Motor Hardware

BurnSim 4 draws the hardware round your grains: the case tube, the forward closure (bulkhead), the aft closure or nozzle carrier, the nozzle, and the snap rings, washer, threads or bolts that hold them. This page explains where each part comes from, how BurnSim works out where each part sits, how the case length can set the ullage, the warnings you can get when parts don't fit, and how to draw your own nozzles and closures. Added in 4.0.0.25 and much extended in 4.0.0.26.

The drawing is supplemental. It shows you the motor and checks whether the parts fit, but the case strength check never reads it. That check uses only the numbers typed on the Case tab, however the parts are drawn. The simulation reads the Motor tab's fields (throat, exit, the cone angles, the ullage), which is why picking a nozzle writes those fields rather than replacing them. A file with no hardware at all simulates exactly as it always did.

Where the parts come from

Part	Set by
Nozzle	The Motor tab's nozzle fields, or a nozzle picked from the throat picker: an AeroTech nozzle, a Loki nozzle number, a 1/64" size, or one of your own. An AeroTech or custom nozzle is drawn as its real shape; the others are drawn as a cone from the throat, exit, throat length and cone angles.
Case tube	The Case tab: outside diameter, wall and case length. With no case defined, the tube is drawn in proportion to the grains.
Forward closure	The Case tab's Closure part row for the forward end. The default is a plain bulkhead whose length is the Motor tab's Bulkhead length (0 draws an assumed 30% of the grain diameter). An AeroTech case uses AeroTech's own forward closure instead, and you can pick one of your own.
Aft closure / nozzle carrier	The Case tab's Closure part row for the aft end: Automatic (the maker's closure for this tube, if there is one), a maker's closure, or one of your own.
Retention	The Case tab's retention method for each end: snap ring, thread or radial bolts, with their dimensions. A snap ring joint can also have a washer under the ring.

AeroTech hardware

The AeroTech RMS nozzles (24 to 98 mm, including the drilled variants AeroTech's reloads ship with), aft closures and forward closures are drawn from the maker's own drawings, traced from RCS Rocket Motor Components' nozzle and RMS assembly drawings and used with RCS's permission. Nozzle throat, exit and outside diameters come from rocketmotorparts.com's listings, and the cone half-angles and throat lengths from the drawings. Choosing an AeroTech case on the Case tab chooses its hardware too: its closures, and its nozzle size first in the throat picker. A built-in case is protected, so its maker's closures stay as they are; use Save as... on the Case tab to make your own copy and change them.

How the parts are placed

BurnSim lays the motor out from the forward end to the aft end, one part against the next. Every drawing - the 3-D view, the Along axis section and the closure editor - reads the same layout, so they always agree.

The forward end

• A plain bulkhead sits in the tube. When the forward end is not threaded or bolted, it is held by a snap ring: the ring's aft face is on the bulkhead's forward face, and the tube runs on past it by the groove and the land.
• An AeroTech forward closure is placed as AeroTech assembles it: its collar against the tube's end, the boss standing forward of the case. The bulkhead length field is not used; the closure's own length is.
• Your own forward closure is placed by its shape. If part of it is wider than the bore, the tube ends against that part (a collar on the tube's end). If nothing is wider than the bore, it sits inside the tube behind a snap ring, and the ring bears on its first shoulder wider than the ring's inside, so a boss narrower than the ring passes through it and stands forward.
• The grains start at the closure's inner face, plus the forward ullage.

The aft end

What holds the nozzle depends on the nozzle and on the aft retention:

• A flanged nozzle (every AeroTech nozzle, and a custom nozzle with a flange wider than its body) is held by an aft closure: its flange bears on the closure's lip, and its body and exit cone stand out past the end of the case. The closure is the one the case names when the nozzle fits it; otherwise it is the first closure for that tube that does.
• A nozzle in the bore (a nozzle drawn from the Motor tab's fields, or a custom nozzle with no flange) is held by a snap ring unless the aft end is threaded or bolted. The ring's forward face is on the nozzle's exit face, and the exit sits the groove plus the land inside the tube's end.
• On a radially bolted aft end, a nozzle in the bore always sits in a nozzle carrier, because the bolts need a part to go into: the carrier the case names when the nozzle fits its seat, or else a stand-in carrier drawn round it. The bolts go into the carrier, never into the nozzle.
• A carrier you named is used on a snap ring or threaded aft end too. A threaded carrier screws into the tube with the thread cut on it. A carrier with nothing wider than the bore is held by the snap ring, which bears on it.
• A washer under the snap ring moves the nozzle forward by the washer's thickness: the nozzle's exit is on the washer, and the ring on the washer.

