Motor Tube for CTI Pro150 or Larger OD Loki?

[kclo4]

My university is cooking up a very large rocket in the mechanical engineering department.
Our plan so far is to use a custom Loki motor, or possibly a CTI O8000 as a backup.
The Pro150 hardware is a lie, and is actually 160mm OD(6.33"), and Loki can turn down the motor tube to 6" but if it helps schedule or cost for us we could opt to not have it turned down. I am guessing(have to confirm with Scott) that his OD is the similar if he us using the same aluminum stock.

If we go this route, what options are there for commercial motor/airframe tubing and couplers? The central motor/airframe tube of our rocket is going to be rather long so we definitely don't have schedule to manufacture 40+ feet of tubes and couplers.

What is available and what have people used for the big projects that have flown on commercial 6" hardware?

 

[Bat-mite]

No experience here. But for any CTI questions of this magnitude, get a hold of Robert DeHate of AMW-ProX: 978-766-9271.

 

[Nytrunner]

I know Public missiles sells 8 and 11 inch Phenolic airframe/couplers. There's probably 8" fiberglass airframes also (maybe carbon fiber).
If you get the main airframe commercially, you could try making your own motor tube to fit the Pro150 and CNC custom centering rings to hold it.

Update: Madcow sells large diameter G12 tubing.

 

[ChrisAttebery]

On a project like this I'd just use centering rings sized for the motor. You could have them cut so that they have stringers between them if you're worried about a single ring transferring thrust to the airframe.

 

[tfish]

kclo4, I was part of the first AeroPac 100K projects. Our Goal was to use the CTI Pro 1500 O5100 motors in the booster. We at the time 2006 (?) could not find any off the shelf body tubes. We ended up making our own. It's been a long time and the only info still around is here https://web.archive.org/web/20090122063148/https://www.to100k.org/.

There is off the shelf 6" body tubes that could work. Depending on how fast your going to be pushing things. Off the shelf stuff can also be strengthened with additional layers of materials.

You can spec your 6" Loki motor to fit inside Brand X body tube.

Curious, what school? And what's the goal?

Tony

 

[kclo4]

I am with the University of New Mexico ME department. I don't want to spoil too many surprises without permission from the team but we will be flying a very large project using lightweight build techniques. It's going to be well over 40 feet tall, 40" diameter and should only weigh in around 200lbs.
Should only hit 3-4K feet.

The plan so far is to have Loki turn the motor to fit in standard 6" airframe tube, but it sounded like schedule was going to be tight so I wanted to make sure the team knew all the options.

The motor tube will run the length of the rocket with a lightweight structure around it to build up the bulk. We could use 7.5" airframe and couple the motor down based on whatever we fly. Centering rings and a thrust plate won't take long to make.Was just trying to see if we could avoid that. I'll present that to the team to see what they think.

kclo4, I was part of the first AeroPac 100K projects. Our Goal was to use the CTI Pro 1500 O5100 motors in the booster. We at the time 2006 (?) could not find any off the shelf body tubes. We ended up making our own. It's been a long time and the only info still around is here https://web.archive.org/web/20090122063148/https://www.to100k.org/.

There is off the shelf 6" body tubes that could work. Depending on how fast your going to be pushing things. Off the shelf stuff can also be strengthened with additional layers of materials.

You can spec your 6" Loki motor to fit inside Brand X body tube.

Curious, what school? And what's the goal?

Tony

 

[jimzcatz]

You don't need a body tube, as previously stated use centering rings.

 

[pondman]

You don't need a body tube, as previously stated use centering rings.

+2 on the centering rings. Doing the same with a Gorilla 152mm case.

 

[Handeman]

You don't need a body tube, as previously stated use centering rings.

+2 on the centering rings. Doing the same with a Gorilla 152mm case.

Exactly! All you need is the CRs. I saw that when a club member brought his L3 cert rocket out for a 10th anniversary flight. He stuck the fins in slots, reached up the motor hole and bolted them to the inside braces, then shoved the motor up the hole that consisted of two CRs to keep it centered. When you can remove the fins like that, it makes for easy transport and storage.

 

[markkoelsch]

Assuming it is not minimum diameter I would skip the motor mount tube. Cut the centering ring hole to match motor od.

 

[bobkrech]

As others have mentioned, no motor tube is not necessary or and not necessarily desired in a minimum weight rocket.

The thrust force is transferred from the motor thrust ring to the lower centering ring then to the airframe. You will want a second centering ring forward to prevent the motor from twisting. You can make an assembly of 2 centering rings attached to each other with longitudinal ribs to transfer some of the thrust load to the second ring. You can also use radial ribs on the forward side of the rings to stiffen the rings and minimize the total ring weight.

 

[bobkrech]

I am with the University of New Mexico ME department. I don't want to spoil too many surprises without permission from the team but we will be flying a very large project using lightweight build techniques. It's going to be well over 40 feet tall, 40" diameter and should only weigh in around 200lbs.
Should only hit 3-4K feet.

