AeroTech aft end closure without thrust ring?

[Chad]

Are there aft end closures for AeroTech RMS hardware without a thrust ring? After googling around a bit i haven't turned any up.

I'm looking at buying my first set of RMS hardware since i've only flown borrowed hardware up to now. I'm building a two stage with a min diameter sustainer and plan to hang the motor out of the bottom to use as a coupler for the ISC. This means there can't be a thrust ring on the sustainer motor which, so far, implies Loki hardware and snap rings. I'd prefer to fly AeroTech reloads since they're the most accessible to me. I'm local to buyrocketmotors.com and can just pick up motors from them to avoid hazmat fees but they do not carry Loki nor CTI.

 

[tfish]

There have been in the past. I have a 98mm rear that is.

Tony

 

[rharshberger]

There are no certified aft closures of the style you are asking about.

 

[Hybrid33]

Modify a std closure
Fly as Research

 

[plugger]

There are no certified aft closures of the style you are asking about.

Rousetech used to make a flush aft closure in 38mm for this exact purpose. I had one, but it was lost with the sustainer when I lost tracking on a J510 sustainer.
What Hybrid33 said, get someone to turn down a standard enclosure to be flush to the casing.

 

[SDramstad]

My understanding is the aft closure keeps compressive loads off of the internal O-rings by transfering the thrust loads directly to the airframe at the aft instead of transferring the force through the body of the motor. By transfering the thrust through the forward closure it can distort the O-rings and cause a failure. Someone posted this some time ago but I dont remember who.

 

[plugger]

My understanding is the aft closure keeps compressive loads off of the internal O-rings by transfering the thrust loads directly to the airframe at the aft instead of transferring the force through the body of the motor. By transfering the thrust through the forward closure it can distort the O-rings and cause a failure. Someone posted this some time ago but I dont remember who.

My experience indicates otherwise. I flew a 38mm MD configuration both in single and two stage configurations with the biggest motor available (J510) and a flush aft enclosure and it wasn't an issue.

 

[waltr]

What about when using a threaded forward closure to secure motor in MD air frames?

 

[tjsnakez]

My understanding is the aft closure keeps compressive loads off of the internal O-rings by transfering the thrust loads directly to the airframe at the aft instead of transferring the force through the body of the motor. By transfering the thrust through the forward closure it can distort the O-rings and cause a failure. Someone posted this some time ago but I dont remember who.

If you use a non floating forward closure (standard) to secure the motor it shouldn’t be an issue.

 

[SDramstad]

What about when using a threaded forward closure to secure motor in MD air frames?

If you install it correctly the motor should screw in far enough that the rear closure comes into contact with the airframe

 

[eggplant]

I've posted this in response to discussions like this one before, but doing some basic calculations shows that exactly how thrust is transferred from the motor really shouldn't impact the operation of the motor as a system:

For a hypothetical 3" motor (2.75" ID) running at 1000 PSI you'd have about pi*(2.75/2)^2 * 1000 ~= 6000 lbf of force keeping the FC against the retention ring or snap ring. I don't care how long your 3" motor is, it isn't producing close to 6000 lbf of thrust. You see the same result for any diameter, where the motor would have to be quite high thrust for the diameter and running at a low chamber pressure for these figures to get at all close.

I originally posted this in a thread about snap ring motors, which have a "floating" closure design, like larger CTI motors and AT ones if using the RAS. If you are using a standard AT hardware set where the threads are on the closures themselves, then it is even less likely to cause problems because the closures cannot move by more than the "backlash" in the threads.

I'm pretty sure this is basically an old wives' tale that probably showed up after someone had a motor fail for another reason.

 

[Titan II]

If you install it correctly the motor should screw in far enough that the rear closure comes into contact with the airframe

Not sure what you are saying. He is using the motor as the interstage coupler. The thrust ring OD cannot exceed the motor OD so it will slide into the booster.

 

[Steve Shannon]

My understanding is the aft closure keeps compressive loads off of the internal O-rings by transfering the thrust loads directly to the airframe at the aft instead of transferring the force through the body of the motor. By transfering the thrust through the forward closure it can distort the O-rings and cause a failure. Someone posted this some time ago but I dont remember who.

If that was a thing I think someone would have warned us. But it’s not. Thrust doesn’t push the aft closure forward. Simply put, pressure inside the motor presses against the rear facing surfaces of the forward closure and the forward facing surfaces of the nozzle and aft closure. The rear facing surfaces have a greater area than the forward facing surfaces which results in more forward force than rearward, causing forward motion.

 

[AlexBruccoleri]

If that was a thing I think someone would have warned us. But it’s not. Thrust doesn’t push the aft closure forward. Simply put, pressure inside the motor presses against the rear facing surfaces of the forward closure and the forward facing surfaces of the nozzle and aft closure. The rear facing surfaces have a greater area than the forward facing surfaces which results in more forward force than rearward, causing forward motion.

