commercial hunter rocket project final report

8/6/2019 Commercial Hunter Rocket Project Final Report

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Final Report

Marine Corps Warfighting Laboratory Commercial Hunter Program

GPS Weapon Guidance System

Josh Wood, Max Tonsi, Gavin Goodson

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Caruth Institute for Engineering Education

Lyle School of Engineering

SMU, Dallas, Tx

May 134, 2010

1. Background

In Fall 2009, the Marine Corps approached engineering students at several universities

with an intriguing proposition. Students of multiple disciplines and backgrounds were

split up into teams andtold asked to disrupt a specific military scenarioin essence,to

play the bad guys. Using only an internet connection, a basic engineering background,

and assuming an unlimited budget, students were encouraged to think outside the box

and use consumer off the shelf products to create sources of mission interference. A

group of SMU students proposed a GPS missile guidance system.

Essentially, tThe students used a frequent and currently used terror tactic as the source

of their COTS disruption. Insurgents in hostile areas are using basic trigonometry and

ballistic physics to aim unguided rockets at specific targets. Due to a number of factors,

including wind and propulsion inconsistencies, these attacks very rarely hit their

intended targets. If this accuracy could be increased even slightly by adding a relatively

inexpensive GPS guidance process, these attacks could instantly become more

devastating. With information about GPS and autonomous flight abundant on the

internet, it is not difficult to understandwhy this is a very possible threat. A cheap





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solution to retrofitting existing missile tech in hostile areas could be devastating, so it isvery viable information to know whether this capability is possible.

The initial report made by these students was a generic application of GPS technology; it

did not include design details or an in depth analysis of the processes behind the

technology. In January of2010, SMU engineering students were once again asked to be

of service by prototyping this technology.



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2. ResearchOne of the key ideas of this project is the influence of the internet not only as a source

to purchase materials from, but also as a knowledge base. Therefore, the research

conducted for this project was performed using only the internet(and limited to

websites that did not require any login credentials or any other registry information).

The research required for this project can be categorized into two components. The

first issue to tackle is the notion of autonomous flight using GPS. as the key guidance

component. In correlation with the recent surge in interest of autonomous electronics,

there exist a number of websites that are dedicated to this burgeoning technology.

DIYDrones.com was the most novice-friendly of these websites, offering beginners

guides, links to simple hardware platforms and sample software, and discussion boards

with troubleshooting. The community support of this website and the hardware it

promotes was probably the key factor in our decision to go with one of their platforms.

The ArduPilot autopilot hardware board was chosen based on several key factors. It

boasted a feature list that encompassed all of the functionality we required:

y Programmable 3-D waypoints the ability to pre-program destinations was key,as we can then translate destination points into targets

y Altitude control via elevator/throttle the elevators can be remapped to thehorizontal control canards allowing for elevation assistance and potential range

extension

y GPS interface for EM406A GPS the board includes the desired GPS interfacey Battery powered a must for portable electronics such as a rockety Small (30mm x 47mm) easily fits into the body tube of our chosen commercial

model rocket

y Extensive sample code repository code variations to assist in our debuggingprocess

The feature set on this hardware platform gave us a cost-effective autopilot system.

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and roll stabilization to the rocket were somewhat clear, so initially just the GPShardware required interfacing and extensive modification. Therefore, we took as many

design elements from the pre-existing R/C controlled rocket as we could.

Due to the nature of the testing that would be required of this system (approximately

15 unrecoverable launches would require the construction of at least 15 of these

rockets) we chose to apply these control surface elements to an existing model rocket.

One of the potential issues with attempting to navigate a rocket in mid-fl ight is the

possibility of roll along the longitudinal axisif the rocket rotates 180 degrees, any up

command will translate to down, and rudder controls will be reversed as well.

Eliminating this roll is the key to a successfully guided flight, and the online plans we

discovered had an elegant solution for solving this problem.

