ASTEROID provides a means to calculate the drag of steps, ridges, grooves and cavities on the external surfaces of aircraft. The external geometry of two aircraft and a number of wings has been loaded into ASTEROID, along with appropriate aerodynamic flow data. ASTEROID provides a graphical interface that enables users to define the excrescences on the aircraft and then calculate the drag increments.

The ASTEROID Tool uses the calculation methods given in Data Items located in Sections 28 and 29 of the Aerodynamics Series. Links to these Data Items and relevant sketches are included in ASTEROID. The Data Items should be consulted for full details and discussion of their derivation and applicability.

The ASTEROID Tool uses surface-flow data generated by the VFP CFD code (available in the Transonic Aerodynamics Series) at flight conditions representative of the cruise. In its application, when an excrescence is located on any of the aircraft surfaces, the program reads from the database the flow conditions corresponding to the the mid-point of the edge(s) in the case of a multi-node excrescence, or the centroid in the case of a small circular cavity. It should be born in mind that where an excrescence extends for a significant distance, the surface flow conditions may vary significantly over the length of the excrescence, and so any calculations restricted to a single set of flow conditions will, inevitably, be an approximation. For regions where the flow conditions change rapidly, such as near the wing leading edge or near the shockwave on the upper surface, accuracy may well be improved for a multi-node excrescence by increasing the number of nodes/segments.

Note – In aerodynamic studies it is common to use 'the equivalent wing concept' to characterize more complicated wing planforms. This technique is effective when representing overall forces and moments and loading distributions on aircraft. However, in the cruise conditions, where excrescence drag is of interest, the transonic nature of the flow (including shockwaves of widely varying strength and shape) renders the equivalent wing concept inadequate for the purpose of representing local surface flow conditions. Therefore, the ASTEROID system includes a number of 'real' wing planforms, including wings medium-range and long-range transport aeroplanes, a business jet and a concept blended wing-body.


Open an internet browser and login to the ESDU web site, using the access details provided by ESDU and navigate to the ASTEROID Drag Tool. If any issues are encountered with ASTEROID users should use the browser's functionality to clear 'cache'.

Getting Started

ASTEROID will open on the 'General' page. To proceed the user must choose (from the drop-down lists): the aircraft type or wing and the units of length; and click continue. The free stream Mach number for the calculations can then be selected from the datasets loaded by ESDU.

If an incorrect choice is made, ASTEROID can be reset using the 'Clear' button at top right.

The management of the geometric data requires that the basic geometry and units are defined at this point. To change the aircraft or wing geometry, or the units, the user must return to the 'General' tab, clear the data using the 'Clear' button and make new choices. This action will delete any excrescence geometry that has been input by the user.

However, the Mach number of the airflow can be changed by returning to the 'General' tab and selecting any available aerodynamics data set from the dropdown list. In this case all of the geometry will be retained. It is simply necessary to click 'Calculate' for each excrescence or aircraft component to obtain results calculated at the new Mach number.

Selecting the various tabs – Wings, Tailplane, etc – displays images of the external surfaces of these aircraft components. The Tool has aerodynamic flow data embedded for each of these surfaces for the chosen Mach number.

The user can 'click' on these graphics to define the locations and dimensions of excrescences. ASTEROID will register the co-ordinates, calculate excrescence length and sweep angle and retrieve the appropriate local aerodynamic data. ASTEROID will then use the appropriate ESDU methods (with further user geometry input) to calculate the drag of each excrescence and the contribution to aircraft drag (based on the wing area).


Node A geometry point marked '+' that can be positioned by the user. A single node defines the centroid of the plan view of a cavity. Two nodes linked by an edge define a straight-line excrescence that may be a step, ridge or groove.
Line excrescence A straight-line joining 2 nodes.
Poly-line excrescence Two or more joined lines that do not form a closed shape.
Polygon excrescence Three or more joined lines that together form a closed shape.
Aircraft component The aircraft structure shown in a single graphic. e.g. Port wing upper surface, fin starboard side, fuselage, etc.

