Possibility of using spring to boost Estes rocket on launch

[WigwamTrail]

Hey y'all

My grandpa and I are getting into model rocketry as a summer project, and he had an idea that's been stumping both of us.

The idea is what if you could use a spring to give a model rocket (Estes Mongoose to be exact) a "boost" before igniting the motor, his logic is that since you use the most fuel to go the least distance at launch, giving a small boost at that stage would improve the efficiency or performance a sizeable bit.

So I wanted to ask if anyone had experience with something like this, or had input of how we could pull this off. I've attached some sketches of initial ideas we had, top one having the spring compressed by a string which is then burned by motor exhaust, releasing the spring. Bottom one having the spring on the guide rail compressed by a servo, which moves to release the spring, pushing against the rockets launch lug.

Thanks everyone

 

[CalebJ]

The problem I see is the spring not getting the rocket off the rod with enough speed to guarantee stability. So the outcome is the motor igniting at a time when it's likely that the rocket may -already- be unstable/not oriented upwards.

And that, of course, is ignoring the technical problem solving of how you're igniting the motor after it leaves the pad.

 

[SolarYellow]

Research piston launchers. Lots of info out there. It's a system designed to do what you're thinking. It's activated by the rocket's motor, so no problem coordinating when the events occur.

 

[WigwamTrail]

The problem I see is the spring not getting the rocket off the rod with enough speed to guarantee stability. So the outcome is the motor igniting at a time when it's likely that the rocket may -already- be unstable/not oriented upwards.

And that, of course, is ignoring the technical problem solving of how you're igniting the motor after it leaves the pad.

Sorry, I should have clarified further, the spring would just be to get the rocket moving, not to get it off the pad/guide rod.

I guess the best way to describe it is the spring would move the rocket a couple inches up at most, as it accelerates from the spring the motor ignites.

Thanks for replying!

 

[WigwamTrail]

Research piston launchers. Lots of info out there. It's a system designed to do what you're thinking. It's activated by the rocket's motor, so no problem coordinating when the events occur.

Thank you! Will do so

 

[lr64]

If you're going to go to that much trouble, suggest you use compressed air instead of a spring. Easier to get some real power, even if pressurizing by hand with a bike pump. It seems to me that having the motor burn through something will insure that it won't launch without a lit motor. The water rocket guys have their own sort of piston launchers, which I think might be adapted. Minus the water, of course, unless you want a water rocket first stage.

To make the burn thing work, you probably need something that's fairly strong, but burns through quickly. Natural fiber soaked in KNO3? (You could use Spectracide stump remover). Of course, once soaked, you'd need to treat the stuff as quite flammable.

 

[Art Upton]

The Rocket Contesters solved this with piston launches way back in the late 70s and 80s. Lookup how to build a model rocket piston launcher.

No Springs needed , the captured thrust is more powerful to run the piston

 

[Alan15578]

Yes, but the spring should be more like a crossbow. Not legal for NAR competition.

 

[BEC]

If you're going to go to that much trouble, suggest you use compressed air instead of a spring. Easier to get some real power, even if pressurizing by hand with a bike pump. It seems to me that having the motor burn through something will insure that it won't launch without a lit motor. The water rocket guys have their own sort of piston launchers, which I think might be adapted. Minus the water, of course, unless you want a water rocket first stage.

To make the burn thing work, you probably need something that's fairly strong, but burns through quickly. Natural fiber soaked in KNO3? (You could use Spectracide stump remover). Of course, once soaked, you'd need to treat the stuff as quite flammable.

This is the whole point of piston launching, though the gases are supplied by the motor. MUCH information out there. Much simpler than doing anything with burning KNO3 or such.

 

[lr64]

This is the whole point of piston launching, though the gases are supplied by the motor. MUCH information out there. Much simpler than doing anything with burning KNO3 or such.

A bit of black powder sprinkled on the piston could really liven things up, but would have to be done with real moderation!

While KNO3 soaked string isn't very hard to do, and would be entertaining, perhaps a fine sewing thread with some kind of mechanical advantage would work as well. Just so the dohickies giving the advantage were low mass and could release quickly.

