Rules of thumb for T:W and speed off the rail?

[R3verb]

Hey All,

I've been getting more and more into simming my flights lately and using thrustcurve to make sure I'm well within the safe range. This is mostly due to overbuilding my Estes kits (mostly Plywood upgrades and epoxy+fiberglass filets).

Are there good rules of thumb for T:W ratio and speed off the rail? I've seen 5:1 is the minimum you want to see for T:W but no good data on speed off the rail. I know this is well discussed for HPR stuff but I haven't found any great guides for MPR/LPR (I assume since most of this is just follow the kit lol).

Thanks everyone!

 

[Azamiryou]

30 mph ≈ 45 fps

 

[Aeva]

Hey All,

I've been getting more and more into simming my flights lately and using thrustcurve to make sure I'm well within the safe range. This is mostly due to overbuilding my Estes kits (mostly Plywood upgrades and epoxy+fiberglass filets).

Are there good rules of thumb for T:W ratio and speed off the rail? I've seen 5:1 is the minimum you want to see for T:W but no good data on speed off the rail. I know this is well discussed for HPR stuff but I haven't found any great guides for MPR/LPR (I assume since most of this is just follow the kit lol).

Thanks everyone!

My rule of thumb, 5:1 is minimum in IDEAL conditions (barely a breeze faster than someone exhaling); 10:1 is target minimum under most conditions, and 15:1 and above is excellent under normal launch conditions (barring G forces).

You should check the thrust curve and make sure it actually reaches that ratio and a desirable speed when leaving the launch rail.

https://www.rocketryforum.com/threads/ezi-65-heavily-customized-on-i115-is-it-too-heavy.182078/

I went through the same thing and got a lot of good feedback in the thread i linked.

 

[MidOH]

I use thrust to weight ratio as well.

If I'm at 6:1, the rocket better be a joke. Like a DRM or Micromag.

8:1 and up. My L1 attempt might be more like 10-15:1.

 

[John Taylor]

Under conditions that are proper a RSO can approve a 3 to1 ratio.
Its in the Tripoli Safety Code
5 to 1 and higher is ideal.
Minimum speed off rod or rail, in no wind conditions, is 50fps. Under special circumstances 40fps will work. Thrustcurve.org is very consertive. It wants to see at least 50fps.

 

[lr64]

The length of the rail and the moment of inertia in pitch or yaw vs the stability should enter into this, but that's not so easy to calculate.

 

[JohnCoker]

5:1 has been the most common T:W ratio, but of course that really depends on the burn. ThrustCurve.org lists the "initial thrust" as a separate value (this isn't published by the cert. orgs., but is calculated from the data file).

15m/s or 45fps has been suggested as a good speed to leave the rail, but that requires running at least a quick sim.

Note that ThrustCurve.org will flag motors failing either of these rules of thumb when using the Motor Guide.