Best rocket-stopping method

[Rocket Maker]

Does anyone have a good method for stopping a model rocket while it is still going up without reverse propulsion?

 

[Flyboy]

Define “stop”. Are we talking canceling your vertical velocity ASAP?

 

[Rocket Maker]

essentially

 

[berkeman]

Tether cable?

 

[Rocket Maker]

do you mean by tethering the rocket to the ground

 

[PeroK]

Inverted parachute?

 

[Rocket Maker]

do you mean using a parachute like a drogue chute

 

[berkeman]

do you mean by tethering the rocket to the ground

Yeah, like fishing line that plays out from a spool until you hit the "brake" lever thing...

 

[berkeman]

BTW, why do you want to do this?

 

[Baluncore]

Have high drag air brakes, that fold out from the sides of the rocket.

 

[Rocket Maker]

BTW, why do you want to do this?

I'm doing the American rocketry competition

 

[Rocket Maker]

Have high drag air brakes, that fold out from the sides of the rocket.

I was thinking of doing this but all of the servos that are light enough are too expensive and not powerful enough

 

[Baluncore]

I was thinking of doing this but all of the servos that are light enough are too expensive and not powerful enough

Hold them in place with a catch, then let them open backwards.

 

[Rocket Maker]

that could work

 

[Rive]

If you have the front side opening, then you can count on the air drag to open the brake.
Can be pretty destructive, though. But make it strong enough and it will stop that rocket like brick wall would.

 

[berkeman]

I'm doing the American rocketry competition

What are the rules?

 

[Rocket Maker]

These are the official rules

 

[berkeman]

These are the official rules

So why are you wanting to stop the rocket dead in the sky at some point? Is that the only way you can meet the flight duration and altitude target numbers?

2.5 DURATION SCORING

The duration score for each flight shall be measured from first motion at liftoff from the launch pad until the moment that the section of the rocket containing the eggs touches the ground (or a tree) or until it can no longer be seen due to distance or to an obstacle. Times must be measured independently by two people not on the team, one of whomis the official NAR-member adult observer, using separate electronic stopwatches that are accurate to 0.01 seconds.The official duration will be the average of the two times, rounded to the nearest 0.01 second, with .005 seconds being rounded up to the next highest 0.01 seconds. If one stopwatch malfunctions, the remaining single time will be used. The flight duration goal is a range of 41 to 44 seconds. Flights with duration in the range of 41 to 44 seconds get a perfect duration score of zero. Duration scores for flights with duration below 41 seconds will be computed by taking the absolute difference between 41 seconds and the measured average flight duration to the nearest 1/100 second and multiplying this by 4. Duration scores for flights with durations above 44 seconds will be computed by taking the absolute difference between 44 seconds and the measured average flight duration to the nearest 1/100 second and multiplying this by 4. These duration scores are always a positive number or zero. The target duration range for flights at the Finals will be the same regardless of the altitude target.

2.6 ALTITUDE SCORING

Rockets must contain one electronic altimeter of the specific commercial types approved for use in the American Rocketry Challenge that will be the sole basis for the altitude score. These approved types are the Perfectflite Pnut or Firefly or the Jolly Logic Altimeter One or Altimeter Two. The altimeter must be inspected by an NAR official both before and after the flight, and may not be modified in any manner. The altimeter must be confirmed by this official before flight to not have been triggered and to be ready for flight. The Jolly Logic altimeters must have all previous flights erased before a scored flight. The peak altitude of the rocket as recorded by this altimeter and sounded or flashed out on its audible or visible light transmission or displayed on its screen post-flight will be the sole basis for judging the altitude score and this altimeter may be used for no other purpose. Other altimeters of other types may be used for flight control or other purposes. The altitude performance goal for qualification flights is 790 feet (241 meters). The altitude score will be the absolute difference in feet between the altitude performance goal in feet and the altimeter-reported actual flight altitude in feet (always a positive number or zero). The altitude performance goal for flights at the Finals will not be 790 feet; it will be 765 feet and 815 feet, with the order for these between first and
second flights determined by a coin toss at the student team pre-flight briefing at the Finals.