The plan so far is to have Loki turn the motor to fit in standard 6" airframe tube, but it sounded like schedule was going to be tight so I wanted to make sure the team knew all the options.

The motor tube will run the length of the rocket with a lightweight structure around it to build up the bulk. We could use 7.5" airframe and couple the motor down based on whatever we fly. Centering rings and a thrust plate won't take long to make.Was just trying to see if we could avoid that. I'll present that to the team to see what they think.

You either have an unlimited budget or your payote is extremely potent.

There is no way a rocket that is 40' high and 40" diameter is going to weigh only 200 pounds! A CTI Pro150 motor weighs between 57 to 72 pounds. A light Loc Warlok weighs 100 oz built stock with tight bond glue. Multiply by a factor of 10 for your rockets length and by 5 for your diameter and you get 5000 oz = 312.5 pounds without motor. Regardless of materials, I can not see a pad weight less than 400 pounds and more likely twice that to account for a thickness increase to maintain the required strength and stiffness.

You should get contact John DeMar in Las Cruces to review your preliminary design. He is a L3 who works for the NM Space launch facility and facilitates the student launches there.

 

[cbrarick]

John:

thin-wall carbon and a carbon-foam nose cone gets you there. Not gonna comment on the strength of such a set of tubes. I would hate to do the actual calculations.......40 inch diameter gives you 1256 square inches of circle, much more when you calculate the area of a complex nose cone...which translates into a lot of surface area for compression forces on the body tube..

 

[bobkrech]

John:

thin-wall carbon and a carbon-foam nose cone gets you there. Not gonna comment on the strength of such a set of tubes. I would hate to do the actual calculations.......40 inch diameter gives you 1256 square inches of circle, much more when you calculate the area of a complex nose cone...which translates into a lot of surface area for compression forces on the body tube..

Do the math.

All by itself a 1 mm wall (0.040") x 40" diameter x 40' long carbon fiber tube weighs 200 pounds! That's without bulkheads, rings, fins, stiffeners, recovery system, motor, or anything else.

Not to mention the cost of fabrication. A professional tube would have 120 pounds of carbon fiber or cloth and 80 pounds of resin. Typical carbon cost is $25/lb for $3000 and $10 pound for laminating resin for $800.

Now you have to fabricate. Filament wind or uni lay-up? Either way it's thousands of dollars. You can wind a 30'+ tube but how do you transport it? Multiple pieces require couplers and now there's more weight can cost.

No way on weight simply due to costs. Doubt you can go thinner due to stiffness issues and fabrication issues.

And yes, I actually have fabricated thin CF tubes and also purchased them. Wonderful stuff but very pricey to fabricate or purchase.

 

[dhbarr]

Thunda V2 was aluminum frame, foam, & glass IIRC. ~200kg from memory?

200lbs not incl. motor weight sounds not completely insane, but certainly challenging.

 

[kclo4]

We are using an aircraft type build structure with spruce runners and a cloth covering. My instructor has done a number of half scale V2's using the same build techniques that were about 50 pounds with a 75/6400 case(M1315 I think but have to double check). We should be 1/4 the weight of the full scale V2 flight. My projected 200lbs was without motor, so our flight weight will be around 250-270lbs. I will have better number when CAD gets closer but so far I think we will hit that.
Our budget is fair enough, but no carbon are aluminum structure required.

We are trying to get our motor to run right around the 6:1 or 7:1 to keep the speeds and air pressure lower on the airframe so hoping Scott can work us up something in the 1400lbs max thrust range. Right now looking at around a 300 mph max speed which is still a little higher than I would like but we are still in the initial stages.

The team pretty much settled on using 7.5" tubing for the inner structure so we can adapt to any motor and give us a little more rigidity.

 

[pythonrock]

If you are using that skin over frame building as in model aircraft, you might consider using heat shrink film instead of cloth. It's easier to work with and lighter and might be appropriate for at least parts of the rocket and fins. Also, spruce as a "natural" product may have significant variability and weak spots. I would consider using thin wall glass or carbon tubes for your stringers. more rigid and far more consistent.

 

[kclo4]

I actually don't recall the name of the product we plan to use for the skin but I think it is full size aircraft grade stuff not monokote for model planes. It does heat shrink though I believe.

I have thought about using small fiberglass tubes for runners but not sure if that would help weight wise. The spruce runners worked well for for the half scale V2, we are just going to add more. With something like 46 of them along the circumference and fixed by a number of light weight bulkheads along the airframe it's actually a surprisingly rigid structure. Remember they don't call it aircraft spruce for nothing, they make planes for people out of the stuff.

 

[Maxwelljets]

Did you ever take a look at Loki's 152mm brochure? It gives you the casing dimensions. Loki's casing is exactly 6" OD, which should be able to fit commercial 6" airframe.

I am a bit concerned here. I know aircraft spruce is strong enough to be used in human rated aircraft, but keep in mind that those aircraft go much slower than the rocket you're building. I don't know your top speed, but I'd expect it to be transonic- at least.

