My rockets are too heavy

[bjphoenix]

I had the idea to collect open rocket files for my newer rockets and run sims with different motors so I would know what I'm getting. I weighed the rockets with my wife's postal scale and found out that mine are heavier than the theoretical weights. My Estes Patriot Missile is 2.0 oz. according to Estes but mine weighs 2.3oz, and that is without parachute. My Estes ESAM is 3.4oz without parachute and the open rocket file I found has is closer to 3 oz. I'm going to have to bump up the mass a little bit in the sims.

I don't put on a lot of glue. I put on a couple of coats of primer and one coat of paint. The ESAM has papered fins, I don't remember about the Patriot.

I'm also surprised that the ESAM is that much heavier than the Patriot. The ESAM has a lot longer nose cone, and more fin area. I think the nose cone has most of the extra weight.

 

[waltr]

This is typical. Actual build weight is usually not the same as maker's spec.
Lots of reasons. Glue, paint, or even manufactors weight is in error or they had changed the parts included but not the specs.

This is why it is suggested to measure weight after the build to get correct weight into the simulation.

 

[teepot]

I don't find the difference to be a problem. Tubes weigh next to nothing. Nose cones on the other hand are heavy. Heavier than I thought they were. For me, all the weight is in the fin can. I use TTW fin attachment, with internal and external fillets. I have a 29mm MD that weighs 8.3 ounces and I would bet that half of the weight is in the bottom 6 inches. The rocket is 36" and needed nose weight.

 

[tsmith1315]

My Estes Patriot Missile is 2.0 oz. according to Estes but mine weighs 2.3oz

That's 15% off spec, and less for the ESAM.

As a reference, model rocket motors are allowed to vary up to 20% (or 1N) from average thrust specified. I'd say you're doing good!

It would be interesting to weigh the kit parts before assembly and see how much it weighs out of the bag.

 

[neil_w]

Keep in mind that the manafacturer has to take a guess at how much glue and paint you'll be adding. I'm guessing that they do what they think is a "typical" build and weigh it out. But everyone builds a bit differently, so it's not really expected to exactly nail the specified weight.

I know my builds *always* come out heavier than the OR model because I don't try to include glue and paint in my models. OR can model fillets but not just the routine glue used to put everything together.

Papering fins definitely adds weight. That's one thing I *do* try to account for in my OR models because it's usually weight added to the rear, which may affect stability.

 

[DaHabes]

Use bigger motors. They're fun...

 

[Bill S]

I end up weighing the parts out of the bag, then allowing 15-20% extra paint and glue weight. So I normally build the rocket, weigh it, add the difference between raw parts weight and built weight as "glue" weight, adjust the CG so it matches the actual CG and weight. Then I paint it, and adjust again. Normally the paint weight doesn't do much to the CG, but the glue weight sure does.

 

[mh9162013]

What a manufacturers say usually means nothing. It's like a pro athletes "official" height and weight or a battery company talking about how long their batteries last.

I just built an Apogee Apprentice, which is supposed to weight less than 29 grams. Guess how much mine weighs? About 46 grams...and no, there's no motor inside. However, I did install Dino Chutes 3x3 inch wadding inside.

Sure, I tend to overbuild my rockets for better durability and ease of use ("why tie a knot when I can use a snap swivel?" for instance). But it's not like I'm going crazy with glue or paint or anything.

 

[spigalau]

Your 0.3 weight difference is probably down to epoxy & paint. Keep in mind that 0.3 oz is less than 2 tea spoons of product applied. How much glue did you use, how much paint, how many layers... it all add's up.

 

[cwbullet]

Or...Is it your motor is too small?

 

[Back_at_it]

The weights in OR are all over the place. You might get one that was created by someone that took all the default materials weights or left parts out etc.

As for comparing them to Estes weights, don't bother. They aren't even close. I built a dead stock Bertha for someone not that long ago and it was 1.2oz heavier than the 2.5oz estimate from Estes. Didn't do anything fancy, Just a couple of coats of sealer on the fins, primed and painted. Weighted 3.7oz loaded minus the motor.

I know that I build heavy. I like to seal my fins, use large fillet, add Kevlar and extra long shock cords. I typically add baffles and sometimes upgrade the centering rings or even larger motor mounts.