 

[Rocket Maker]

I think it would be the most effective way to meet the altitude goal

 

[Flyboy]

So, you have a time of flight and an altitude target to aim for, with an egg on board. So, that constrains some of the design parameters.

I would argue that you should design your payload section first, then a suitable booster section, select a motor that will provide the correct amount of total impulse to get you to that target height and the correct amount of ejection charge delay, and finally size your parachute to give you the correct amount of hang time under the parachute to hit your time goal.

 

[berkeman]

I would argue that you should design your payload section first, then a suitable booster section, select a motor that will provide the correct amount of total impulse to get you to that target height and the correct amount of ejection charge delay, and finally size your parachute to give you the correct amount of hang time under the parachute to hit your time goal.

So you're saying that you don't like my hillbilly fishing pole idea, huh?

 

[Flyboy]

So you're saying that you don't like my hillbilly fishing pole idea, huh?

I’m not saying that it won’t work, it absolutely will. But once the time component was mentioned, it strongly suggests that they’re looking for carefully designed, tested, and tuned rockets.

 

[berkeman]

Yeah, I like how you used the size of the parachute to fine-tune the total flight time.

 

[erobz]

Yeah, like fishing line that plays out from a spool until you hit the "brake" lever thing...

(Not trying to be a Debbie downer) I think this violates the “externally generated signal” rule in flight control section?

 

[Flyboy]

(Not trying to be a Debbie downer) I think this violates the “externally generated signal” rule in flight control section?

… y’know, that’s a good point.

 

[Baluncore]

Does anyone have a good method for stopping a model rocket while it is still going up without reverse propulsion?

When a fast moving explosive device, detonates in the atmosphere, the fireball effectively stops immediately. That is because the sphere of combustion products has a huge cross-sectional drag.

You could achieve the same effect on a small rocket, by detonating the passenger or driver's airbag from a vehicle. The attachment of the bag to the rocket would be important, since the bag is the rocket's connection to the volume of combustion products. You would need a cheap supply of airbags recovered from a vehicle wrecker.

An explosion in the atmosphere is followed by a partial vacuum, due to outward momentum. That partial vacuum is not present in an airbag, as the bag is designed to become a momentary support cushion. The bag could stop the rocket sooner, if there was sufficient space for the greater volume of the hot combustion gasses, even without a following depression. It will come down to balancing the internal gas pressure against the external dynamic pressure at that velocity. The tension in the bag fabric, and the speed of sound in the fabric material, may be important in keeping it all together. Expansion of the combustion gasses should cool them quickly to below the bag material melting point.

You might use the squib from a vehicle SRS, but make your own toroidal bag, that fits into the neck of the rocket. If the bag was sealed, it could also reduce the free fall velocity, and cushion the landing of the rocket, possibly avoiding the need for a parachute.
https://en.wikipedia.org/wiki/Airbag#Spacecraft_airbag_landing_systems

When correctly handled, airbags are no more dangerous than rockets.

 

[Vanadium 50]

@Rocket Maker you need to be a lot more forhcoming - peope are trying to help you and you're only willing to type terse, nearly information free answers.

I don't see how stopping a rocket in flight will help you reach altitude or duration goals. You should start by explaining that before moving on to the how.

