# RMS vs SU

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#### rocketsonly

##### Well-Known Member
Are there any disadvantages in using a reloadable motor system (RMS) as opposed to a single-use (SU) motor? Reliability? Difficulty in assembling motors? My TARC team is considering to use the RMS because of the lower cost.
Thanks.

#### solrules

##### Well-Known Member
If you loose the casing (read: you can't find your rocket), you are out quite a bit of cash. It takes more time to assemble, but otherwise they are the same as SU motors, except cheaper.

#### DynaSoar

##### Well-Known Member
Originally posted by solrules
If you loose the casing (read: you can't find your rocket), you are out quite a bit of cash. It takes more time to assemble, but otherwise they are the same as SU motors, except cheaper.
Zackly. For RMS to actually be cheaper, you have to have flown and recovered and reused the same motor enougn times.

Here's a rough guesstimate:

AT 29/40-120 casing $35.00 AT F22 loads$6 each.

AT F20 S/U $10 each It takes 10 successful RMS motor recoveries (regardless of flight success) to cost less ($95.00) than S/U motors ($100.00) to fly the same amount. Even after this point, if you lose the RMS motor it's a major replacement cost. You wouldn't want to go into this with the possibility of losing a motor and having a set back. You'd want to start with two at least. I'd start with 3. That ups the break-even point to 23 flights. This does allow for potential loss of up to 2 motors during this time. With RMS motors, even the most experienced can make a mistake putting it together causing a CATO or at least a failed flight attempt. It is far less likely for this to happen with S/U motors. It may cost more to use S/U in the longer run. Considering that it reduces the chances of failure, I'd consider the extra cost to be insurance premiums. It might be easier to pay more in the long run by buying fewer at a time rather than coming up with the big chunk up front. (It's a lot easier to ride the bus 10,00 times than to buy a$5,000 car).

If it's for the TARC team, consider contacting a motor seller and asking for a bulk discount based on buying 20 or whatever 2 packs of Econojets over the next year. They might even send you them in bulk based on payment over 90 days or something. If Rockeguts is doing well, this might be an option.

#### Missileman

##### Well-Known Member
RMS has certain other advantages, IF you don't lose you case.
Wider range of motors
Built in thrust ring (aft closure)
More knowledge learned about how motors work and
a certain satifaction of putting your own motor together.
You will of course have to fly about 4 or so flights to break even on cost, after that you are saving money with each flight.
EDIT: I was off on number of flights to break even but that depends on the motor. The larger the motor the faster the break even point.

#### solrules

##### Well-Known Member
Oh, here is a disadvantage: cleaning. You have to clean your hardware after every flight, which is time consuming and messy.

#### rocketsonly

##### Well-Known Member
Originally posted by DynaSoar
Zackly. For RMS to actually be cheaper, you have to have flown and recovered and reused the same motor enougn times.

Here's a rough guesstimate:

AT 29/40-120 casing $35.00 AT F22 loads$6 each.

AT F20 S/U $10 each It takes 10 successful RMS motor recoveries (regardless of flight success) to cost less ($95.00) than S/U motors ($100.00) to fly the same amount. Even after this point, if you lose the RMS motor it's a major replacement cost. You wouldn't want to go into this with the possibility of losing a motor and having a set back. You'd want to start with two at least. I'd start with 3. That ups the break-even point to 23 flights. This does allow for potential loss of up to 2 motors during this time. With RMS motors, even the most experienced can make a mistake putting it together causing a CATO or at least a failed flight attempt. It is far less likely for this to happen with S/U motors. It may cost more to use S/U in the longer run. Considering that it reduces the chances of failure, I'd consider the extra cost to be insurance premiums. It might be easier to pay more in the long run by buying fewer at a time rather than coming up with the big chunk up front. (It's a lot easier to ride the bus 10,00 times than to buy a$5,000 car).

