SpelSim Description

Spelsim 1.0.0
December 28, 2007

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SpelSim is a Space Elevator calculator and simulator. You can select values for all of the key parameters of the system, and the program will calculate and display the key values of the space elevator cable under those conditions. You can then simulate the rotation of the cable with the planet, with a visual presentation of the cable plus additional graphs displaying cable parameters such as stress. You can simulate cable breakage events and see what happens to the pieces.


All numbers are expressed in the MKS system, using meters (m), kilograms (kg), and seconds (s). Units for a field are shown to the right of the value. Fields which display no units to the right of the value are unitless. The notation (m^2) means meters squared; m/(s^2) means meters per second squared.

Interface Basics

SpelSim presents four tabs: Cable, Elements, Events and Simulate. Start with the Cable tab to set the parameters for the planet, material, and cable to allow SpelSim to calculate the rest of the cable values. After setting up the cable, you can inspect the individual elements of the cable using the Elements tab, or start the visual simulation in the Simulate tab.

Cable Tab

The Cable tab is divided into three panels, one each for

Each panel is headed by the corresponding keyword in bold.

To the right of the bold label in each panel is a preselected set of known items from which you can choose. If you wish to set your own values for that panel, select the "custom" entry. Editable fields in that panel will turn white. Output-only fields will stay gray, and have italic labels. These fields contain values which are calculated based on your inputs.

To change a value, select the "custom" entry for that panel, edit the value in the desired field, or delete it and type in a new value, and press Enter. When you press Enter, all of the output-only values will be recalculated and displayed. If the values you select do not give a sensible answer, you may see question marks in some fields. For example, if you select a weak material on a planet with strong gravity and slow rotation, the required values may exceed what can be represented in a double variable (about 10^300), so you will get question marks for the cable values.

The Planet

You can select values for the following planet parameters:

The following values for the planet are calculated and displayed:

For comments about the values used for the known planets, download the kit, unpack the source, and look in the file src/net/jimmc/spelsim/Planet.java.

The Material

You can select values for the following material parameters:

The following values for the material are calculated and displayed:

The Cable

You can select values for the following cable parameters:

Most of the below values for the cable are calculated numerically rather than functionally. The cable length is divided into a large number of separate elements which are treated as point masses connected by a massless cable. Today's computers are fast enough to do this calculation quickly even when using many thousands of elements to model the cable.

The following values for the cable are calculated and displayed:

Elements Tab

The Elements tab allows you to inspect the individual elements of the table. You can examine the static values for element mass and cross-section, and the dynamic values (of the simulation) for stress (at the bottom of the list), acceleration, velocity, and position. This panel also displays the "ideal" acceleration, velocity and position, which are the values that this element "should" have if the element calculations and simulations were "perfect". These values are calculated by taking the initial values and rotating them by the amount of rotation of the planet. The difference between the ideal and actual values are listed as the drift values. By looking at these values, you can get an idea of how well behaved the simulation is running. All vector values are displayed in both rectangular and polar coordinates, where the center of rotation of the planet is the origin of both coordinate systems.

The Elements panel gets a list of elements and displays one of them. You can change which one is displayed by entering an index number into the Index field, or by using the + and - buttons to move to the next or previous element in the list.

You can modify which elements appear in the list by changing the values in the Filter Parameters panel on the left side of the tab.

Events Tab

The Events tab allows you to specify events that affect the simulation. For each event, you can specify the time at which that event occurs, the type of event, and the arguments for that type of event.

There is currently only one type of event: Break. This event type takes a single integer argument, which is the index of the cable element to be broken at the specified time point.

You can delete events by pressing the Delete button next to the event, or add new events by pressing the Add Event button at the top of the tab. Events are executed in time order, independent of the order they appear in the list.

Simulate Tab

The Simulate tab allows you to simulate the rotation of the planet and cable. This tab is divided into four areas:

Simulation Parameters

You can control the following parameters of the simulation:

Simulation Line Charts

The Line Charts area lets you see various simulation parameters as a function of the element number. The small circle on the left end of the chart represents the planet. The horizontal axis is the element number, with the right end of the axis representing the last (topmost) element of the cable.

To select which data items to display in the chart, click on the Setup button, then click on the checkboxes to toggle them on for the items you want to display. Click on the Chart button to view the selected data.

Each item is displayed in a different color and labeled at the top of the Line Chart. To bring a specific line to the top, click on the label for that line.

The number to the right of the label for each item is the value of the top of the chart (on the Y axis) for that item. For all angle data, the max Y value is always PI. For all other data, the max Y value is auto-scaled so that the maximum magnitude value of that data is at the top or bottom of the chart.

Simulation Notes

This simulation is done using discrete elements. This introduces artificial high frequency components into the cable dynamics, which in turn causes inaccuracies in the results. There are two parameters which allow for some compensation: the Damping Factor and the Counterweight Derating. If you change any of cable parameters, the number of cable elements or the simulation step size, you can play with these two values to see if you can improve the accuracy of the simulation. In any case, using larger step sizes and smaller element lengths will result in less stability, so the simulation is likely to diverge soon. This typically results in spectacular mass breakage, which can be fun to watch on the screen, but is probably not realistic. (Try running with 1000 elements, step size 10, damping 0 and CW derating 0.) In general, you should view these simulations with a grain of salt.

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