The method we use to gain roll stability is simple and works well. It

requires launching only on a clear day with the sun low on the horizon. A

control circuit is used to sense the sun and generate pulses sent to a

model servo to correct any roll. Then the operator can concentrate on

guiding the rocket by the joystick on the transmitter. The circuit that

changes the CDS photocell resistance into different pulse widths for the

servo is a simple 555 timer thatcan be assembled on an experimenter's

circuit board.

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Figure 3.1. Photo roll stabilizer circuit

There are several boards to choose from at Radio Shack. Other parts are

.01, .22 and 10 mf capacitors, a silicon diode (276-1122), a CdS cell (276-

116), a 68k, 1/4 watt resistor, and the proper connectors for your make of

servo.

Figure 3.2. Roll stabilizer circuit diagram

If you are firing the rocket vertically it is best to shoot when the sun is low

on the horizon during morning or evening. Set upthe launch pad so the

sun shines onto half the photocell thru the window in the body tube. As

the rocket turns, the servo must move the control surfaces so as to turn

the rocket back to keep the sunlight on half the cell. If light totally floods






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the cell, it should roll back to be half-covered. If it is shaded entirely, itshould roll to be half-lighted again.

Figure 3.3. Roll stabilizer window and control surfaces

Due to the age of the website containing these instructions, some of the parts theyused

in their design were discontinued. The CDS cell we used to replicate this system had a


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smaller internal resistance, which had to be accounted for in the circuit design. Otherthan that issue, this circuit was copied and initially verified as functional.

Figure 3.4. Modified design using existing model rocket

Figure 3.5. Light-sensing roll stabilizer circuit

With roll stabilization addressed, attention then turned to recreating the RC rocket and

modifying the ArduPilot hardware. For the control surfaces, we decided to emulate the

RC rocketit was a proven design (according to the websites author), and would




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therefore possibly save us some development time compared to devising our ownsystem. We developed the autopilot software while assuming we had functional radio

control of the control canards; this allowed for the a lignment of the development

schedules of both systems.

The website for the R/C rocket lacked detail when describing the linkages between the

front servo motors and the control canards. The best approximation can be obtained by

simply looking at the photo the author provided. It is these linkages that would prove to

be the most complex mechanical design element, as the non-uniformities of our

manufacturing process became more and more evident.

Figure 3.6. R/C rocket layout

The software modifications we decided to implement on the ArduPilot board were

intuitive for the application of model rockets as weapons. Using a fully functional

sample code provided on the DIY drones website (originally configured to an

inexpensive Styrofoam model aircraft), we simply remapped the rudder controls on the

autopilot to the vertically-oriented control canards on the rocket. The same process

was applied to the elevators, remapping them to the horizontal canards.



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There is one last item ofnote regarding the design of our modified rocket system. Thenotion of testing this illegal guided weapon system raises several logistical

complications, most notably testing locations. By raising our maximum potential flight

range, it would severely limit the number of testing sites within a reasonable geographic

distance. Therefore, we decided to power the unguided and guided rockets with a D

size engine capable of20 Newton seconds of thrust, which is one size smaller than what

the rocket was originally designed to handle. This engine/rocket combination was rated

for a 600 ft. maximum altitude when launched straight up. Since our launches were

going to be around 50 degrees from the ground, we needed a radius of approximately

1000 ft in all directions. An increase in motor size would significantly increase the

required testing real estate, so the decision to use the D engines was made.

4. Navigation Control and Testing

The first phase of testing consisted of confirming the functionality of the initial

hardware purchase. For this, we constructed a basic electronics tray much like what

would eventually go inside the body tube of the rocket. From there, we wired the board

up as per the assembly instructions page on the ArduPilot website. Two servo motors

were added, but not attached to any control fins; for this test, we were only concerned

with accurate motor turning and proper direction of turn. These were connected to the

rudder/elevator-OUT ports on the GPS-interface ArduPilot board, with the R/C control

coming through the proper IN ports.



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Figure4.

1.

