety margin.

We have found that a bond to the outside of the case is not sufficient, so we use a tab that is embedded inside the nozzle epoxy for extra strength. The ring could be machined from alternate materials but we are trying not to increase costs greater than they already are.

 

[Tractionengines]

Since it's 75mm, and it's literally just holding the weight of the motor and not having to handle an ejection charge, I would say it would be fine. It's not going to take much stress.

I disagree : While it is NOT firing an ejection charge, the altimeter is. The body tube gets pressurized internally the same on all surfaces. So the same pressure that is acting on the bulkhead of the coupler to deploy the recovery, is also pushing on the frontal area of the motor trying to push it out the rear.
(In the case of MD rocket these are equal. For non-minimum diameter, the force on bulkhead is greater than the motor by the difference in area )

 

[cwbullet]

It has amazed me how much force a 3D-printed bulkhead will take. I suspect the result you get from a ring will surprise your.

 

[AeroTech]

Can someone measure the I.D. of the aft opening of the Aero Pack 75mm retainer cap please?

 

[jmasterj]

@AeroTech, I know you said 29mm RMS Dark Matter won't happen, does that also apply to Metalstorm?

 

[jjwb22101]

Can someone measure the I.D. of the aft opening of the Aero Pack 75mm retainer cap please?

(This is for the flanged retainer) The section that the thrust ring is retained by is 3.270" in diameter and 0.434" deep on mine, and the opening in the screw-on cap is 2.828" in diameter.

 

[cautery]

Y'all might want to consider using a PICTURE to label the dimensions so that no one gets confused....

 

[GalantVR41062]

I have a aeropack tailcone retainer I can measure this weekend and get back to you. I would want the tailcone to be tight to the airframe with the motor in place.

On the bonded thrust ring the motor thrust will be a lot more stress on that joint then the ejection charger.

The surface area on a 75/76mm motor is about 6.83 square inches, so 15psi (could be more or less depending on many factors) in the air frame would be about 102lbs trying to push the motor out.

 

[Theory]

I disagree : While it is NOT firing an ejection charge, the altimeter is. The body tube gets pressurized internally the same on all surfaces. So the same pressure that is acting on the bulkhead of the coupler to deploy the recovery, is also pushing on the frontal area of the motor trying to push it out the rear.
(In the case of MD rocket these are equal. For non-minimum diameter, the force on bulkhead is greater than the motor by the difference in area )

Quoted as this is accurate and important to understand

 

[SDramstad]

Would a tapped forward closure help insure against losing the motor?

 

[rharshberger]

Would a tapped forward closure help insure against losing the motor?

In what way? You can thread a bolt directly into, put a eyebolt in it, even have a bulkhead with a bolt mounted to it similar to the MD retainers from Aeropack.

 

[SDramstad]

In what way? You can thread a bolt directly into, put a eyebolt in it, even have a bulkhead with a bolt mounted to it similar to the MD retainers from Aeropack.

If you are not sure the standard aeropac retainer wont shake loose, if it is also secured at the forward end at least you wont have a spent casing falling out of the sky

 

[rharshberger]

If you are not sure the standard aeropac retainer wont shake loose, if it is also secured at the forward end at least you wont have a spent casing falling out of the sky

Sure, I usually use some form of motor retainer and use the eyebolt on the forward closure as recovery anchor point as its very strong, but you could also use other methods to bolt the closure in from the interior side too.

 

[AeroTech]

(This is for the flanged retainer) The section that the thrust ring is retained by is 3.270" in diameter and 0.434" deep on mine, and the opening in the screw-on cap is 2.828" in diameter.

Thank you!

 

[loopy]

I disagree : While it is NOT firing an ejection charge, the altimeter is. The body tube gets pressurized internally the same on all surfaces. So the same pressure that is acting on the bulkhead of the coupler to deploy the recovery, is also pushing on the frontal area of the motor trying to push it out the rear.
(In the case of MD rocket these are equal. For non-minimum diameter, the force on bulkhead is greater than the motor by the difference in area )

Not as much stress/pressure as if the ejection charge was being fired by the motor itself, though. This is my point. Yes - there is the pressure from the gas filling the body tube, which is nearly immediately relieved by the booster separating from the rest of the rocket. There is significantly more rear-ward force when that gas is being generated by an ejection charge firing in the forward well of the motor case itself. Similar to kickback when firing a gun.

 

[rharshberger]

Not as much stress/pressure as if the ejection charge was being fired by the motor itself, though. This is my point. Yes - there is the pressure from the gas filling the body tube, which is nearly immediately relieved by the booster separating from the rest of the rocket. There is significantly more rear-ward force when that gas is being generated by an ejection charge firing in the forward well of the motor case itself. Similar to kickback when firing a gun.

I don't believe its as much pressure as you think, main reason is that its the same volume of space to be pressurized and the charges are not as tightly contained as the environment inside a gun. Might be worth someone testing out, not sure how the test rig would go but interesting no-less.

 

[SDramstad]

just put a bulkhead just forward of the motor

 

[Titan II]

There is significantly more rear-ward force when that gas is being generated by an ejection charge firing in the forward well of the motor case itself. Similar to kickback when firing a gun.

Not really. Recoil is due to the bullet being fired. The law of physics says the bullet will exert an equal force in the opposite direction. You are not firing an object out of your motor.

 

[loopy]

Not really. Recoil is due to the bullet being fired. The law of physics says the bullet will exert an equal force in the opposite direction. You are not firing an object out of your motor.

The gas would still exert pressure pushing the case backwards against the retainer.

 

[cautery]

Not as much stress/pressure as if the ejection charge was being fired by the motor itself, though. This is my point. Yes - there is the pressure from the gas filling the body tube, which is nearly immediately relieved by the booster separating from the rest of the rocket. There is significantly more rear-ward force when that gas is being generated by an ejection charge firing in the forward well of the motor case itself. Similar to kickback when firing a gun.

No.... Just.... no. Don't make me teach a math/physics seminar here.

I am on vacation!

 

[Wayco]

What happened to the Aerotech thread?
Nevermind, it's just a typical TRF sidetrack, right?

 

[Banzai88]

What happened to the Aerotech thread?
Nevermind, it's just a typical TRF sidetrack, right?

The discussion about ejection charges and body tube pressurization is germane the the overall thread as AT has announced that they MIGHT decide to use a 3D printed thrust ring that the rocketeer MIGHT have to glue themselves onto a DMS motor. Understanding the forces involved and the materials chosen speaks directly to the customer's trust in the choices provided by AT to work properly.