Working with excrescences

The calculations of the drag of excrescences are set up by 'drawing' the excrescences on the aircraft components using a mouse or similar device.

Choose a location

In all cases the first action is to display on the screen the intended location of the excrescence at a convenient size, as follows:

  • Choose the aircraft component tab (e.g. Wings).
  • With the cursor outside of any sketch/graphic window, scroll down until the appropriate surface is shown (e.g. Port wing, lower surface).
  • By default, the graphic will be in 'Drag/zoom sketch mode'. Put the cursor on the image. Hold down the left button to drag the sketch to the desired position (or use the arrow keys at the bottom of the image window). With the cursor on the image use scrolling to zoom in or out (or use the + and - keys at the bottom of the image window).

Drawing single line, polyline and polygon excrescences – steps, grooves or ridges

  • From the drop-down list at the top of the image window select 'Add/edit multi-node excrescence'.
  • Put the cursor on the image to locate the first node and left click.

    • For a 2-node, single-line excrescence, move the cursor to the desired location for the 2nd node and left click.
    • For a poly-line excrescence, hold down the Ctrl key while moving the cursor and using left click to define the node locations. Release the Ctrl key before locating the final node.
      If you forget to release the Ctrl button, left click again to get one extra node then right click on that node and select 'Delete node'.
    • For a polygon (closed) excrescence, hold down the Ctrl key while moving the cursor and using left click to define the node locations. Release the Ctrl key and hold down the Shift before locating the final node – this will add a line between the final node and the first node.
      If you forget to hold down Shift, you can still close the shape by holding down Ctrl and clicking on the last (or first) node. This will join the last node to the first.


    • Nodes can be repositioned at any time by dragging using the cursor; (hold down the left click button).
    • An additional node can be added between two nodes: place the cursor on the line joining the two nodes and left click.
    • A node can be deleted by placing the cursor over it, right clicking and selecting 'Delete node'.
    • The complete excrescence can be deleted by placing the cursor over any node, right clicking and selecting 'Delete shape'
    • The complete excrescence can be repositioned by placing the cursor over any node, holding down Shift and left click and dragging to the desired location.

  • As the final node is placed the right-hand side of the screen will be populated.
  • Text can be added under 'Title and Notes' as a reference for the excrescence.
  • Under 'Nodes' the co-ordinates for each of the nodes is shown. Clicking on 'edit' allows the user to type in precise values (and the nodes will move on the image to the positions defined by the amended co-ordinates).
  • Under the heading 'Edges' you will find, for each line, the co-ordinates of the mid-point, the local aerodynamic flow data at the mid-point, and an 'Edge details' form to be completed by the user.
  • There is the option for the user to input their own local aerodynamic data, by selecting this under 'Flow Data Source'.
  • For each edge select the 'Type' of excrescence from the drop-down list; i.e. step, rectangular ridge, square ridge or groove. Then enter the dimensions, edge treatment etc. as specified.


    • Heights are measured relative to the local skin surface: a positive value means the point projects the distance h from the surface into the airflow.
    • Height variation – The program allows the geometry at the ends of each line to be different (e.g. a misalignment or rigging variation). The average of the dimensions is used for the calculations provided that they are all of positive sign. In the case of a step the two heights entered may be of opposite sign, but the program will then require the step to be split into sections. It offers users the choice of doing this manually or allowing the program to split the step where the height is zero.
    • There is the option to copy the same edge features and dimensions to all of the lines – while retaining the ability to edit any individual item.