Wiring for RC/

S Tes

Our original thought was to si ly obtaina GPSlockan walkaroun relati e to a pre-

progra e waypoint an obser e theser o motor movement for therudder. This

testing method was quicklyrevised upon furtheranalysis of thecode, as thelocation

resolutionand refresh rate of the GPS wasnot high enough to obtaina quality bearing

determinationat walking speeds. To solve this, we programmed anew waypoint and

hit the freeway to obtainsolid GPS data. At highwayspeeds, the GPS output our

coordinatessuccessfully.

5. Wi

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Due to the unrecoverablenature of our design, simulationisakeycomponent in testing.

In order to realistically test the two mainareas of our design (in-flight longitudinalroll

stabili ationand controllable flight viacanards) weneeded to simulate the high speed

flight of therocket. This would require theconstruction ofa wind tunnel.

The originalconstruction planconsisted ofa 6x2x2chamber with a transparent side

and lid foreasy observationand to eventually test therollstabiliersunsensitivity. The

source of wind was originallyanarray ofseven ducted fans powered byDC power

supplies. The feasibility of thisconstruction was quicklynixed, as the powerneeded for

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the fans to output the required wind speed (approx. 175

mph) would require enoughpower supplies to fill a room.

Figure 5.1. Original wind tunnel design

In the spirit of the commercial of the shelf theme of this project, we opted instead to

power the wind tunnel with readily available electric leaf blowers. The model we chose

boasted a 210 mph wind speed, a 2 way speed selector switch, and a standard 120V AC

plug.

In an attempt to avoid significant power-related issues in our wind tunnel construction,

we hit a minor snag. The electric leaf blowers each draw 10 amps, which would blow

any normal circuit if two or more were connected at start-up. To circumvent this issue,





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Figure 5.2. Roll Stabilizer test setup

We reconfigured the tail section to be further toward the front of the wind tunnel

chamber, thus increasing the wind exposure of the control flaps. We again experienced

little to no rotational influence cause by the control flaps. Instead, we noticed a pattern,

which is explained by the fact that our rocket had three stabilizer fins (compared to the

four in the template we were following). This creates a weight imbalance, creating a

natural tendency to have two fins on at the bottom and on at the top. To combat this,

we added weight at the tip of the bottom fin, as to maximize the effect of the weight on

limiting the roll of the rocket.



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Figure5.3. Repositioned tail fins

Figure5.4. Originalstabilier fins

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Figure5.5. Weighted fin.

With this weight added, and byshifting the batteries to the bottom of the tray, we

significantlylowered thecross-sectionalcenter of gravity. In the high wind conditions,

the weight influencecaused therocket to stay properly oriented effectively. From

there, we decided that wecould conquer theroll-stabilityissue with asimplere-

distribution of weight.

Our final wind tunnel test was to confirm theability of the front canards to steer the

flight of therocket. For this, weconfigured thecanard control motorsinto the RC

mode, so that wecould manually drive therocket whilein the wind tunnel. Thoughwe

experienced some dead zones when trying to maneuver therocket through the

airstream, thecanards functionality wasconvincingproven. There was potential for

more wind tunnel testing before westarting our production, but the generator used to

power theleaf blowers wasrented, and therefore we had alimited window to do our

testing.

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Figure 5.6. Free-moving wind tunnel guidance

6. Flight Tests

Our flight test plans began with choosing an appropriate location to perform these

potentially dangerous activities. We settled upon a ranch in rural Oklahoma, with

enough flat land to safely test and keep our rockets in a reasonable range of visibility.

We set up our launch pad and anemometer and determined a launch direction. For the

first set of tests, we were simply attempting to gain unassisted ballistic launch data.

Using D engines in unmodified rockets (save for weighting the nosecones to ensure a

strong ballistic trajectory), we launched a rocket at a 45 degree angle from the ground.

It was assumed that this would maximize range, but with the extended burn of the

rockets, we felt that a higher trajectory would extend the range and flight time

considerably. At 60 degrees, a noticeable jump in range is achieved. Therefore, we

continued with nine more (10 total) launches with this configuration in relatively similar

wind speeds. The results can be found below in the test results section.

