Thrust to weight ratios rocketry calculator

[Adam3836]

just wondering has anyone found the website rocketry calculator reliable/accurate for determining thrust to weight ratios

and as far as general rule of thumb does these numbers look right


calm 5:1
Slight breeze 10:1
breezy 15-20:1


thanks
Adam

 

[Bat-mite]

What you really care more about is rail/rod exit velocity. TTW is a way of making an educated guess that rail/rod exit velocity will be sufficient; but it is not really that accurate. The thrust numbers given for motors are their average thrust over the duration of the burn. What is not taken into account is the thrust curve of the burn. Two motors with identical average thrust, but with significantly different thrust curves, will perform differently.

The key factor is the amount of thrust in the first quarter-second or so of the burn, while the rocket is riding the rail/rod.

ThrustCurve.org, RockSim, and OpenRocket will identify safe motors with rail/rod exit velocity that meets or exceeds that needed to escape the rod/raill at sufficient speed to be stable as soon as the rocket leaves the rail/rod.

 

[JohnCoker]

I don't know of any program that does only that, but the simulators will use more detailed analysis to determine if a motor "works" for a rocket.

The ThrustCurve.org motor guide will include that ratio in its report of motors that work.

 

[Nytrunner]

Which website? There are several (Thrustcurve being most popular. I've found it very useful)

The guideline is that rail exit velocity of the rocket should be 4 times the current the windspeed. After you've built your rocket, you should be able to simulate and determine what thrust motors are needed to reach those exit velocities (assuming a standard rail length. Shorter or longer rails would modify your needed thrust)

 

[Bat-mite]

Here's a concrete example. Aerotech has two G79 motors: both G impulse, both 79 Ns average thrust. But notice that the White propellant gives you a lot more thrust while on the rod than the White Lightning:

White Lightning


White



At 0.2 seconds, you've only gotten about 60 N of thrust out of the WL, but over 90 N out of the W.

 

[BABAR]

Something that might help when looking at thrust curve graphs.

For a Six foot rail, assuming rocket reaches minimum standard acceptable launch velocity, how long does it take to get to the end of the rail (we will assume the full six feet is used, although effective length is reduced by distance from forward launch lug and any standoff used to keep motor off the blast plate) ?

How about 4 foot and three foot rods (commonly used to low power)?

Reason I am asking is when I look at the graphs, many motors have a reeeaaalllly heft spike at ignition that drops off pretty quickly. I am curious how much of that “spike” is spent with the rocket still on the guide rail or rod. So it seems from a safety standpoint that any impulse imparted AFTER if leaves the rail is USELESS from a stability standpoint, although it certainly affects altitude, maybe weathercocking, and delay selection for black powder motors, anyway.

My suspicion (which outside of saucers may be wrong) is that draggy Rockets are STILL likely to get to that velocity over that short length of travel, as there won’t be that much drag effect until the rocket GETS to rail exit velocity. Once you are off the rail, seems like stability is either effectively established or not. Meaning that if it isn’t stable by the time it leaves the guide, your are up poop creek, because the rocket will likely tip over and go Helter skelter. In fact, if it isn’t stable by the time it leaves the rod you kind of HOPE it DOESN'T BECOME stable, as it stays unstable it skywrites but doesn’t build of any velocity to really go anywhere. What’s really bad is if it leaves the rail unstable, goes off axis while still accelerating, and BECOMES stable in a non vertical direction.

 

[JohnCoker]

The guideline is that rail exit velocity of the rocket should be 4 times the current the windspeed.

If this is widely accepted, I should add it to the motor guide (instead of using the fixed 15m/s). Do you recall where this came from?

 

[Nytrunner]

If this is widely accepted, I should add it to the motor guide (instead of using the fixed 15m/s). Do you recall where this came from?

I believe its in one of the safety codes

Unfortunately I can't recall which one at the moment

 

[JohnCoker]

If 20mph is the maximum for launching, 4 times that would be 36m/s and 15m/s would account for an 8mph wind.

I always thought the guide speed was an estimate of the minimum velocity for the fins to have a corrective force. It's not obvious how weathercocking is related. (I mean the fins could be working fine, just bending the rocket into the wind.)