  • Clicking on the 'Highlight' button emphasises on the sketch the line that is detailed in that section. (Clicking again removes the highlighting).
  • Clicking on 'Calculate drag' will present the results for the particular edge as a local coefficient and an increment to aircraft drag. The drag increment at aircraft level is also shown in 'drag counts' (CD × 104). Clicking on the 'Calculate' button on the excrescence title line will calculate the drag for all edges that form that excrescence.
  • To add another similar excrescence, right-click on the symbol on the excrescence title line and select 'Duplicate'. A second copy will appear on the graphic/sketch with the same geometric data. Drag this to the required location (using shift with left click), amend the dimensions (if required) and click on 'Calculate'.
  • Left-clicking on the symbol on the excrescence 'Type' line provides links to the relevant ESDU methods and to diagrams for the definitions of the local excrescence geometry.

Local geometry for excrescences comprising forward-facing and rearward-facing steps.

When using the Tool to calculate the drag of steps, it is important to note that the software makes decisions about whether each particular edge is a forward or rearward facing step.

Polyline steps – In the case of a polyline shape (not a closed polygon) the software assumes that any step with positive height is forward facing and any step with negative height is rearward facing.

Polygon steps – If the shape is a polygon (a closed shape) the software will make assumptions about whether the steps are forward-facing or rearward-facing based on the complete shape.


  • If a rectangle is drawn with all node heights positive this will be treated as a plate on the outside of the skin of the aircraft. Based on the direction of the freestream airflow, 'leading edges' of the shape will be determined to be forward facing and 'trailing edges' will be rearward facing.
  • If a rectangle is drawn with all node heights negative this will be treated as a cavity. Based on the direction of the freestream airflow, 'leading edges' of the shape will be determined to be rearward facing and the 'trailing edges' will be forward facing.

The software states the assumption made for each edge and gives the user the ability to override the automated decision and make any edge forward or rearward facing. This is accessed by left-clicking on the symbol on the excrescence 'Step facing' line.

Another scenario to consider is where, after drawing a polygon, you want to look at the effect of removing one of the edges. This could be done by re-defining the shape, but the software enables this by means of the 'Omit this edge' button - the symbol at the start of the 'Edge' section and adjacent to the highlight button. The classification of the edges as forward or rearward facing steps and indeed, the calculations of drag for all the edges, are not changed by this. The 'Omit this edge' option simply removes the contribution of the selected edge from the total drag increment calculated.

For all calculations involving steps, it is strongly recommended that users check that the forward and rearward facing classifications are as intended.


A cavity can be modelled by creating a multi-node, multi-line excrescence and setting all of the node heights to negative values. The program then assumes that the area enclosed by the polygon is recessed from the aircraft skin – i.e. a cavity.

However, for small circular cavities there is a quicker alternative method. For circular cavities of small diameter, the system offers a calculation based on one chosen aerodynamic data point. This is set up as follows:

  • From the drop-down list at the top of the image window select 'Add/edit cavity'.
  • Put the cursor on the image to locate the centre of the cavity and (left) click. The right-hand side of the screen will become populated.
  • Text can be added under 'Title and Notes' as a reference for the excrescence.
  • Under 'Location' the co-ordinates of the centre (the node) are shown. Clicking on 'edit' allows the user to type in precise values (and the node will move on the image to the position defined by the amended co-ordinates).
  • The aerodynamic data extracted for the node position are shown under 'Local flow conditions'.
  • Under 'Cavity details' select 'Circular Cavity' from the drop-down list.
  • Enter cavity diameter and depth; (e.g. 100mm diameter, 3mm deep).
  • Click on 'Calculate'.
  • The results are displayed including drag based on cavity area and aircraft wing area – and as aircraft drag counts.
  • If required use the cursor to drag the cavity node to a new location. Edit the diameter and depth as required and click on 'Calculate' to get a result for the amended location/geometry.

Moving, replicating and deleting excrescences

Once an excrescence has been created, it can be re-positioned by:

  • holding down 'Shift';
  • putting the cursor over any node;
  • holding down the left button; and
  • dragging.

An excrescence can be replicated by going to the symbol on the right-hand side at the start of the section for that excrescence and selecting 'Duplicate'.

To delete any excrescence either click on the symbol and select 'Delete' or 'right-click' on any node in the excrescence and select 'Delete'.

