# Motor size limits: A Discussion

### Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

#### Kirk G

##### Well-Known Member
Hey guys,

As a low power kinda guy, the whole discussion of launch heights limited by (or permitted by) the FAA is kinda fuzy for me.
I know certain motors are expected to be able to reach certain heights, and therefore, if your club launch is limited by the waiver or the FAA, above a certain size motor cannot be allowed.

Is there a chart somewhere that describes heights reached or expected by motor size or class? (Hope I'm describing this correctly. A, B, C, D, E, F, G, H, I, J, K and upward?)

So, hypothetically, if you had a motor or rocket that was promoted as able to reach, say, 6000 feet... and your elevation above seal level launch point was 700 feet... does this mean you could only launch if the FAA waiver read permitted up to 7000 feet?

Or am I doing this backwards? Would an FAA waiver up to 7000 feet then exclude the use of any motor or rocket expected to exceed 6,300 feet above your 700 foot MSL launch point?

I've also heard that the clearance area around the launch point must be at least a quarter of the expected elevation reached by the motor/rocket. In this example, a motor expected to reach 8000 ft would then require at least 2000 ft clearance on the ground to any inhabited building?

Is there a simple primer for calculating this or educating oneself on what the formula/restrictions on height is/are?

Again, I'm just picking numbers out of the air here. I have no idea if these values are possible or even accurate.

There's a theoretical maximum efficiency limit one can approach w/ min dia birds. But think about a saucer design w/a coffee-can-k, it's going nowhere in a big hurry.

Model and HP codes do tell us quite a bit about launch site dimensions vs. impulse; and the FAA tells us plenty about how high we can go, and when. But there's not an easy chart to relate all of this, because Cd varies so much.

Last edited:
well, not really close to the issue.

Class I rockets, AKA model rockets, do not require a waiver. Think of these flying A-G motors (and a few H's as well). Most don't have altimeters so no one really knows how high they go. The FAA isn't interested in these, but remember the NAR rules about flying Class I rockets.
Class II rockets, AKA "High Powered" rockets require a waiver. This is H (with a few class I exceptions) through O motors. Generally we use AGL (above round level) but you may see MSL (height above sea level). You also need to meet the requirements for NAR/Tripoli high powered rocketry rules. The heighest waiver you can put in is twice the smaller the length and width of your recovery area. Distance from inhabited buildings are also discussed in the code
Class III rockets, AKA Amateur Rocketry Activities. These are P motors and larger. This is a whole different ball of wax, and the paperwork is HUGH. I still have nightmares about my class 3 submission, and the committee was actually extremely helpful to me.

Class I rockets, AKA model rockets, do not require a waiver. Think of these flying A-G motors (and a few H's as well). Most don't have altimeters so no one really knows how high they go. The FAA isn't interested in these, but remember the NAR rules about flying Class I rockets.

Well, the FAA *is* interested in Model Rockets, even though those do not require a waiver. One still has to observe the Safety Codes such as not launching when there is an aircraft in the area, and not launching thru clouds (An aircraft flying thru clouds would have no way to see and avoid a model coming down slowly by parachute). Even WITH a waiver you have to observe those (I've seen too many claim that by having a waiver, any aircraft in the area is at fault and so rockets are fine to launch in that case. 100% WRONG! )

Also gets a bit sticky when there is an airport nearby.

But in general, as mentioned in other posts, there is no altitude restriction for Model Rockets as there is for HPR (HPR limited to whatever the launch waiver limit is).

Last edited:
Hard to put a chart together for a perticular engine.
Because an engine in a light weight, min dia rocket will go much higher than the same engine in a short fat heavy rocket with lots of drag.
I'm sure there is a formula using thrust vs weight, but that math is beyond me.
I have no knowledge of what the symbols mean, so I'm lost.
I've read here on the forums that a waiver will state from sea level, or ground level at the location.
Most rockets comes with a list of engines it uses, and an estimated altitude.

motor sizes typically determine minimum launch site size. the FAA gets involved when you want to fly anything other than a class 1 rocket. waiver altitudes are given in 'Above Ground Level', it is the responsibility of the flyer to not exceed this altitude(this is where rocket design software comes in handy). from what I have seen it would take some doing to get a class 1 rocket over 9000'. as you mentioned rockets differ so a simple chart will not work. the estimated altitudes listed in the catalogs are determined by; rocket design software and or testing, as I recall most of the rocket companies use 'Rocksim' and test, however your results may vary.
Rex

Replies
59
Views
2K
Replies
1
Views
724
Replies
2
Views
684
Replies
0
Views
602