At the end of the day, weight your rocket, ensure the CG is in a safe location and run the sims to find out what motors you need to run. Also don't be shy about upgrading the motor mounts. Rockets like the Big Bertha, ESAM and Citation are just screaming for a 24mm upgrade.

 

[RoyAtl]

I had the idea to collect open rocket files for my newer rockets and run sims with different motors so I would know what I'm getting. I weighed the rockets with my wife's postal scale and found out that mine are heavier than the theoretical weights. My Estes Patriot Missile is 2.0 oz. according to Estes but mine weighs 2.3oz, and that is without parachute. My Estes ESAM is 3.4oz without parachute and the open rocket file I found has is closer to 3 oz. I'm going to have to bump up the mass a little bit in the sims.

I don't put on a lot of glue. I put on a couple of coats of primer and one coat of paint. The ESAM has papered fins, I don't remember about the Patriot.

I'm also surprised that the ESAM is that much heavier than the Patriot. The ESAM has a lot longer nose cone, and more fin area. I think the nose cone has most of the extra weight.

Paint and glue add a surprising amount of mass. Back in the old days (60's and 70's) we used model airplane dope instead of enamel paint because it was much lighter.
Papering fins can also add significant weight, especially in an area which is problematic for smaller rockets. When I do it, I find the lightest bond paper I can find, and carefully use thin CA glue. I tried the Avery labels for awhile but found them too heavy and the adhesive required preparing the wood surface anyway for best adhesion.

 

[RoyAtl]

Also, I don't think the catalog weights are as accurate now as they were back in the day (and I'm not sure how accurate they were then, but they were definitely closer)

 

[Joekeyo]

They don't fly as high as the sim either. Build strong and prosper.

 

[bjphoenix]

Your 0.3 weight difference is probably down to epoxy & paint. Keep in mind that 0.3 oz is less than 2 tea spoons of product applied. How much glue did you use, how much paint, how many layers... it all add's up.

I use Titebond II and I start out using a good amount but as the stuff dries you lose most of the bulk and mass. I've put on 2 generous fin fillets and after it's all dry the fillets are still fairly small. You can put a lot of Titebond in there when you are building but most of the weight goes away. As for paint I typically put on a couple of coats of primer then one coat of paint. With multicolor paint jobs some parts naturally get more than one coat of paint. I was surprised to see that much difference in weight but I do understand that papered fins are heavier. In some that I've kit bashed I found open rocket files for the original rocket then modified to match my redesign. In the process I weighed my fins after papering and calculated new density to put into the sim.

At the end of the day, weight your rocket, ensure the CG is in a safe location and run the sims to find out what motors you need to run. Also don't be shy about upgrading the motor mounts. Rockets like the Big Bertha, ESAM and Citation are just screaming for a 24mm upgrade.

I fly a lot on small fields so the ESAM will have to be happy with 18mm BP motors. It flies well enough on a C. My 2 recent builds were kit bashed from a HiFlierXL and a MeanMachine so they have 24mm mounts. I already have a HiFlierXL built stock and it flies high enough for me on a D12-7. I actually built an 18mm x 3.75" adapter so I could easily fly a C6 in these 2 rockets. I haven't simmed them yet, I was getting to that when I started discovering issues with weights. Both of these kit bashes have different size fins so I was wanting to be sure I was good on stability before flying them.

 

[neil_w]

I use Titebond II and I start out using a good amount but as the stuff dries you lose most 50% of the bulk and mass.

Fixed. I measured once for Titebond II.

 

[jimzcatz]

Or...Is it your motor is too small?

Exactly!!!, It is my mantra there is no such thing as a rocket that is too heavy, It just allows for a bigger motor!!!.

 

[bjphoenix]

Fixed. I measured once for Titebond II.

That's interesting. I might have guessed that you lose more than 50%.

 

[neil_w]

That's interesting. I might have guessed that you lose more than 50%.

I posted the details about my experiment somewhere in the past, but I can't find it right now. Mind you, 50% is the reduction in mass, I had no way to measure the shrinkage.

 

[brockrwood]

Use bigger motors. They're fun...

In thrust we trust.

 

[Ez2cDave]

Bottom line . . . I never trust manufacturer's specs or "sims" ( actually, I never use sim programs, at all ).

"Real World" numbers are all that matters and, no matter what you have been told by a "sim" or a manufacturer, the end product is the "truth".

Dave F.