Basics of Space Flight Section 3

Building the Gravity Assist Mechanical Simulator


Overall Description

The device almost resembles a pin-ball machine.

A clean glass tabletop is slanted slightly from the horizontal using an adjustable wedge. A smooth, spherical steel bearing ball (BB) is launched down a little ramp so that it rolls up and across the glass in an arc. A revolving magnet, driven by a motor, is introduced in such a way that the magnet passes near the BB in its arc, moving a little faster than the BB, and in generally the same direction.

Small view of Simulator

The magnet is labelled with a small image of Jupiter. In the image here, the small, flat, powerful magnet is entirely hidden by the cutout Jupiter image. The motor at the center of the Jupiter-magnet's "orbit" is labelled with a cut-out image of the Sun. An image of Saturn is placed on the glass surface, roughly twice as far from the Sun image as the Jupiter-magnet's orbit, up at a higher elevation on the slanted glass surface.

Paper clamp as cam
The BB is launched a number of times, first with the magnet out of the way, to calibrate and mark its default trajectory. Then the Jupiter-magnet is adjusted and set revolving. The BB's interaction with it is observed and noted.

An important part of the simulator is a trip mechanism that releases the BB down its ramp at a point in time based on the Jupiter-magnet's angular position as it revolves. This trip mechanism must be adjustable through a wide range, so that BB launches can take place at various times relative to the Jupiter-magnet's location in its orbit. The trip mechanism is a moveable paper clamp, whose handle strikes a trip arm, lifting a pivoted ball-release gate.

Mechanical Options

There must be many different ways to create and use a Gravity Assist Mechanical Simulator like this one. This website describes, in agonizing detail, one way to build and run it, lists all the components needed for this version, and suggests typical sources for the components*. It is possible to reproduce this very device, by following instructions on this website.

However, the individual who tackles a project such as this is necessarily a motivated person with some skills and resources who enjoys tinkering with gadgets. Therefore, your version may differ greatly. You might decide to put the revolving magnet under the glass rather than above it. You might have all sorts of different approaches or refinements. Your device will, though, probably need these basic components:

  1. An inclined clean, flat, smooth surface,

  2. A smoothly rolling BB,

  3. A BB-launcher capable of producing a repeatable trajectory,

  4. A variable-speed motorized* revolving Jupiter magnet supported in a suitable way,

  5. An adjustable trigger to release the BB on its trajectory automatically, based on Jupiter's position in "orbit" (if this mechanism were to be omitted, achieving good "gravity assists" would require too many BB launches to be practical),

  6. Labels for Jupiter (the magnet), for the Sun (the motor), for a target more distant than Jupiter, such as Saturn, and for the ecliptic plane (glass tabletop). The launch ramp gets an "Earth" label. It's important to label things, lest the participants or observers miss the whole point.

*Instead of necessarily employing a motor, some innovative person might want to work out a way to make a massive Jupiter magnet revolve freely, maintaining much of its momentum once set revolving (as does the planet). Such an arrangement might even show a noticeable decrease in Jupiter's revolving momentum during "assists" to the BB, illustrating in an exaggerated way the momentum tradeoff between planet and spacecraft.

Here's how to build the device

All the components for it appear in the Parts List below.

  • Obtain a flat glass surface such as a tabletop, about a yard or a meter in diameter if circular. It should have no dangerous sharp edges or corners. Make sure it is flat enough and clean enough, that when you tilt it and roll a BB on it you can observe a highly repeatable trajectory. This flat surface will represent the solar system's ecliptic plane, in which the planets mostly revolve.

  • Introduce an adjustable slant. Place your flat glass surface on a table. Make a tapered shim and insert it under one edge of the glass, so you can adjust the slant of the glass surface gradually. Its maximum thickness should be a little more than 1.5 inches or 4 cm. For this purpose, you can tape together a bundle of four or five tapered wooden "shingle" shims, obtained from a home improvement center (these are normally intended for window and door installations). The slant crudely illustrates the Sun's gravity.

    Whole Simulator, with components labelled

  • Obtain a 12- or 24-volt DC geared motor capable of turning from zero to about 40 RPM.

    Attach the motor to a flexible 16-gauge steel strap, about 2.5 feet or 75 cm long, using the motor's mounting screws. Bend the strap's other end around a heavy object such as a 12-inch or 30-cm square concrete brick to serve as a support. Bend and arrange the strap so the motor will suspend its attached Jupiter disk parallel to the slanted glass surface, about 3/8 inch or 1 cm above it. Be careful of any sharp edges on the steel.