A good few pages to read for more details are 1-4 of this lecture. https://ocw.mit.edu/courses/16-512-rocket-propulsion-fall-2005/resources/lecture_2/

 

[mholmes]

I have some questions, and this old thread seemed like the best place to start. I take it as a given that the entire motor, when buttoned up with closures on both ends, acts like a monolithic thrust source. One could tap into that thrust at the forward or aft end, e.g. with the female threaded connection at the forward end or the thrust ring at the aft end (excepting the floating forward closure; that would probably be a bad day).

However, even so…

1) In a minimum diameter flush aft closure setup, is the thrust really transferred entirely through the threaded rod to the forward bulkhead retainer? I can think of no other options. In which case: doesn’t the much farther forward thrust point change the flight dynamics? The thrust point would be much closer to the COM, and thus torque caused by slight axial misalignment between the thrust and COM would be amplified by thrusting much closer to the COM, right? When trying to balance a broom on its end, the length of the shaft is crucial…

2) If this is a perfectly adequate method, e.g. pushing on the forward threaded rod only, with no thrust ring, why isn’t it used as a matter of course? It would entail only one dual-purpose thrust + retention connection, rather than a thrust ring aft and a motor retainer forward. Does the flush aft closure setup have drawbacks or negatives in comparison to thrusting from the aft-most edge of the airframe tube? Ya, you can’t use motor ejection, but aside from that?

It seems to me like the forward threaded rod thrust path is not as robust as the aft thrust ring and might affect stability to boot, but there are a fair number of informed comments suggesting that it’s perfectly fine. Thx

 

[waltr]

How the thrust is transferred to the rocket from the motor does NOT effect any flight dynamics. The entire rocket and motor acts as a single rigid body.

What is "COM"?

This forward threaded rod to bulkhead is a bit harder to build plus the condition that motor ejection can NOT be used is the reason it is not a common method. It does then require electronics to eject and deploy recovery.

 

[Titan II]

What is "COM"?

My guess is center of mass.

 

[waltr]

Ahh, Center of Mass which we typically cal CG (Center of Gravity) which is the same thing.

The motor should be retainer radially it entire length. Therefore, the Thrust is at the NOZZLE.

 

[SolarYellow]

My understanding is the aft closure keeps compressive loads off of the internal O-rings by transfering the thrust loads directly to the airframe at the aft instead of transferring the force through the body of the motor. By transfering the thrust through the forward closure it can distort the O-rings and cause a failure. Someone posted this some time ago but I dont remember who.

No. During the assembly process, the forward closure (at least of an AT case) is bottomed against the case, so it's metal-on-metal. Transferring the thrust load to the airframe through the forward closure cannot have any effect on any o-ring. The preload of certain O-rings is set by the installation of the aft closure, but the combination of the o-ring compression and the internal pressure pushing rearward on the aft closure dwarfs the inertia of the rocket pushing rearward on the closure ring.

CTI motors are a little different, where the forward closures (delay/eject modules) are captured internally by the rolled feature at the end of the case. However, the delay/eject module seats against the liner of the reload with a shoulder, so pushing on the delay/eject module just increases this seating force without causing any relative movement. The sealing o-ring will not see any movement. With combustion pressure present, the force pressing upward on the delay/eject module will be much greater than any force the rocket's inertia could apply to the delay/eject module.

Because they are phenolic, if you tried to thread into the CTI delay/eject module and use a long threaded fastener to transmit thrust force as discussed above for AT threaded forward closures, you could possibly stab the fastener down into or through the delay grain and cause a rupture. But if you had a wider thrust feature that would at least capture the flat perimeter of the ejection charge well (better yet, capture the rolled rim of the case), you should be able to transfer force through the front of a CTI without any issue.

 

[3stoogesrocketry]

My understanding is the aft closure keeps compressive loads off of the internal O-rings by transfering the thrust loads directly to the airframe at the aft instead of transferring the force through the body of the motor. By transfering the thrust through the forward closure it can distort the O-rings and cause a failure. Someone posted this some time ago but I dont remember who.

Do threads conpress inside the case?

 

[Rschub]

Do threads conpress inside the case?

Metal acts as a spring, so even one ounce of pressure will compress the case, even if minutely.

 

[3stoogesrocketry]

Metal acts as a spring, so even one ounce of pressure will compress the case, even if minutely.

On a atomic level everything moves. Not what I'm asking and you know it.

 

[Rschub]

I don't know what you are asking, so I took the question literally.

 

[bobbyg23]

Are there aft end closures for AeroTech RMS hardware without a thrust ring? After googling around a bit i haven't turned any up.