 

[amiles]

I think there is a problem with the premise of this question, and the idea that the initial thrust is somehow less effective if the rocket starts from rest. The rocket motor provides a fixed amount of thrust, and a corresponding fixed acceleration. The motor does not know or care whether the rocket's speed at ignition is zero and the rocket on the ground, or the speed at ignition is a few feet per second with the rocket a couple feet above a spring. Neglecting drag and other details, the end result in peak speed should only be that initial few feet per second that the spring provided. And if anything, the initial thrust would seem to be more effective if starting on the ground, where it gets to push against a blast deflector.

 

[Steve Shannon]

Sorry, I should have clarified further, the spring would just be to get the rocket moving, not to get it off the pad/guide rod.

I guess the best way to describe it is the spring would move the rocket a couple inches up at most, as it accelerates from the spring the motor ignites.

Thanks for replying!

The problem with doing that is that it results in less guidance from the guide rod.
A very important aspect for safety is the airspeed of the rocket as it leaves the rod.
If the spring moves the rocket closer to the end of the guide rod you lose some of the guidance and could leave the rod before the rocket has built up airspeed. It must be going fast enough that air moving over the fins causes the rocket to fly stable. A spring (or even a rubber band like a slingshot) could certainly augment the acceleration, but increasing the impulse of the motor does so more simply.
It’s great that you and your grandfather are flying rockets and engaging in very interesting thought experiments.

 

[Art Upton]

Again, the Contesters have worked for decades to get the best results of boosting the rocket to win.
No need for pyro augmented strings or springs, they wont even work near as well as a piston.

Just use a Piston Launcher

Not all piston launchers are complex, there are many really simple ones.

 

[Art Upton]

A bit of black powder sprinkled on the piston could really liven things up, but would have to be done with real moderation!

While KNO3 soaked string isn't very hard to do, and would be entertaining, perhaps a fine sewing thread with some kind of mechanical advantage would work as well. Just so the dohickies giving the advantage were low mass and could release quickly.

That would be dangerous and frankly would be classified as EX-Research. It would not be legal for Model rockets under fire protection codes and/or insurance. Design posts of such things are Level 2+ HPR research even if used on a B motor; and restricted to the Research forum.

Thank you for understanding and not taking me as an old grumpy fart

 

[jadebox]

As the guy who built and launched a six-foot tall, fur-covered, spider-themed rocket, I feel the need to point out that we often do things just for the challenge and enjoyment of doing them. Using spring might not be the most effective way of imparting a little extra momentum to the rocket, but it sounds like an interesting and fun project.

 

[rcktnut]

If you are new to model rocketry my suggestion would be to forget about that for now. Research piston launchers and get to know how they work and are constructed.

 

[ksaves2]

If you are new to model rocketry my suggestion would be to forget about that for now. Research piston launchers and get to know how they work and are constructed.

Yup sounds like a complete PITA to do except for an introverted engineer who doesn't have anything else to do. It's like a lot of complexity with minimal gain. Plus..........The spring would get "cooked" every time and affect the launch ability next time it was used. O.K. could use a "single use" spring but
the consistency between "different" springs would be in question. Sound familiar fellow fliers when dealing with rocket motors?
Sometimes get a "nominal" commercial motor flying and next time one of the same advertised impulse seems "hot". Been there, done that.

 

[shockie]

Way back in the day (early to mid 60 's), Paul Hans created a rubber band launcher, activated by burn string.

 

[lr64]

That would be dangerous and frankly would be classified as EX-Research. It would not be legal for Model rockets under fire protection codes and/or insurance. Design posts of such things are Level 2+ HPR research even if used on a B motor; and restricted to the Research forum.

Thank you for understanding and not taking me as an old grumpy fart

I have been a bit fuzzy on the limits and was under the impression that I could mention things if I was vague about them. Should I go back to writing "I did something which must not be discussed" on the rocket wise today thread?
It's possible to be dangerous even with a 34NC, using an Estes motor, painted with an American flag, and launched on a 5 foot rod. What I mentioned is not NECESSARILY dangerous, depending on just how it was done.
I admit to being mystified and miffed by the conflation of HPR and DIY motors. 5 grams is much less hazardous than 500. I might even be interested in some sort of certification if it didn't involve driver ed in Indy cars.

Meanwhile, I will comply with my understanding of the forum rules, adjusting my understanding if corrected. I did not intend to violate them.