To delete all of the excrescences on the component (e.g. wing lower surface), click on 'Delete All' at the bottom of the sketch.

If you make a mistake:

Too few nodes added Position the cursor on a line between any two nodes and left click to add a node. All nodes can be re-positioned by clicking and dragging (or entering co-ordinates in the right-hand window).
Too many nodes added Place the cursor over the extra node, right click and select 'Delete node'.
To delete an excrescence Either:
  • click on any node, then the symbol and select 'Delete this excrescence' or
  • 'right-click' on any node and select 'Delete shape'.
To delete all excrescences from a component To delete all excrescences from an aircraft component (e.g. port wing upper surface) click on the 'Delete all' key just below the sketch.
To delete everything and start again Click on 'Clear' at top right.

Calculating the drag for defined parts – steps or grooves around an aircraft panel or moving surface.

When the cursor is placed over a panel or a moving surface, the perimeter of the part may be highlighted in red. This indicates that there is a 'short cut' available to define the co-ordinates of the nodes for the panel.

Ensure that 'Add/edit multi-node excrescence' is selected. Holding down the Ctrl button. Then, clicking with the right button when the part is highlighted creates a polygon excrescence matching the shape of the part. The user can then enter the appropriate local excrescence geometry and run the calculations.

When using this function there may be occasions when you want to omit one of the edges from the calculations. (e.g. one of the surfaces available for as a defined part is a slat, for which the foremost line is the wing leading edge, not an excrescence). This can be done by locating the edge within the data on the right-hand side, clicking on the symbol at the start of the 'Edge' section and selecting "Omit this edge".

Excrescences on the fuselage and engine nacelle

The area for defining excrescences on the fuselage or engine nacelle represents the skin of the fuselage or nacelle as a flat surface. The airflow is from the left (aircraft nose / nacelle intake lip). Excrescences should be positioned at the appropriate distance downstream and be of the correct linear dimensions for the excrescence. i.e. Any differences between Z co-ordinates are distances measured on the skin 'around the curve' of the fuselage / nacelle.

The vertical positioning of a complete excrescence on the sketch/graphic has no effect on the calculations as the local flow is assumed to be the same around the circumference of the (axisymmetric) body and dependent upon downstream distance only. Cavities, line, poly-line and polygon excrescences may be applied.

Help and ESDU information for individual excrescence calculations

Select the 'Help' tab for this user guide and tutorial videos .

For help with any particular excrescence when setting up calculations – such as a step or cavity – scroll to the calculation of the specific excrescence. Next to the 'Calculate drag' button for the excrescence is a button. Clicking will give the options to open the excrescence-specific help and the related ESDU Data Item containing the method for the calculation.

Saving and reloading input data

The geometry that has been entered to define excrescences can be saved so that it can re-loaded when the software is used again. To save the excrescence input data click on the 'save data' button at top right. This will create a file containing the geometry that can be loaded again.

The location of the downloaded file is determined by the settings of your chosen internet browser. By default, this will normally be the 'Downloads' folder of your computer. Most browsers allow the user to change this. If using Google Chrome the default folder can be changed by going to 'Settings'; 'Advanced'; 'Downloads'; and then selecting the option to choose where to download the files to. Where available, it is probably best to use the 'ask' option when using ASTEROID as this will enable the user to store the data in a folder appropriate to the task in hand. The file to be saved can be given any convenient name.

When ASTEROID is re-opened the user can click on 'Load data', navigate to the appropriate folder and select the input file. The previously stored geometry data will then be loaded. Note that this only re-instates the input geometry; the user must click 'Calculate' where appropriate to reinstate the answers.

Working examples

Clicking on the 'Examples' button at top right gives options to load example excrescences for either the single aisle airliner or low the low speed commuter aircraft.

Once the data are loaded, clicking on calculate will generate answers. Once loaded the examples can be modified / customised by adding or removing excrescences, changing the position etc.