We then attempted a guided launch with the same setup and a GPS destination set to

the right of where the average unguided launch landed. The results were




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underwhelming, as we had grossly underestimated the impact of the added weight ofthe electronics tray, control canards, and batteries. The first guided flight managed

only 25 yards or so. We decided to return the next week with larger engines.

Figure 6.1. Unguided launch w/ anemometer

The largest off the shelf available rocket engines is size E which has a maximum thrust

of40 Newton Seconds., which is what o Our rockets were originally intended to use E

size rockets for propulsion. We reestablished our range difference (betweenguided and

unguided) by launching two unguided, unloaded rockets with size E motors. With this

data recorded, we attempted four more guidance test launches. The purpose of these

was originally to establish a maximum loaded rocket range as well as fine-tune the

control loops involved in the guidance process.



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The team came to the conclusion that the system was not operating properly, but it wasour last day of testing, and we needed to acquire some comparable data. Of the ten

remaining launches for the fully configured guided rockets, we experienced mechanical

(improper alignment whenglue dried) and electrical (ArduPilot board simply would not

power on) issues on two of them. This left us with eight rockets that were available for

a viable launch. The data from these launches is below.

The point repeatedly confirmed throughout our fully loaded rocket launches is that

these rockets, for their new weight, are still significantly underpowered. The flight

times we recorded were in the range of four to six seconds; this truncated in-flight

duration allows for only 3-4 potential course corrections, which is far lower than our

original prediction of30. It is also important to know that, in their current

configuration, the ArduPilot boards were unreliably switching between operating

modes. Our rockets were only designed to operate when the autopilot board is in

Waypoint mode; unfortunately, these boards were not holding their states as reliably

as the initial 3 boards we purchased from the manufacturer.



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7. Unguided Flight Results

The following table shows the GPS location information for the launch station and the

subsequent 11 unguided launches. The firstlaunch behaves as an outlier because it was

before we adjusted the launch angle from 45 degrees to 60 degrees.

Table X7.1. Unguided Launch Info

Below is the application of these GPS landing coordinates into the GoogleEarth

program. The CoM tagged icon is the center of mass of the landing zones. The standard

deviation of this landing data is 27.45 meters.


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Figure 7.1. Unguided rocket landing zone spread

8. Guided Flight Results

The following table denotes the landing GPS coordinates for the guided test launches.

Note the significant difference in the range of an unburdened rocket with a D size

engine vs. the fully equipped guided rocket with the more powerful E engine. Also,

several of these flights experienced inconsistencies that are worth noting.

Flight F one of the tail fins became detached loose in the transportation process.

Flight G one of the more mechanically sound rockets we produced, the launch stand

began to fall down at the exact moment G was being launched. This resulted in a

roughly 30 degree ballistic angle vs. the ideal 60 degree angle.




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Flight I Mechanical issues plagued flight I; the control linkages and canards wereinterfering with one another prior to launch, and the rocket was missing its top flight rail

guide

Flight M in an attempt to compensate for the wind issues, we ended up having the

launch rod angle too high,basically launching Mnearly vertical directly up.

.

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Figure 8.1. Guided rocket landing zone spread Formatted: Font: +Body


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Figure 8.2. Cumulative test data

10. Design Critique

As a complete system, the GPS guided rocket design was susceptible to several sources

of error. The wind-tunnel testing earlier in the schedule provided a certain degree of

confidence that a simple weight r e-distribution could eliminate or severely reduce

longitudinal roll during flight; however, this characteristic was inconclusive during our

test flights. On multiple occasions, the rocket rotated almost immediately after leaving

the launch guide rod, but did not cover enough distance to self-correct. From the

testing done so far, the flight times have not been long enough to determine if the

weight distribution effectively limits the body roll.

Another significant factor in the discussion of sources of error lies with the variation in

manufacturing of these rockets. From the electronics tray to the control canards and

steering linkages, custom fabrication was required to successfully modify these model

rockets. Unfortunately, variability was impossible to limit given the number of rockets

constructed and the complexity of the control system we were attempting to create.

