difference between LPR & MPR?

[TwoWalks]

Being new to this - I see the difference between Low Power Rockets & Medium Power Rockets as far as the engine size designation. What are the other differences between the two?

 

[eugenefl]

Originally posted by TwoWalks
Being new to this - I see the difference between Low Power Rockets & Medium Power Rockets as far as the engine size designation. What are the other differences between the two?

Typically the type of materials and methods used to construct rockets changes drastically. Balsa is replaced by plywood, plastic chutes are replaced by nylon, yellow glue is replaced by epoxy. Mid power is a great preview into HPR. Newer techniques such as "fin can" building or through-the-will fin mounting are learned. In some circumstances such as extreme altitude/speed or minimum diameter rockets, lightweight fiberglass is used for strengthening joints. There's also a grey area in there where some rockets such as the Estes Executioner can be considered LPR and MPR. In these situations it's typically the motor selection that determines the type of flight and not the type of rocket.

So, ya getting the MPR bug yet? Needing more power? Have fun!

 

[Stymye]

epoxy,plywood and fiberglass are typically required on a midpower rocket ?

 

[sandman]

Easy, bigger motors need tuffer rockets.

sandman

 

[KenParker]

Originally posted by stymye
epoxy,plywood and fiberglass are typically required on a midpower rocket ?

Well, as usual, it depends on the circumstances.

I can take, as an example, an Estes Broadsword (pretty much the same as an Executioner), and put a 24mm RMS F12 motor in it. The Broadsword (Executioner) will do fine with that motor even if you just build the kit stock. It will just go very very high. You can consider the F12 as just a very long burning D12.

Now... take the same rocket, but load the 24mm with an F24 reload or.... an F39 reload. Most likely, if you built the rocket stock, you are probably going to shear those balsa fins off just where the filets end. If you fiberglass the balsa, you'll be OK.

Then... take the Executioner and put a 29mm motor mount in it. Plenty of room for it. And, say you decide to put a 29/40-120 RMS with a G64 reload in it. Ya better have reinforced the heck out of that airframe and those fins, or when that G64 lights you will get to witness your first shred.

So... as I said at the start.... it depends on the circumstances.

 

[lalligood]

Just like there are rockets that bridge the gap between LPR & MPR (like the forementioned Estes Executioner), there are plenty of rockets that can handle MPR & HPR motors. Kits like the PML Phobos, which I built stock & flew a couple of times on G35s before attempting my L1 with a Pro38 H153. According to the reviews & flight log on EMRR, the Phobos (if built lightly) can handle some Fs all the way to a J350 (with an extended 38mm MMT & fiberglassed airframe).

There are some folks who will fiberglass darn near every rocket in their fleet regardless of size (yeah, even their LPRs!) Me, I'm in no hurry to use that much epoxy & do that much sanding! haha There are even plenty of large LPR rockets that don't require fiberglassing...

Epoxy on the other hand is reasonably necessary for assembling most MPRs & all HPRs. While it seems daunting to have to mix & apply it, it's really not that hard. Until you get the hang of mixing epoxy & the actual amount of time you have to work with it, I'd recommend starting with 30-minute epoxy. 12- or 15-minute might be OK (I started with 12) but I've learned since getting my hands on some of the longer curing epoxy that my work comes out better when I have more time to work with it & I'm more relaxed when using it. That's a personal choice though...

The last change, balsa -> plywood, usually isn't a choice with kits... The wood you get is the wood you get. Again as a personal choice, I prefer working with plywood because it's much harder, more resilient to impact, & not nearly as porous as balsa.

Lastly, I'd recommend to start small & on scrap pieces to get the hang of these new materials/adhesives/techniques. It'll save you headaches & money. Good luck & keep us posted of your decisions & progress!

 

[astrowolf67]

Originally posted by TwoWalks
Being new to this - I see the difference between Low Power Rockets & Medium Power Rockets as far as the engine size designation. What are the other differences between the two?

The body tubes are thicker and heavier. Nose cones are thicker and heavier. Fins and centering rings are usually made of 1/8" ply instead of card stock or balsa. Most MPR, and HPR kits do not use thrust rings or motor retention. If you want those, you have to add your own. Recovery components use nylon chutes, and tubular nylon, kevlar, or heavy elastic shock cords. The use of wood glue, or epoxy is needed, instead of white glue.

Also, some companies use very different materials and techniques. Such as Aerotech, they use a fin lock system, comprised of heavy plastic centering rings, with tabs that molded plastic fins snap into. PML uses a plastic type tube called Quantum tube (which is excellent for finishing), fiberglass fins, and piston ejection systems. LOC Precision MPR kits, are just like Estes kits, only using the heavier components.

 

[rstaff3]

My 2c:

For E powered rockets, you may get by with Estes materials. I have several thin tube/balsa fin birds that do just fine on E15s and E30's.'s with no mods.

However, I generally use epoxy for MPR, but as long as you have wood/cardboard bonds you can use yellow glue if you build properly.

Like someone later said, you usually get plywood in MPR kits. For scratch builts I generally would recommend plywood also, but I have even used reinforced foam board. G10 isn't bad either, but you will need Epoxy in this case.

You don't need fiberglass on mid-power. I have to think, but I don't think I've fiberglassed any of my mid power rockets. On my Big Brute, with its surface mounted fins, I wish I had.

The theme here is that the answer to the materials question depends a lot on the building techniques and the application. Thrust to weight plays a big part, which is what Ken was getting at in his example. You also have to remember that more rockets break on landing than during boost.

BTW, I know of one M/N powered rocket that used no epoxy or fiberglass, and another that was built entirely with Liquid Nails...what will work just d'pends....

STILL, if you are unsure, epoxy and thru the wall plywood is the best bet.

 

[TwoWalks]

Want to thank everyone for taking the time to answer my question. Got a ton of great info out of this not only on technique but also materials and transition kits.

eugenefl "So, ya getting the MPR bug yet? Needing more power? Have fun!"

Don't know if I am needing more power because I have yet to finish my first kit. I do know that when I seen the Discovery show, I wanted "More and More power" Figured would start with LPR just to get the hang of it. My goal is scratch MPR/HPR in time. Also I figured may as well start the learning curve now.

 

[Stymye]

the journey is alot of fun!