Maybe rules of thumb are better unexamined.

 

[mbeels]

It's not obvious how weathercocking is related. (I mean the fins could be working fine, just bending the rocket into the wind.)

The amount of weather cocking can be calculated from the tangent of the rocket velocity divided by windspeed:

https://www.grc.nasa.gov/WWW/k-12/rocket/rktcock.html

 

[Adam3836]

What you really care more about is rail/rod exit velocity. TTW is a way of making an educated guess that rail/rod exit velocity will be sufficient; but it is not really that accurate. The thrust numbers given for motors are their average thrust over the duration of the burn. What is not taken into account is the thrust curve of the burn. Two motors with identical average thrust, but with significantly different thrust curves, will perform differently.

The key factor is the amount of thrust in the first quarter-second or so of the burn, while the rocket is riding the rail/rod.

ThrustCurve.org, RockSim, and OpenRocket will identify safe motors with rail/rod exit velocity that meets or exceeds that needed to escape the rod/raill at sufficient speed to be stable as soon as the rocket leaves the rail/rod.

this makes perfect sense thank you

what would you say about wind conditions I would think you can have a motor that’s giving plenty of thrust to safely exit the rail/rod but then still have to face some wind condition ?
I have had a motor that on paper was right for my rocket but appeared to not have enough thrust to make it through the wind To keep a straight flight and ended up weather cocking

 

[DaHabes]

If 20mph is the maximum for launching, 4 times that would be 36m/s and 15m/s would account for an 8mph wind.

I always thought the guide speed was an estimate of the minimum velocity for the fins to have a corrective force. It's not obvious how weathercocking is related. (I mean the fins could be working fine, just bending the rocket into the wind.)

Maybe rules of thumb are better unexamined.

Formulas don't always work either. 4 x 0 MPH is zero m/sec off the rail. OOPS!

Also, I find 15m/sec too slow. I sim out rod/rail length to a minimum of 50mph (22m/sec). That avoids the worst of the weathercocking. Too slow and you are guaranteed to weathercock. As you increase the relative speed V of the wind speed vector, it overcomes overcomes the horizontal component w (angle b gets very small).

 

[Bat-mite]

this makes perfect sense thank you

what would you say about wind conditions I would think you can have a motor that’s giving plenty of thrust to safely exit the rail/rod but then still have to face some wind condition ?
I have had a motor that on paper was right for my rocket but appeared to not have enough thrust to make it through the wind To keep a straight flight and ended up weather cocking

Every rocket weather-cocks to some degree, and it's true that the faster it goes upward, the less time the wind has to alter the flight path. So yes, higher thrust is necessary to escape the effects of the wind on the flight path.

People who have studied hobby rocket flight have come to the conclusion that an exit velocity of about 48 fps (32 mph) is sufficient for average wind conditions. But definitely more thrust is recommended for higher winds. And then, of course, the safety code prohibits flights in winds > 20 mph.

TTW ratio is a quick and dirty means to give an RSO a feel-good about letting a rocket launch. A really good RSO will also be familiar with motor thrust curves, and an even better one will ask the flyer, "Tell me what this rocket is going to do when it leaves the pad, and why."

 

[Scott_650]

Just an out loud sort of thought experiment but where do we go in the (possibly very near) future when active thrust vectoring is used in hobby rocketry? The safety codes governing passive guidance, fin controlled rockets may not be a good fit if thrust vectoring becomes commonplace. Perhaps a separate set of general rules/guidelines will be needed - maybe an additional category specifically for active guidance rockets? Thrust vectoring may be the next big leap in hobby rocketry - comparable to the rise and acceptance of high power.

 

[Rocket501]

Also, a lot of this depends on how your rocket's physical response is (moment of inertia, damping ratio, etc.). Depending on how these factors change, your rocket may be more or less vulnerable to weathercocking and thus have different minimum velocities.

 

[Saint_Tiki]

These rules of thumb don't account for a particular rocket's stability margin either. A highly overstable rocket with a high thrust motor on a windy day will still want to weathervane more than a rocket that is barely stable. A rocket with 0 cal stability won't weathervane at all but will simple be pushed downwind while maintaining it's orientation... though said rocket shouldn't make it passed safety in the first place.