 

[SolarYellow]

Neglecting drag and other details, the end result in peak speed should only be that initial few feet per second that the spring provided.

Yes, but whatever that additional speed is, it is there for the entire time of the flight. So it could add considerably to apogee.

And if anything, the initial thrust would seem to be more effective if starting on the ground, where it gets to push against a blast deflector.

The motor ejecta that has already separated from the rocket pushing against the blast deflector has zero effect on the rocket. The inertia of the ejecta products being "shot" out of the nozzle is balanced by the impulse applied to the rocket at the front of the motor.

The problem with doing that is that it results in less guidance from the guide rod.
A very important aspect for safety is the airspeed of the rocket as it leaves the rod.
If the spring moves the rocket closer to the end of the guide rod you lose some of the guidance and could leave the rod before the rocket has built up airspeed. It must be going fast enough that air moving over the fins causes the rocket to fly stable. A spring (or even a rubber band like a slingshot) could certainly augment the acceleration, but increasing the impulse of the motor does so more simply.
It’s great that you and your grandfather are flying rockets and engaging in very interesting thought experiments.

I had assumed the point of the spring would be to increase the velocity of the rocket by the time it reached the end of the rod, not just kick the rocket up the rod before the thrust curve kicks in.

 

[Tom Flint]

Back in the late 80s I launched a NCR Phantom 4000 on a D12-0 using a catapult launcher that was made from a huge rubber band stretched between two posts driven into the ground on each side the launch pad. The rubber band went under the back of the rocket which was held down to the pad by a piece of string that passed under the nozzle to be burned through upon ignition. I apparently reinvented this system independent of Paul Hans, as I had never heard of him or his system. The catapult was necessary to get the heavy rocket up to speed as the D12 didn't have enough thrust all by itself. It was a lot of work for a 100 foot flight.

 

[SolarYellow]

If a spring is linear and follows the formula f = kx, where the force to compress it is equal to a constant k times the distance it is compressed x, then the energy stored is E = (1/2)kx^2. It's interesting in the context of a model rocket because the force accelerating the rocket will be maximum at the start and decrease linearly with distance traveled. The thrust curve necessarily starts at zero and increases to some higher level. A linear spring assisting at the start would give increased acceleration right at the beginning, tapering as the thrust rises.

It might be interesting to try this in a tower with the motor exhaust releasing the spring.

Relative to competition, the stored energy of the spring is increasing the energy imparted to the rocket, so it would logically have to considered in the impulse class. That's overly complicated, so prohibition is the easiest and probably best answer. Pistons use the rocket motor's own energy more effectively, so they are allowed, generally.

 

[rocket_troy]

If a spring is linear and follows the formula f = kx, where the force to compress it is equal to a constant k times the distance it is compressed x, then the energy stored is E = (1/2)kx^2. It's interesting in the context of a model rocket because the force accelerating the rocket will be maximum at the start and decrease linearly with distance traveled. The thrust curve necessarily starts at zero and increases to some higher level. A linear spring assisting at the start would give increased acceleration right at the beginning, tapering as the thrust rises.

It might be interesting to try this in a tower with the motor exhaust releasing the spring.

Relative to competition, the stored energy of the spring is increasing the energy imparted to the rocket, so it would logically have to considered in the impulse class. That's overly complicated, so prohibition is the easiest and probably best answer. Pistons use the rocket motor's own energy more effectively, so they are allowed, generally.

I also posted this tool in the recovery section 5 or so years ago.

www.propulsionlabs.com.au/spring_stuff/Spring_Estimator.xlsm

TP

 

[Gus]

I just did a one hour presentation on pistons for the Juniors hoping to make the next U.S. Spacemodeling team.

It covers both fixed and floating head pistons.

You can see the presentation here:

 

[amiles]

Yes, but whatever that additional speed is, it is there for the entire time of the flight. So it could add considerably to apogee.

A few feet per second for ~10 seconds is a few tens of feet, which is probably a few percent of apogee. "Considerably" is admittedly subjective, but to me that seems like a marginal effect.

The motor ejecta that has already separated from the rocket pushing against the blast deflector has zero effect on the rocket. The inertia of the ejecta products being "shot" out of the nozzle is balanced by the impulse applied to the rocket at the front of the motor.

