Ground hit velocity - what's practical?

Hey Y'all;

A design being checked out in Open Rocket ( am new user of program) shows in flight simulation result tables a "Ground hit velocity" of 12 point something feet per second for a 5 oz rocket with an 18 inch diameter plastic parachute.

That gives rise to a question about descent - what is a practical range for ground hit velocity?

I look at my height and think something falling 6ft per second is going to land with a thump, let alone twice that per second.

"Practical" - That depends on a large number of factors:

• Size and weight of the rocket
• Fin and body tube materials
• Construction methods - TTW fins, Epoxy, Foam fin can, etc.
• Surface of impact - sod, mown hay, young soybeans, crushed gravel, asphalt, dry lakebed, etc.

Any and all of these work together to determine your upper velocity limit for recovery, but current wind speed will determine your lower velocity limit.

My Wildman Drago (Fiberglass TTW fins with internal and external epoxy fillets) when landing on either mown hay or cut grass, I'm comfortable with as much as 22fps, it's fairly bulletproof. According to OpenRocket, the supplied parachute slows it down to about 17fps, so I'm OK with that.

My Madcow Jayhawk (Plywood TTW fins into kraft airframe tube) has those huge finlets that snap off under a hard or off-angle landing, so I prefer to keep the descent to between 12 and 15fps. Any slower, and prevailing winds will give it a significant amount of drift which would guarantee a snapped finlet.

I generally don't give much thought to LPR, as most of my fleet is mass-market enough that the flaws are well-publicized. I just try to ensure that my parachutes will open fully, and that the shroud lines aren't pre-tangled prior to launch. Well-constructed LPR rockets under about 9oz or so should bounce on impact with most surfaces even at velocities as high as 19fps.

I apologize for further confusing the issue.

G.D.

Rule of thumb is 17 feet per second.

There are a few tables that will give you a good idea of what size parachute you should use based on your rocket's weight.

Top Flight Parachutes recommends 17-22 fps

Sunward also has a nice table

But it really depends on the design/construction as much as the weight. For example, my LOC Lil' Nuke comes in at around 13 oz, which is stock for the kit and according to Semroc's table would need at least an 18" parachute - preferably a 24". But it came with a 14" parachute. The design of it is such that the fins are mounted about an inch up from the aft end and when it lands, it lands on the motor. IIRC, the Open Rocket sim is somewhere around a 30 fps descent rate. I've not had any issues with it yet.

Then I have a scratch built LPR where the fins extend down below the aft end of the rocket and are the first thing to contact the ground. Anything faster than about 15 fps results in a break and I've had breaks even at that speed when it landed on asphalt (it's been repaired several times).

Ahh.
Construction on this baby is standard cardboard body tubes with TTW fins. Fins will most likely be 1/8 inch balsa or 3/32 with paper overlay.
Either way will have to go buy some appropriate hard balsa, what's on hand turned out to be too soft.

quickburst said:

Rule of thumb is 17 feet per second.

Click to expand...

Rules of thumb are like both rules and thumbs - they are made to be broken. :kill:

Actually, I just checked a lot of my altimeter data and that is a good descent rate. Your rocket will likely not suffer much damage no matter what you land on if you are near that speed.