 

[ECayemberg]

Did you ever take a look at Loki's 152mm brochure? It gives you the casing dimensions. Loki's casing is exactly 6" OD, which should be able to fit commercial 6" airframe.

I am a bit concerned here. I know aircraft spruce is strong enough to be used in human rated aircraft, but keep in mind that those aircraft go much slower than the rocket you're building. I don't know your top speed, but I'd expect it to be transonic- at least.

My Loki 152mm hardware does not fit commercial filament wound glass, nor PML Phenolic airframe. I know for a fact the Gorilla 152 doesn't fit the glass tube either...ask Crazy Jim and the Aussies! As others suggest, for 6" hardware most just use rings sized to fit the motor and not a motor tube. Simple and effective!

 

[Maxwelljets]

My Loki 152mm hardware does not fit commercial filament wound glass, nor PML Phenolic airframe. I know for a fact the Gorilla 152 doesn't fit the glass tube either...ask Crazy Jim and the Aussies! As others suggest, for 6" hardware most just use rings sized to fit the motor and not a motor tube. Simple and effective!

Out of curiosity, how far off is the case from fitting? My university is also looking into 6" casings (we are planning to make our own, but Loki would be our backup if we are unable to finish in time). I don't want to give much away, but it will suffice to say that for our design, the motor must be able to fit into a tube.

 

[ECayemberg]

Out of curiosity, how far off is the case from fitting? My university is also looking into 6" casings (we are planning to make our own, but Loki would be our backup if we are unable to finish in time). I don't want to give much away, but it will suffice to say that for our design, the motor must be able to fit into a tube.

It is close. In most cases, a dedicated attempt at sanding the ID will get the casing to fit. This is not to say its encouraged nor easy....it will take elbow grease and a lot of flapper discs! One can also make your own motor tubes...wrap the motor casing in a layer of release film and wrap it in glass cloth and an epoxy that can take some heat.

 

[dixontj93060]

Don't use motor tubes. Just use centering rings only.

 

[Maxwelljets]

Centering rings is hard with minimum diameter...

 

[dixontj93060]

Centering rings is hard with minimum diameter...

Oh, my bad.

 

[Len B]

Oh, my bad.

OP did not say minimum diameter and part way through the thread describes a skin on runners design.

 

[kclo4]

The rocket is so big and draggy that even with an O8000 it will only hit about 300mph. If Loki can get us closer to our thrust goals(1400-1500lbs) it might be as slow as 250mph. We are still trying to dial in our final weight estimation so that we can get the motor to run around 6-7:1 thrust to weight.

I think the motor mount and retention is pretty well sorted at this point. The class is working out some other matters but I will start a new thread for those.

Did you ever take a look at Loki's 152mm brochure? It gives you the casing dimensions. Loki's casing is exactly 6" OD, which should be able to fit commercial 6" airframe.

I am a bit concerned here. I know aircraft spruce is strong enough to be used in human rated aircraft, but keep in mind that those aircraft go much slower than the rocket you're building. I don't know your top speed, but I'd expect it to be transonic- at least.

 

[bobkrech]

Warning. Do not cut your thrust to weight margins to anywhere near the minimum because you will almost always have a crosswind at your launch site. The rule of thumb is a recommended minimum T/W ratio of 5:1 for a stable model rocket lift-off from a standard launch rod/rail length with fins that are aerodynamically effective at 25 mph in a cross wind of 5 mph or less.

An underpowered 40' rocket will have cross wind issues and will severely weathercock. The practical rule of thumb for any rocket is that the T/W ratio of the rocket-motor combination equals the maximum cross wind velocity in mph that is allows you rocket to maintain a less than 15 degree angle of attack at rail separation velocity. If the angle of attack is more than 15 degrees, the fins are not aerodynamically effective and the rocket will commence a gravity turn from which is may not recover.

For example a rocket launched in a 10 mph cross wind should have leaving velocity of 50 mph to maintain a 15 degree of angle of attack when launched from a standard length guidance rail/rod. If the T/W < 5 the guidance length must be increased to insure this leaving velocity. Failure to follow this rule of thumb usually produces a land shark flight. I personally witness nearly a dozen Estes Maxi-V2s suffer this fate when launched on a E9 motor from a Maxi-Pad length rod.....

I think you need to make sure you have enough thrust in you motor. While drag increases with V^2, the max velocity is proportional to V^3 (the power of the motor). I have attached a motor selection file generated from thrustcurve.org for you rocket based on a 200 pound dry weight (this I believe is optimistic). You really need a Pro150 O8000 equivalent for you rocket. If your rocket is heavier, the extra thrust is important to keep the pointy end up.

View attachment Forrty_Footer-results-all.xls

 

[WILDMANRS]

if you are going to use only rings keep in mind the CTI case is not strait on the outside. You will need to put your rings on a landing of the case.
https://www.pro38.com/pdfs/Pro150_dimensions.pdf
We can get this size MMT if you want it.

 

[markkoelsch]

Centering rings is hard with minimum diameter...

Pick the motor. Get exact dimensions. Roll custom tube to those dimensions. Custom nose cone.