Thrust to Weight & Getting Off The Pad Calculation

On youtube there are some great NASA HPR video's one of them goes thru all the math to easily compute feet/meters per second speed off the rail. If I remember the minimum speed is 40 feet a second once the rail is cleared. THe math is not hard.

here's the link
[video=youtube;G6JXY1ol53U]https://www.youtube.com/watch?v=G6JXY1ol53U[/video]

Handeman said:

At BattlePark, our RSOs have a graph with 3:1, 5:1 and 7:1 lines on thrust vs. weight. As long as your average thrust is over 5:1, you're good to go on TTW. If you're under 3;1 that is almost always a no-go, if it's under 5:1 then we take a closer look. it doesn't mean you can't fly, it just means it will take a while for the RSO, LCO, BOD, etc. to decide if, where, and/or how we will let you fly.

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I've sent people away trying to fly some 38mm FG rocket with a 29mm motor mount on an Estes F15. I've also let some people fly with questionable rockets because they gave the right answers when I asked them about stabilty, rod velocity, and estimated altitude. If the flyer obviously did their homework and knows what they're doing, it's all good. That K261 with a 13# rocket works out to a decent 5:1 thrust:weight ratio pretty much through the first 4 seconds of burn, no reason to turn it away as long as the winds aren't too nasty.

As mentioned by Jeff above, here's a copy of the 5:1 average thrust to weight ratios spreadsheet I developed for use at Battle Park. Another tool we use is the free TTWCalculator program available from the Google Play store. Perhaps someone knows if there is a similar program for the Apple devices.

As Jeff mentioned, these are guidelines and not hard and fast rules.

Ben

View attachment 5 to 1 Thrust 2 Wgt.xlsx

math sucks

ben said:

As mentioned by Jeff above, here's a copy of the 5:1 average thrust to weight ratios spreadsheet I developed for use at Battle Park. Another tool we use is the free TTWCalculator program available from the Google Play store. Perhaps someone knows if there is a similar program for the Apple devices.

As Jeff mentioned, these are guidelines and not hard and fast rules.

Ben

Click to expand...

A search on Google play has no return

Sorry .. search for thrust calculator in Google play

I am glad I found this thread. I like playing with TTW ratio on some of my projects. Usually I build at minimum 5:1 but I have a project I am working on that is 16ish lbs when finished. A G250 VMax. And very high drag. The math says a g250 Max Thrust287/4.45=64.49lbs / 16lbs = 4.09) will get it airborn at 4:1. Going for a 10 foot 3/4 inch pole with lugs internally along the motor mount.
I ordered a minitimer 4 to engage separation of the nose and the body to come down on separate chutes. It will have a stable margin of stability, and if I time the separation just after burnout since it won't be going that high anyway, there should be time for the chutes to inflate. And land safely.
Right?

ChrisLentz said:

I am glad I found this thread. I like playing with TTW ratio on some of my projects. Usually I build at minimum 5:1 but I have a project I am working on that is 16ish lbs when finished. A G250 VMax. And very high drag. The math says a g250 Max Thrust287/4.45=64.49lbs / 16lbs = 4.09) will get it airborn at 4:1. Going for a 10 foot 3/4 inch pole with lugs internally along the motor mount.
I ordered a minitimer 4 to engage separation of the nose and the body to come down on separate chutes. It will have a stable margin of stability, and if I time the separation just after burnout since it won't be going that high anyway, there should be time for the chutes to inflate. And land safely.
Right?View attachment 323149

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That on a G motor? How high is it supposed to go? What delay?

50 feet ish. .45 sec burn time. Set the minitimer to .9 sec after liftoff and there should be enough velocity to help with separation and chute inflation. Hoping to keep the weight down to 14lbs. If it all works..it should be up and down in about 3.5 secs.

ChrisLentz said:

50 feet ish. .45 sec burn time. Set the minitimer to .9 sec after liftoff and there should be enough velocity to help with separation and chute inflation. Hoping to keep the weight down to 14lbs. If it all works..it should be up and down in about 3.5 secs.

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By its weight it will not be a class one rocket. So why are we wanting to stay with a g motor. Does not seem safe to me.

Test of the system before going higher. why not safe? The launch leads are twice the distance of the test altitude. Precision experiment with timer. If it crashes...the rocket suffers. I won't lose sleep.

Man I'd go with a bigger motor on that sucker!!!

Dwatkins said:

By its weight it will not be a class one rocket. So why are we wanting to stay with a g motor. Does not seem safe to me.

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Because it's there?

Because its there. The math says it will work. The technology exists to make it happen. The safety factor is there because it can't go farther than the lead lines. The next step upon success of this mission is to up the ante and go with an H410. Another short burn quick thrust motor to go a little higher. Eventually an I500 years down the line. I am not trying to reach the moon, only safely successfully do the near impossible. That being, turn a table and chair stackable rocket and make it fly. Baby steps. And come on, if it works, think of the children around the world who will benefit with a little joy in their hearts.

ChrisLentz said:

Because its there. The math says it will work. The technology exists to make it happen. The safety factor is there because it can't go farther than the lead lines. The next step upon success of this mission is to up the ante and go with an H410. Another short burn quick thrust motor to go a little higher. Eventually an I500 years down the line. I am not trying to reach the moon, only safely successfully do the near impossible. That being, turn a table and chair stackable rocket and make it fly. Baby steps. And come on, if it works, think of the children around the world who will benefit with a little joy in their hearts.