If it's for the TARC team, consider contacting a motor seller and asking for a bulk discount based on buying 20 or whatever 2 packs of Econojets over the next year. They might even send you them in bulk based on payment over 90 days or something. If Rockeguts is doing well, this might be an option.
We just purchased a 29/40-120 case for $35, and are about to get 23x G64-(x) reloads for a little less than$10 each (complimentary of our sponsor). A G80 is about $17 each, so we're saving about$7 per flight! Five flights will payback for the motor!

EDIT: motor case

#### n3tjm

##### Papa Elf
RMS motors offer me more security... I can visually inspect each component... which reduces the chance of a cato.

Since I started using RMS in 1995, I have called Aerotech requesting replacement parts for various problems... voids in propellant usually. One time the reload had the wrong nozzle. These problems you can solve before you fly it. Disposable motors... you'd never know.

I like RMS motors A LOT more then disposables... They ARE more work, but to me, it is worth it... Then again... I fly A LOT!!!

Disposables do have their place though... high altitude, where you can't garuntee finding it again.

#### gerbs4me

##### Well-Known Member
I agree with Doug
its messy but worth it

#### DynaSoar

##### Well-Known Member
Originally posted by n3tjm
RMS motors offer me more security... I can visually inspect each component... which reduces the chance of a cato.

Since I started using RMS in 1995, I have called Aerotech requesting replacement parts for various problems... voids in propellant usually. One time the reload had the wrong nozzle. These problems you can solve before you fly it. Disposable motors... you'd never know.

Visually inspecting each piece reduces the chance of a CATO compared to using an RMS and not doing this. It is definitely a fact that more people have more RMS motors fail than S/U motors, and the problems are most often their fault. Any maunfacturing process as controlled and regulated as motor making is going to produce very consistent results as compared to anything that involves people in the process. For sport flying, RMS is cheaper over the long run, more fun for those who enjoy this part, and more educational. For consistency of results and lowest possible failure rate, S/U is better. If it's for a competition which requires flying the same bird over and over and getting enough data to predict results and then fly it to the specified result and not CATO, as I said, S/U motors are cheap insurance.

#### n3tjm

##### Papa Elf
Sorry. I was speaking from an experienced rocketeer P.O.V.

I have seen more disposable motors fail then RMS motors... and you are right; whith RMS failures, many are user error failures. However... many people fail to read the directions, and the most common user error failure is the delay element... Also... every failure that can happen in a RMS, can also happen in a disposable...

These failures I seen in both include:

Blown nozzles, case burn through, ejection not functioning, thrusting from both ends, etc.

There is another major benifit to RMS... They don't detonate. They are designed to blow the nozzle or closure if they overpreasurize. Disposables blow. I seen an Initiator loose its tail on a G80... my G-Froce and Arcas both got damaged motor mounts from Econojet Catos...

#### Stymye

##### Well-Known Member
I've had bad luck , I've had far more SU failures than RMS.
but atleast they were replaced by Aerotech and usually with a 2 pack.

#### Missileman

##### Well-Known Member
I have only had 1 CATO and that was a SU motor.

#### hokkyokusei

##### Well-Known Member
Cesaroni ProXX hardware mitigates a few of the drawbacks of RMS in general, in that they are very easy to put together, and the clean up is minimal.

Of course, there's still the investment in the hardware, and, the smaller size reloads tend to be relatively expensive.

#### loopy

##### Well-Known Member
I love RMS! Wider variety of motors, and I feel more "in control" of the motor. This way I can be (more) reasonably sure that if it fails, I have no one to blame but me.

As for cleanup, it's not that bad. As soon as I get it back to my prep table, let it sit for about 5 minutes, then take the closures off, dump it out in the trash, then hit it with some baby wipes. Takes about 5 minutes,

I'd suggest having an experienced person help you with the first one or two. After that, you'll never forget how to do it - it will become second hand.

The other reason I went RMS (aside from cost considerations) is the fact that at some point, I want to get into high power rocketry, where the cost benefits of RMS are MUCH more immediate. I wanted experience with smaller motors/rockets so that my L1 flight wasn't my first reload. Too much risk in that.