 

[Flyboy]

Okay, having looked at the full criteria listed in the rules, I think you have to rule out any sort of assisted deceleration:

2.3 ROCKET REQUIREMENTS

Rockets must not exceed 650 grams gross weight at liftoff. The overall length of the rocket must be no less than 650 millimeters (25.6 inches) as measured from the lowest to the highest points of the airframe structure (including fins) in launch configuration. They must use body tubes of two different diameters in their exterior structure, the upper one of which must have sufficient inside diameter to hold an egg of up to 60mm length sideways and the lower one of which must be no greater than 57 millimeters in outer diameter (T-70 tubing). Each of these body tubes must be no less than150 millimeters (6 inches) long. The rocket must separate into two non-connected pieces for recovery, with one piece containing the eggs and altimeter and the other containing the rocket motor. Each piece must recover safely by parachute. Rockets must have the team’s official American Rocketry Challenge entry number written on them. Rockets flown at the Finals will be required to have a paint or other decorative coating applied to any wood, paper, or fiber exterior surface of the rocket and will be assessed a 5-point flight score penalty on their first flight at the Finals if they do not. Rockets may not be commercially-made kits designed for this event or kits designed to carry egg payloads with the only modification being the addition of an altimeter compartment. They must have only one stage. They must be powered only by commercially-made model rocket motors of “F” or lower power class that are listed on the American Rocketry Challenge Certified Motor List posted on the ARC website and provided in the ARC Handbook. Any number of motors may be used, but the motors used must not contain a combined total of more than 80 Newton-seconds of total impulse based on the total impulse ratings in the ARC list and they must be used only for flight propulsion. Motors must be retained in the rocket during flight and at ejection by a positive mechanical means (clip, hook, screw-on cap, etc.)and not retained simply by friction fit in the motor mounting tube, but they must be removable post-flight. Rockets must not contain any pyrotechnic charges except those provided as part of the basic commercially-made rocket motor used for the flight, and these must be used only in the manner prescribed in the instructions for that motor.

So, as a TL;DR:

• Rockets must not exceed 650 grams gross weight at liftoff.
• They must use body tubes of two different diameters in their exterior structure, the upper one of which must have sufficient inside diameter to hold an egg of up to 60mm length sideways and the lower one of which must be no greater than 57 millimeters in outer diameter (T-70 tubing).
• The rocket must separate into two non-connected pieces for recovery, with one piece containing the eggs and altimeter and the other containing the rocket motor. Each piece must recover safely by parachute.
• They must be powered only by commercially-made model rocket motors of “F” or lower power class.
• Rockets must not contain any pyrotechnic charges except those provided as part of the basic commercially-made rocket motor used for the flight, and these must be used only in the manner prescribed in the instructions for that motor.

That rules out pretty much every reasonable source of extra drag that you could deploy based on a precise altitude. No drag brakes, as they would be too heavy and may violate the dimensional rules, and would certainly violate the parachute recovery requirement. No ballutes as @Baluncore was suggesting on their last post, as that violates the pyrotechnic charge rule, as well as the parachute recovery requirement.

I think you're going to have to do design optimization for the flight profile, like I first suggested. As cool as it would be to design a "stop on a dime" kind of rocket, I think they're looking for careful design and construction, not innovative thinking.

Nothing says you can't do one on your own time, though. I would advise much more caution as you're probably going to run into a lot of challenges with weight, size, and complexity, and possibly running into the realm of high-power rocketry.

 

[Flyboy]

So, a little bit of a follow-up, more to answer the question initially asked... There's a few ways to encourage the rocket to rapidly decelerate in a controlled manner, several of which have seen use by the military for low-altitude, high-speed bomb delivery. The deployable airbrakes mentioned by Baluncore early on is a particularly well-known example, best demonstrated by the Snakeye tail kit for the Mk80 family of low-drag general purpose bombs:

Image from dcsworld.pro

Image from NMUSAF Cold War Gallery

The bomb has four petals that are restrained at the forward end by a strap, to which a lanyard is attached. Depending on the fusing option selected by the pilot, that lanyard is either allowed to depart with the bomb, leaving it in a closed, low-drag mode, or retained briefly to pull the strap off the bomb after it clears the aircraft, allowing the petals to deploy into their high-drag configuration.