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h399, i600r, & similar motors just really call out for bowling ball drag races ( at the appropriate three-or-more distance ).

If concerned, one can reach out to the club before loading up the car and see what rail lengths are available... if an 8-foot won't do, inquire about the availability of a 10, if that will suit. Or secure your own that's compatible with their particular mounting system and bring it with...


Later!

--Coop

ChrisLentz said:

50 feet ish. .45 sec burn time. Set the minitimer to .9 sec after liftoff and there should be enough velocity to help with separation and chute inflation. Hoping to keep the weight down to 14lbs. If it all works..it should be up and down in about 3.5 secs.

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Chris, if I were RSO you would need to bring documentation for me to look over-sims etc before I would let this fly. The flight profile is so short that I doubt you will get the chute open before hitting the ground.

Frankly, if you fly this on less than a high thrust H you are asking for trouble in both the actual flight and quite possibly with the RSO.

ChrisLentz said:

50 feet ish. .45 sec burn time. Set the minitimer to .9 sec after liftoff and there should be enough velocity to help with separation and chute inflation. Hoping to keep the weight down to 14lbs. If it all works..it should be up and down in about 3.5 secs.

Click to expand...

I think you should do a complete sim of the flight. I doubt the chutes will open before it impacts the ground. Thrust for the G250 lasts 0.5 s, just enough for the mini timer to detect launch.
0.9 s later the timer blows your charges. Although it's short that still takes time, especially with a heavy body and nosecone. Let's say 0.1 s; that's 1.5 seconds. It's a heavy rocket which requires a larger chute. Because it's a large chute on a rocket that's traveling slowly the chute won't open quickly. The first second of descent the chute won't even try to open. One more second and you're down, using your numbers.
You have too many things which are too close to theoretical minimums and no margin for error.
Why not fly it on something that makes sense for the weight? You have to be L1 to fly the G250 anyway.

I say fly with an H and let it go higher first. Use a Magnetic Anomaly Detector for the deployment charge(s). https://www.tindie.com/products/ZeptoBit/zeptomag/

Prove the system first allowing for a large interval between the rocket and the crowd. Once proven, shoot for the "World's Shortest Safe Flight" with your G motor. I'm not so sure a timer would be that accurate on such a tight flight plan. The advantage to the MAD unit would be on the G flight if the rocket
goes WAY off profile by tipping sideways, the MAD will blow the deployment charge and slow the thing down. As long as a fair distance is kept away from a crowd, the flight should be safe no matter what the outcome. I've flown the Galejs MAD unit like crazy but out of production. The ZeptoMag has some great safety features like turn on the thing and tip it over, nothing happens! The charge won't blow! Have to be up to speed and "G" before the device arms. Nice project otherwise. With the right venue the RSO's shouldn't have a cow.
If at a crowded launch with the first flight to be the G motor, ok, cause to be cautious. Prove it first then go for the G at a sparsely attended launch. Take the video with you to a major launch to soothe the safety concerns and then go for "WOWING" a crowd!
Why rush it?

Kurt

It went up to about 10,500 feet, and came down without a scratch. Just a follow up.

Good job! From 50 feet to 10,500 feet. Obviously something changed with your plan.

Handeman said:

At BattlePark, our RSOs have a graph with 3:1, 5:1 and 7:1 lines on thrust vs. weight. As long as your average thrust is over 5:1, you're good to go on TTW. If you're under 3;1 that is almost always a no-go, if it's under 5:1 then we take a closer look. it doesn't mean you can't fly, it just means it will take a while for the RSO, LCO, BOD, etc. to decide if, where, and/or how we will let you fly.

Click to expand...

Battle Park was the first site is been to, where there was a box to fill in the flight card for thrust-to-weight ratio. I was asked to calculate it for a rocket I had flown many times before with no issues... It was about 15:1. Not a bad check but a surprise question that I had to stop and do some quick math to answer. As long as it's not applied as a hard and fast rule, I think asking for it is a good idea for RSO checks. I've been too many places with questionable RSO practices... that lead to questionable or even unsafe flights.

Normally, I only use TTW ratio as a ballpark number for starting to pick out motors and see if I'm getting too heavy while the rocket it's still just a concept or under construction. For new builds, larger projects get simmed, small stuff may just go through the thrust curve app.

AdAstraPerAspera said:

Battle Park was the first site is been to, where there was a box to fill in the flight card for thrust-to-weight ratio. I was asked to calculate it for a rocket I had flown many times before with no issues... It was about 15:1. Not a bad check but a surprise question that I had to stop and do some quick math to answer. As long as it's not applied as a hard and fast rule, I think asking for it is a good idea for RSO checks. I've been too many places with questionable RSO practices... that lead to questionable or even unsafe flights.

Normally, I only use TTW ratio as a ballpark number for starting to pick out motors and see if I'm getting too heavy while the rocket it's still just a concept or under construction. For new builds, larger projects get simmed, small stuff may just go through the thrust curve app.

Click to expand...

I tend to use the TTW the same way you do with design and sim. I'm happy to say that BattlePark doesn't use the TTW as a go/no-go decision point, they use is as a flag for the LCO for "take a closer look at this". With that said, if you have a rocket with a 5:1 or less ratio, coming with the thrust curve for the motor would be a very good thing.