Loopy

#### Chilly

##### Well-Known Member
Well, if you're talking high power there's no question RMS is the way to go. For mid-power it depends. I don't get to fly that often, therefore I like to squeeze as many launches into a day as possible. Using RMS makes that harder. On the other hand, I can take smaller rockets to our neighborhood's soccer field and put them up on E & F reloads with a decent chance of getting them back. That helps scratch the AP itch between Tripoli launches.

##### Well-Known Member
The others have already answered about the relative cost, cleaning time/hassle, wide variety of reloads, etc.

I don't think anyone has mentioned the following quoted directly from the TARC handbook:

motors (and "G" power class motors of any kind) are not available for sale to persons under age 18, per
U.S. Consumer Products Safety Commission regulations. Also, the metal casings that reloadable motors
use are quite expensive. But if the performance of these types of model rocket motor happens to be
exactly what you need for your design, your supervising teacher/adult advisor can purchase them and
supervise your use of them.

COMPOSITE-propellant motors, approved only for single-stage models or
for the bottom (booster) stage of two-stage models.

You probably knew this already, but others may not have read all the rules.

Originally posted by rocketsonly
Are there any disadvantages in using a reloadable motor system (RMS) as opposed to a single-use (SU) motor? Reliability? Difficulty in assembling motors? My TARC team is considering to use the RMS because of the lower cost.
Thanks.

#### n3tjm

##### Papa Elf
Personally, I don't like the ProXX motors. The only propellant formula that I would like to use is their Smoky Sam motors, yet ever single one of their Smokey Sam loads requires an LEUP to purchase... and so far I have not found someone willing to part with one at the field...

And until you get up to the bigger motors.... the costs are way to expensive....

at one launch I was looking to buy a motor for my EZI to give my altimeter a test ride. I had two choices. An Aerotech I357 (3 grain), or a Pro38 H153 (2 Grain). Both motors prices were pretty much the same; I would of gone with the I357... except... I did not have a case, and no one brought one I could borrow... So I paid a 3 grain price for a 2 grain motor

#### rocketsonly

##### Well-Known Member
Originally posted by shreadvector
The others have already answered about the relative cost, cleaning time/hassle, wide variety of reloads, etc.

I don't think anyone has mentioned the following quoted directly from the TARC handbook:

motors (and "G" power class motors of any kind) are not available for sale to persons under age 18, per
U.S. Consumer Products Safety Commission regulations. Also, the metal casings that reloadable motors
use are quite expensive. But if the performance of these types of model rocket motor happens to be
exactly what you need for your design, your supervising teacher/adult advisor can purchase them and
supervise your use of them.

COMPOSITE-propellant motors, approved only for single-stage models or
for the bottom (booster) stage of two-stage models.

You probably knew this already, but others may not have read all the rules.
Yup, I read the rules before they were posted on the rocketcontest.com home page

I got my parents to pay for it.

#### LMazza

##### Well-Known Member
Since I bought some custom centering rings from your team I should probably weigh in. Unless there is a specific SU motor that you need because of its thrust characteristics I would go with reloadables. It seems you'll be flying a whole bunch so you will save money. Good luck!

#### Rick James

##### Well-Known Member
I've been thinking about a cluster of 3 29mm... I could probably borrow the casings I would need for the other two, but I was also thinking of going with SU.

But it seems I heard something about the igniting reliability of SUs not as good as RMS. Anyone else have this experience?

#### n3tjm

##### Papa Elf
Part of the problem is the case design. I don't think the RMS 29/40-120 case can handle a BT G. Also, the shape of the grain is different. Usually, disposable motors don't have a liner. RMS motors use liners. So an RMS G grain will be skinier and longer compaired to the disposable G motor.

There is a RMS motor equivalent to then G80... Aerotechs BRAND SPANKIN NEW RMS 29/120 with its G79 White lightning loads .