Sprung and unsprung weight.

[David Schwantz]

When choosing chute size and using all up weight do you include the weight of your main? Since the main is supporting the weight of the rocket when deployed, I do not use it to determine the size needed. What do you do?

 

[roc_boy]

Being an old moto guy, I've pondered this myself. I figure the chute is also having to support it's own weight, so I add it in. IMHO, YMMV,

 

[Banzai88]

I've never found the weight of the recovery gear to be enough of a % of the overall weight to really care.

That said, I build the model with actual weighed parts in OR, then check any necessary overrides for glue&paint, and then size the recovery gear for the max overall weight for 22ish fps for fiberglass or 15ish fps for cardboard/plywood.

I find that gives me enough wiggle room to go up or down one size as conditions dictate.......and changing the recovery gear and adding or subtracting the weight differential hasn't ever made any realistic difference in a nominal flight.

 

[David Schwantz]

Some of the chutes I am using weigh several pounds. I have a 10' main in the V 2. Same in the Bull Pup.

 

[SolarYellow]

That said, I build the model with actual weighed parts in OR, then check any necessary overrides for glue&paint, and then size the recovery gear for the max overall weight for 22ish fps for fiberglass or 15ish fps for cardboard/plywood.

I weigh parts and subassemblies with a gram scale as I build, then add mass elements to the OR file to represent various quantities of glue and paint. Finished rocket doesn't need any overrides.

 

[Banzai88]

Some of the chutes I am using weigh several pounds. I have a 10' main in the V 2. Same in the Bull Pup.

Curious as to what % of the overall weight of the fully loaded rocket does that represent?

 

[David Schwantz]

V 2 weighs 68 lbs wet. I'd have to get chute out but if I remember correctly I think it weighed in at 4 or 5 lbs. So that would be about 14%.

 

[Banzai88]

V 2 weighs 68 lbs wet. I'd have to get chute out but if I remember correctly I think it weighed in at 4 or 5 lbs. So that would be about 14%.

Yeah, that crosses into a threshold beyond my experience with weights like that, but I'd say 15%+ is rather significant.

In that case, I'd take a look into some of the L3 documents and see how some of those larger rockets compare, since some of those projects get into weights like that.

At that weight level, my gut instinct is probably wrong. I'll take whatever the more experienced folks have to say on the matter as a learning point.

 

[cerving]

If you take a chute and drop it, it will fall. It has mass that the drag works against, so you have to include it.

 

[mtnmanak]

I include the weight of the recovery gear in my calculations.

To David's point, the bigger the rocket gets, the more difference it makes. On my 12" diameter Darkstar, the parachute and deployment bag weigh in at about 35 lbs. On my 4" diameter Darkstar, the parachute and nomex pouch weigh in at about 11 ounces. Part of that differential is the fact that I can afford a much more expensive (cost per square inch) parachute for smaller chutes, but when I get up to chutes measuring 40 or 50 feet, as much as I would love to spend the price of a small car on a massive Fruity Chute Iris, I have to make due with a less efficient, bigger, heavier, albeit cheaper, solution from Rocketman or military surplus. Valkyrie looks like they are getting into the big parachute game with some efficient chutes (a 40 foot chute weighing less than 10 pounds - nice!), but their prices are roughly double the Rocketman equivalent, so still pricing things out of the range I will spend right now.

Agree with Cris, include all the mass you are going to launch in your calculations.

While I enjoy the discussion and am interested in the physics answer, from a practical standpoint, you will always be better off with the safety margin you get by including the full weight of the recovery gear in your chute sizing calculation.

 

[mtnmanak]

I will caveat my previous post with the consideration that is my answer for my everyday, fun flights at the field. If you are going minimum diameter and shooting for that world record altitude attempt, then I am shaving every ounce off I can, going drogueless, choosing a chute at the bleeding edge of safety/rocket survivability, etc. In those situations, as we used to say when packing a rucksack for long range surveillance missions, ounces make pounds.

 

[teepot]

I add the weight of the chute to get total weight. Then I get an accurate answer from Thrust Curve as what motor to use.

Edit; when I saw the title I thought you were talking about cars.

 

[David Schwantz]

that is where the idea comes from. But I was not talking about motor size. I was referring about chute size.

 

[mrwalsh85]

The chutes for Marvin the Martian @ LDRS 38 were twin 30' Cargo Parachutes.

Not exactly negligent in weight there. I'm just ballparking here but they could've been 20 pounds or more each.

 

[Lord Rory Gin]

-- blanked --
My question was already answered.