The core tubular structure provides an easy place to park a motor, especially a long, thin motor, making it somewhat more approachable for hobbyists.

At least one model rocket enthusiast has successfully taken this concept and applied it to rockets, albeit as a recovery mechanism, where the loads are lowest.

There's a nice breakdown of the system in this article from Apogee Rockets. But again, emphasis on deploying it at low speeds to minimize structural loads on the linkages.

Baluncore's second suggestion, an inflatable airbag-type device, has also seen use, albeit more limited, by the military as well. Meet the Air Inflatable Retarder tail kit:

Image from dcsworld.pro

Image from Wikipedia

This is a ballute (combination of balloon and parachute), and it's actually well suited for very high speed (transsonic at sea level to low supersonic at altitude) deployment, conditions that would tear apart the mechanical petal brake designs. It uses a small gas generator charge, not unlike an airbag initiator, to provide initial ejection and inflation, before the air scoops built into the ballute continue to provide inflation pressure.

Note that the ballute approach requires an unobstructed exit to the rear to allow deployment, and the relatively large diameter of the casing that it's deployed from. If you use this with a rocket, you'll need to do some weird work with the rocket motor to get it to fire without burning through the ballute housing. Either a reversed motor with exhaust reversing ducts, or splitting the flow between several nozzles to exit just ahead of the ballute housing. Both options pose significant challenges for manufacturing.

Further challenges to the ballute approach are:

• The need to package it in a manner to allow safe deployment and inflation
• The relatively high diameter-to-length ratio needed for proper storage and deployment
• Design and fabrication of the ballute itself
• Reusability of the ballute is unlikely

I've not been able to find a single example of a small-scale ballute being made for anything other than subscale wind tunnel tests, and I don't know if it was even deployed by pyrotechnics or not. The smallest one I can find any info about is from Copenhagen Suborbitals.

This is a system that could scale up well for something larger, but that's also intruding into high-power rocketry turf just from size and weight increases needing high-power motors.

At the end of the day, both options will impose significant weight and complexity penalties, especially if they're designed to stop the rocket quickly at high speed, but if you wish to pursue an option, then the mechanical petal brakes are probably the easiest option.

 

[berkeman]

Is there any way to throttle or extinguish those amateur rocket motors, or are they on-until-burned-out?

 

[Vanadium 50]

They run until empty.

You can, however, select how long they run when you buy them. This is one reason I advised the OP to fly a lot of rockets,

 

[Flyboy]

You generally have several options in a given “class” of motor to tune your performance.

Most of these motors are black powder derivative fuel grains that are pressed inside the cardboard body tube of the motor and use an end-burning design.

And the burn time is usually no more than 2-3 seconds even on the big motors. Tuning rocket apogee height will be much easier by adjusting the weight, not the motor.

 

[Rocket Maker]

Thank you for all of the suggestions.

@Rocket Maker you need to be a lot more forhcoming - peope are trying to help you and you're only willing to type terse, nearly information free answers.

I don't see how stopping a rocket in flight will help you reach altitude or duration goals. You should start by explaining that before moving on to the how.

The altitude goal is we lose one point for every foot we go over 790 feet, so this is why I am asking and for the time limit the faster we can start going back down the better because we have to up and down within 44 seconds.

They run until empty.

You can, however, select how long they run when you buy them. This is one reason I advised the OP to fly a lot of rockets,

I have been flying Estes rocket kits so I have been using mostly C6-5s with the Riptide rocket kit.

 

[A.T.]

... must have sufficient inside diameter to hold an egg of up to 60mm length sideways ...

I wonder why the egg must be put in "sideways". It definitely doesn't help with keeping the center of mass on the axis.

 

[Vanadium 50]

It just says there needs to be space to hold it sideways. no?

Anyway, if you want less altitude, a sudden stop is bad for the egg. Better to use a smaller engine. One could also add weight.

So, what is the variation in height for "identical" launches? Seems you would want to know that.