The ejecta speed is much higher than the rocket speed, so has time to reflect off a fixed surface below and create some back reaction on the rocket. Essentially an additional pressure source from below. A small effect due to lack of lateral confinement and probably insignificant, but not zero.

I had assumed the point of the spring would be to increase the velocity of the rocket by the time it reached the end of the rod, not just kick the rocket up the rod before the thrust curve kicks in.

A reasonable assumption, but I was reacting to the original post, which said "since you use the most fuel to go the least distance at launch, giving a small boost at that stage would improve the efficiency or performance a sizeable bit."

 

[SolarYellow]

I just did a one hour presentation on pistons for the Juniors hoping to make the next U.S. Spacemodeling team.

It covers both fixed and floating head pistons.

You can see the presentation here:

Excellent presentation. Thanks!

 

[Spacedog49Krell]

I just did a one hour presentation on pistons for the Juniors hoping to make the next U.S. Spacemodeling team.

It covers both fixed and floating head pistons.

You can see the presentation here:

I just watched your Piston Launch presentation; it was very informative.

I received a request, last year, for a 1000Hz micro accelerometer to study piston launchers. After building a test prototype, the requester backed out. From your presentation it appears that little scientific data has been accumulated to determine what works best for piston launchers. My preliminary research indicated that piston launchers accelerate rockets at between 30-50 G's and that almost all of the data has been collected from high speed video and film. Five years ago, I launched a series of 500Hz flight computers in micrograin fueled rockets launched by the Reaction Research Society. The returned digital flight data showed that the previous 70+ years of photographic, movie, and high speed video data on the acceleration of micrograin rockets was low by 20%-25%. I wonder if digital piston flight data might show similar visual interpretation errors with previous acceleration data?

 

[shockie]

It's my understanding that the Russians Regularly used "a pinch of something" in their pistons, for a while , until it became obvious.

 

[Gus]

I just watched your Piston Launch presentation; it was very informative.

I received a request, last year, for a 1000Hz micro accelerometer to study piston launchers. After building a test prototype, the requester backed out. From your presentation it appears that little scientific data has been accumulated to determine what works best for piston launchers. My preliminary research indicated that piston launchers accelerate rockets at between 30-50 G's and that almost all of the data has been collected from high speed video and film. Five years ago, I launched a series of 500Hz flight computers in micrograin fueled rockets launched by the Reaction Research Society. The returned digital flight data showed that the previous 70+ years of photographic, movie, and high speed video data on the acceleration of micrograin rockets was low by 20%-25%. I wonder if digital piston flight data might show similar visual interpretation errors with previous acceleration data?

"... it appears that little scientific data has been accumulated to determine what works best for piston launchers."
Actually, there has been a very large amount of research done over many years. The problem is that piston function is just extremely complex and multi factorial. The net effect of a particular piston depends on a large variety of factors such as particular motor, rocket weight, piston tube length, piston tube weight, fixed vs. floating head, piston head shape, piston head length, piston head weight, piston head surface friction, tube internal surface friction, internal vs. external igniter leads, piston tube deceleration, forcefulness of motor fit in piston tube, etc. etc.

One of the most interesting studies done several years ago showed that for a particular motor type and rocket weight, best tube length is not a straight linear function but actually looks more like a sin wave, improving up to a certain length, then declining to a nadir then improving again with more length. Just incredibly complex.

As I have often said about another ridiculously complex competition rocketry item, streamers, until the U.S. military comes up with a serious need for this item we will never really know the answer, LOL. But that's what makes it so much fun. And pistons, they are just fun. If you've never launched one, watch the video and build a simple floating head piston with the rod stuck in the ground and launch it.

Steve

 

[ChrisFlanigan]

There has been good research on piston performance, particularly for 13 mm and 18 mm motors used in NAR and FAI competition events. Two somewhat recent (2017) R&D reports are available at:

https://drive.google.com/file/d/1VLWVZHnJu7Ms9xdP9AGYA1tUMIAQtxIt/view


The piston performance was measured using a Featherweight Raven with a 75G axial accelerometer at a 400 Hz sampling rate. The test data showed that piston performance is primarily driven by fairly low frequency characteristics. It also showed pretty good agreement between analysis predictions and flight results if sufficient data is available to characterize the initial thrust buildup of the motors.

Additional reports about piston performance are available on the NAR website.

-Chris Flanigan