I'm looking at buying my first set of RMS hardware since i've only flown borrowed hardware up to now. I'm building a two stage with a min diameter sustainer and plan to hang the motor out of the bottom to use as a coupler for the ISC. This means there can't be a thrust ring on the sustainer motor which, so far, implies Loki hardware and snap rings. I'd prefer to fly AeroTech reloads since they're the most accessible to me. I'm local to buyrocketmotors.com and can just pick up motors from them to avoid hazmat fees but they do not carry Loki nor CTI.

Like this?

 

[mholmes]

there are a few separate convos here but I'll stick with just my question...

It's not as simple as saying the thrust is at the nozzle; thrust is against the inside forward chamber of the motor. If thrust was just at the nozzle, it would be pushed up into the chamber. The nozzle pushes on the chamber pressure, but does not exceed it; the chamber pressure pushes on the interior forward wall of the chamber. When a balloon ejects air, the thrust is not solely on the rim of the "nozzle", and is not transferred through the wall of the balloon (which would buckle the balloon); the net unbalanced force is on the inside of the forward wall of the balloon as the mass escapes the "nozzle". So too with a rocket motor.

However, that debate isn't relevant to the question. I'm granting (granted, granting, continuing to grant) that we can treat the entire motor as a solid unit, so it doesn't matter where the thrust is generated on the motor itself. Imagine the entire motor case is being accelerated upwards, without regard to the source of the acceleration. A magical moving cylinder. I am inquiring about the transfer of thrust from the magical moving cylinder to the airframe. In one scenario, the motor pushes on a forward bulkhead/retainer; in the other scenario it pushes on the bottom end of the airframe. Consider the thought experiment in which the threaded rod is centered in the motor, but is dramatically offset to one side on the bulkhead. Imagine it all the way at the edge, so the threaded rod is on a stupid diagonal. My picture is that the motor pushing on one side of the bulkhead would lead to a torque around the center of mass. In practice, the threaded rod is nicely lined up, so this wouldn't be a problem. But my thought experiment is intended to illustrate what I believe is a difference between the two scenarios, e.g. thrusting on the forward bulkhead vs thrusting on the aft airframe edge.

Now, both the motor and the bulkhead are constrained by the airframe to axial alignment, so maybe even a stupidly diagnoal rod can't exert a torque; perhaps that is the answer to my thought experiment. But maybe it would exert a torque, if the rod were stupidly misaligned. I can't sort that part out in my head.

Obviously it works, because people do it, as reported multiple times in this thread. Not debating that, just trying to understand the details of the load path.

Also, I guess it requires a significantly stronger forward bulkhead/retainer when it is to be used as a thrust path to handle 100's-1000's of N's, than if it only needed to handle the force of retaining the motor from dropping out. Do people take extra efforts to strengthen the forward bulkhead when it's used for the thrust path?

 

[SolarYellow]

Go tear apart a piston engine and come back when you've thought about it more.

 

[waltr]

Estes rockets have the Thrust ring above the motor.
AT & CTI have the thrust ring at the bottom of the motor/Body tube.

Both accelerate the rocket the same .

 

[mholmes]

Go tear apart a piston engine and come back when you've thought about it more.

been there, done that. Don't be rude, I'm asking reasonable questions.

 

[Rschub]

Loads always follow the stiffest path, if there is a choice. If the stiffest path fails, alternate load paths may take up the load, this would be a fault tolerant design.
The Jesus bolt is an example of a non fault tolerant design. In this example so would be the threaded rod.

The threaded rod is considered a column in stress analysis, it is very stiff, but can buckle if not sized or supported correctly. There are things like end conditions that effect the load at which the column will buckle, but for a reasonable length and diameter of threaded rod and a motor constrained by the motor tube this shouldn't come into play. The load will move through the rod and be applied to whatever the threaded rod is attached to. Supporting the rod in the center with a bulkhead that stops deflection [bowing] would be in order for very long rod lengths.

The rocket body is generally a tube, which is also a column. For extremely high thrusts, people have allegedly failed the body column from buckling when pushing on the column from the motor end. I say allegedly because no one instruments' their rocket with strain gauges, but it is quite plausible.

If you push on the rocket body from nearer to the center, the "back" half of the rocket body is loaded in tension, which does not have a buckling problem.

The "front" half of the rocket body will be much shorter, which will increase its buckling resistance greatly for a given load.

This is the theory, in practice, I believe that most failures occur at the joints in the body tube for the nose cone or electronics bay, for reasons more complicated than simple buckling.

 

[mholmes]

Thank you, I had not thought of buckling. Working through the euler buckling calculation for a 5/16-18 threaded rod, using an I600R motor with peak thrust of 812 N and a modulus for the steel of 200 GPa, the slenderness ratio works out to 8815, which means that the rod would have to be ~93 ft long before worrying about buckling from that load. That's a sanity check that the threaded rod has enormous margin for the buckling question. Buckling of the tube is a different question. I'll look at that when I get a chance, just to do the exercise, though as you pointed out the buckling of the tube is exacerbated not alleviated by pushing from the very aft edge.