    Wrap the end of the steel strap around a concrete block to serve as a base.

    Mounting the Motor

  • Obtain an acrylic disk, about 12 inches or 30 cm in diameter and 1/8 inch or 3 mm thick. Attach the acrylic disk's center to the motor shaft. One way to do this is to obtain an aluminum pulley or flange that attaches to the motor shaft with a set screw, then to drill and tap two holes in it either side of center. The acrylic disk can then be drilled and attached with machine screws.

    Use double-sided foam tape to secure a small, powerful, flat magnet to the upper side of the disk, out near the edge. Be sure the magnet is securely attached so it can't fly off when revolving.

    Insert a pencil-size object between the steel strap and the brick, that can be moved to adjust the height of the motor and disk. This is the "motor height shim" shown in the illustration. A piece of aluminum channel appears in the photo.

  • Connect a variable DC power supply to the motor. Obtain a suitable DC power supply that can vary its output from zero to about the motor's rated voltage. For the 24-V motor used here, the power supply is variable from 0 - 24VDC with a maximum current of 1 ampere. Connect the DC output to the motor using 16-gauge lamp cord. Secure the cord to the steel strap using nylon wire ties. Test the motor's operation: it should run the Jupiter magnet around counter-clockwise when viewed from above, to simulate viewing the planets from north of the ecliptic. Reverse the connection's polarity if necessary to reverse the motor.

    If you're skilled in safe, legal electrical work, follow this link to build your own power supply.

  • Select a BB. To represent a spacecraft on an interplanetary trajectory, obtain a bearing ball (BB) about 3/8-inch or 1-cm in diameter. It must be heavy, smooth, and spherical so it will roll steadily in a highly repeatable manner. It must be made of ferrous metal so it can be deflected by a magnet. BB Launch Ramp

  • Manufacture a launch ramp. Obtain a piece of aluminum U-channel about 2 inches or 5 cm in length. Its width must be a little smaller than the BB, so the BB can roll along its "rails." File down one end of the channel so the BB can roll smoothly off one end of the ramp. You might also need to file down the inside-bottom of the U-channel with a round file to permit the BB to roll smoothly off the tapered end. In this illustration, there's a little detent filed into the high end at left, useful for holding a BB temporarily. The photo shows the BB held in place on the ramp by the release gate, described below.

  • Devise a BB-release gate. The release gate, or escapement, is perhaps the trickiest part of the simulator to build and adjust. It has to hold the heavy steel BB on the ramp, and then release it suddenly when tripped.

    Start with a chopstick made of bamboo. Drill a hole through the chopstick about 1.5 inches or 4 cm from the thick end, for its fulcrum pivot. The hole must be wide enough to loosely accommodate a machine screw. The hole's width may therefore occupy much of the chopstick's diameter, requiring a bit of care in drilling. Carve the thin end of the chopstick down to about 1/16 inch or 2 mm diameter as shown, so that the BB can be restrained above its center by a very thin piece of bamboo.

    Drill another, smaller diameter hole very near the thick end. The "Trip arm" wire will need to attach by force-fitting into this hole. Force a small piece of copper wire, about 14 gauge, 1 inch or 2.5 cm long, into this hole, and bend it over into a loop as shown.

    BB Launch Gate

    Small metal L-brackets, about 3/4 inch or 3 cm on a side, are used: one to support the fulcrum, another as a "keeper" to steady the thin end of the chopstick. You'll need a third one (not shown in this illustration) to limit the gate's downward travel. Eventually, all this hardware is attached to the glass surface with double-sided foam tape, but only after their final positions are established by trial and error during construction. Until then, vinyl electrical tape can be used to hold down the components temporarily.

    Attach the gate to its fulcrum support as follows: Insert a machine screw of appropriate size through the L-bracket that will serve as the fulcrum support, and tighten a nut to it. Then slip the chopstick's fulcrum hole over the screw, where it should fit loosely. Finally, cinch two nuts against each other to lock the chopstick in place, but without tightening them onto the chopstick. Adjust for easy rotation about the fulcrum with minimum side-to-side play.

    Lay out the components as illustrated above. Place the "keeper" L-bracket against the thin end of the chopstick gate to keep the BB from pushing the gate out of place.

    The gate mechanism is working properly when a BB can be placed on the ramp behind the gate, and then when the trip wire is depressed a little, the thin end of the gate rises and releases the BB swiftly and surely. It may take several iterations and adjustments to get it to function correctly. In fact, it might take all Saturday afternoon.