This led to inconsistencies in axel placement, control arm throw distances, tray locking

mechanisms, and other areas.

Our decision to modify an existing rocket system for our prototyping was also

potentially detrimental to the design. The cardboard tubes we were using had a

significant amount ofgive and flex, and the holes cut for the canard axels did not stay

round for very long. Working with these existing rockets also made construction very

tedious.

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The last significant source of potential design flaw is linked to the GPS update rate. TheGPS that the ArduPilot board was designed for, the EM406A, has a 1 Hz update rate.

This We originally did not find issue with a 1Hz update rate, as the desired flight time

was to be around 30 seconds. This would provide roughly 30 course corrections over

the flight of the rocket. Unfortunately, due to the severe overestimation of flight time,

the rockets were unable to receive more than a few course corrections per flight.

11. Next Steps

At this point in the project, we have determined what has and has not worked, and

what areas of improvement are required. The most significant of these is the overall

platform from which to prototype this guidance system. From the teams perspective,

the next logical step in the design process is to design a rocket from scratch. The hobby

rocket platform was severely limiting in what designelements we could integrate into it.

A custom designed rocket would solve a significant amount of the manufacturing issues

(primarily the repeatability and interchangeability of processes).

Building upon a custom platform, the roll stabilization system needs an overhaul.

Where our initial wind tunnel tests provided a reasonably high confidence level in the

weight/balance modification solution, it became clear that addition systems were

needed. Our next approach would be to actually incorporate the sun-sensing roll

stabilizer with a modified weight distribution.

Though it has been mentioned repeatedly throughout this report, the flight time still

stands as a large barrier to our progress. It is difficult to pinpoint the sources of error

when the test sample (in this case, a test flight) is so small. To counter this, the future

plans should most certainly include a more powerful engine.



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The last notable redesign element is to improve the electronics configuration. Thecurrent system was rigged to be toggled by an RC controller, with no internal timer or

mechanism to initiate guidance at a specific point in flight. Some of the inconsistencies

we experienced on the hardware/software side of this project can be addressed through

the use of hard-coding and persistent connections.

12. Feasibility of Deployment

Our experiences with developing this system led us to a series of conclusions about the

feasibility of retrofitting GPS guidance onto existing missile systems. Our team strongly

believes that this capability is in fact possible, but that it is also a very difficult

accomplishment. The notion of adding such a precise level of control to an existing

platform calls for a great deal of ingenuity, patience, and a great deal of resources. With

more testing and design revisions, this is a distinct possibility.

13. Countermeasures

One of the aspects that makes GPS guided weapons so troublesome is the ability to fire

and forgetthis basically means that once the trigger is pulled, there are very few

avenues to go about stopping a weapon of this type. To be able to disrupt or confuse a

GPS guided autonomous missile, GPS signals would need to be blocked in the area of

interest. This action would render all GPS useless though, both friendly and foe.

Instead of focusing on how to disrupt this system after it has been launched, we suggest

interfering before the first prototype is even built. This would require the monitoring of

large purchases of hobby gear (servo motors, engines, autopilot hardware), as well as

screening the community interactions for any suspicious conversations.



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Lastly, the nature of this type of system lends itself to significant amounts of testing.Therefore, the surveillance of potential testing sites could potentially thwart this threat.

It is difficult to hide a missile launch, much less 15 launches.


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14. Acknowledgements

We would like to acknowledge the following individuals and organizations for their

contributions:

Marine Corps Warfighting Laboratory- for providing this opportunity to assist them in

their missions to defend and protect this Nation

Nathan Huntoon for his guidance and support throughout all phases of this project

Kristine Reiley for the constant trips to the hardware store and judicious use of her

Chevrolet Extended Cab Pickup

Mel Lively for the use of his land, as well as his home, as a rocket testing range

Mikes Hobby Store for prompt hobby component orders and great customer service

Hobby Town USA for providing timely rocket engine restocking and lessons in

removing the ejection charge of said engines




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