  • Install a cam on the motor-driven disk, to trigger the BB-release gate. Clip a small paper clamp onto the edge of the acrylic disk that holds the Jupiter magnet, at any point on its perimeter. Slide the clip all the way up against the edge. Fold its lower wire-arm inward toward the disk's center. Fold its top wire arm so it extends outward from the disk.

  • Set the components in place. Set the motor-strap-brick assembly near the bottom edge of the inclined glass surface. See the large photograph above.

    • Adjust the position of the motor-driven acrylic disk so that it is parallel to the inclined glass surface, and at a height above the surface equal to about 125% the BB's diameter.

    • Place the BB launch-ramp and escapement temporarily, using vinyl tape.

    • Adjust the motor-assembly's position so that the paper-clamp cam arm can strike the trip-arm on the escapement and release the BB down its ramp.

    • Run the motor, and adjust the positions, until the escapement is working well.

  • Establish the BB's default trajectory. Turn off the motor and rotate the Jupiter magnet out of the way. Launch the BB several times by manually depressing the escapement's trip arm. Observe the normal path the BB takes. Adjust the inclination of the glass surface, the direction the ramp points, and the location of the motor and brick support. The objective is to have the BB follow an arc that meets the edge of the Jupiter disk at the height of the BB's arc, and then continue to roll off the side of the glass surface at about the "latitude" of the ramp. Place a "bucket" there to catch the BB. The plastic lid to a margarine tub makes a good "bucket."

  • Attach the ramp and escapement components permanently to the glass surface. Run the Jupiter motor and experiment for a while. Once everything is working as described above, it will be possible for the BB to "crash" into Jupiter (that is, stick to the magnet) once in a while as the BB rolls in its arc. It will also be possible to observe the magnetic interaction between the Jupiter magnet and the BB. At this point, tape ramp and escapement components in position using double-sided foam tape.

  • Label everything. Use cut-out images from a magazine. Label the BB launch ramp as Earth. Label the motor as the Sun. Label the magnet as Jupiter. Place an image of Saturn to act as a target twice Jupiter's distance from the Sun.

  • Tune, adjust, calibrate. Determine by experimenting what is the best speed for the Jupiter magnet to revolve. It should be slightly faster then the BB's natural speed in its arc. Determine the best position for the paper-clamp release cam, to increase the number of interactions between magnet and BB.

  • Prepare for participants' involvement. Print and reproduce the Handouts provided. Have participants read the Gravity Assist Primer and the Video, if possible.

  • Run simulations! Follow the instructions on the Handout.

Sample Parts list

Typical parts-source references are provided as a convenience only. No endorsement by JPL, NASA, or Caltech, of any kind, is made or implied.
Component Spec Quantity Typical Source Approximate Cost
Glass Tabletop 30-inch diameter 1 Pier 1 Imports $36
Jupiter "orbital" disk 12-inch diameter acrylic disc, 1/8-inch thick 1 PlasticMart $9
DC Gearhead Motor 19 RPM CANNON #EN35-H1 3G1 3B 24 VDC permanent magnet, reversible (or any low-rpm, low-voltage, reversible motor) 1 C & H Sales (a surplus store) $8
Motor Power supply 24VAC 1-Amp Transformer, 1-Amp Silicon Diode, Lamp dimmer inline switch (Detailed spec) 1 C & H Sales (a surplus store), or an electronics store such as Radio Shack $12
Jupiter-magnet Flat, powerful rare-earth magnet 1 C & H Sales (a surplus store), or a science supply or novelty store $2.10
Channel for launch ramp Aluminum U-channel, about 1/2 inch or 1 cm wide One length of about 3 inches or 7 cm Hardware store $.25
Bearing Ball (BB) Steel, 3/8-inch or 1-cm diameter 1 or more Surplus store $.50
BB Release gate Handmade from bamboo stick, L-brackets, hardware. 1 Various $1
BB-release cam Universal-brand #10200 Small Binder Clip 1 Office Supply Store $.10
BB-catching cup 2-inch or 5 cm diameter 1 Plastic jar lid -
Base for Motor Support 12-inch or 30 cm square concrete brick 1 Home improvement store $1
Motor Support Arm 16-gauge soft steel strap 1 Hardware store $1
It's not finished until the paperwork is done!
Labels Sun, Earth, Jupiter, Saturn 1 each Magazine image cutouts -
Participant's Handout See samle 1 for each participant Sample Here -

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2 Reference Systems
3 Gravity & Mechanics
4 Trajectories
5 Planetary Orbits
6 Electromagnetics

7 Mission Inception
8 Experiments
9 S/C Classification
10 Telecommunications
11 Onboard Systems
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