Where the tube ends: the inset

Each end of the tube is placed by what holds that end:

• Snap ring: the groove depth plus the land length, from the Case tab.
• An entered inset: the Case tab's Inset from the tube end for that end, which keeps its place.
• A closure part with a collar on the tube's end sets where the tube ends itself, and the inset box says "set by the closure part". Pick another part to set an inset there.

Threads are drawn as rings of the 60-degree thread profile over the engaged length, on the closure and in the tube's bore. Radial bolts are drawn as flush circles on the case, lined up on the bulkhead, closure or carrier they go into, for every ring of bolts entered.

Threaded, snap ring and bolted ends

The same three ways of holding the ends, each shown in the Along axis section (top) and the 3-D view with Cut hardware ticked (below). The nozzle end is on the left in both.

Threaded - an AeroTech RMS case. Both closures are AeroTech's, drawn from its drawings, and screw into the tube; the thread is drawn over the engaged length at each end. The flanged AeroTech nozzle sits in the aft closure, its body and exit cone standing out past the case, and the forward closure's boss stands forward of it.

Snap ring - a case with a snap ring at each end. At the aft end a nozzle carrier of the user's own holds the nozzle in the bore, and the ring bears on the carrier. At the forward end a closure of the user's own sits in the bore behind its ring, its boss passing through. The rings are the small gold blocks at each end of the tube.

Radially bolted - two rings of bolts at each end, passing through the tube wall. On a bolted end the nozzle always sits in a carrier, here the gold part on the left, and the bolts go into the carrier, never into the nozzle. At the forward end they go into the plain bulkhead. In the 3-D view the bolts are flush circles on the case.

The case length and the ullage

With a Case length on the Case tab (or a commercial case that knows how much propellant it takes), BurnSim can work out the motor's ullage from it. On the Motor tab, under the ullage fields, tick:

Set the ullage from the case length (the case, its closures, the bulkhead, the nozzle and the grains decide it)

and the two ullage boxes are grayed out and set for you. BurnSim adds up what has to fit in the tube with no ullage at all:

• the forward closure's length in the tube, and the forward inset;
• the grains, end to end;
• the nozzle (or the aft closure or carrier) as far as it reaches into the tube, the washer, and the aft inset.

Whatever is left of the case length is the ullage, split evenly between the forward and the aft end. A change to any of those parts moves the ullage, and the free volume and L* with it. Untick the box to type your own ullage.

If you type your own and the tube is then too short for the ullage plus everything else, BurnSim says so in a dialog, by how much, with two ways to settle it: clear the ullage and let the case length set it, or change the case length to fit. A longer tube than needed is ordinary, and is not questioned. A grain stack longer than the case takes gets a Case fit: WARNING line on the Case tab, the Results tab and the report.

When parts don't fit

• A grain wider than the case bore raises a dialog, a warning on the Case tab and a Case fit: WARNING line on the Results tab. Change the grain diameter, or the case's outside diameter or wall.
• A closure a little too wide for the bore - wider than the bore, but by less than half the wall - can't go into the tube. It is drawn resting on the tube's end with no snap ring, and a dialog, Closure doesn't fit the tube, says so once for each part and bore. This is usually a closure drawn at the wrong diameter; set its diameter at or a little under the bore.
• A case too short for what is in it: see above.

Free volume

The free volume (and with it L*, the igniter estimates, the ignition transient and the tail-off) counts the ullage and the nozzle's convergent section. With a nozzle drawn from the Motor tab's fields, the convergent section is a cone from the chamber wall to the throat at the convergent half-angle. With an AeroTech or custom nozzle, it is that nozzle's own bore, from its forward face to its throat, as of 4.0.0.26. Motors without such a nozzle are unchanged.

Custom nozzles

The Custom Nozzles window, editing a copy of the AeroTech 54 mm 0.455" nozzle. The line under the drawing reads the shape back: throat, mouth, exit, expansion ratio, cone half-angles and flange.

Open it from the throat picker (the small arrow inside the Motor tab's Throat dia field): Custom Nozzles → New / edit custom nozzles..., or Save current nozzle as custom... to start from the nozzle the motor has now. The Save as custom nozzle... button in the Motor tab's Nozzle heading does the same.

A nozzle is a half-section with a bore through it, revolved about the motor's axis. The forward (chamber) face is on the right and the exit on the left, as in the 3-D view. You can:

• draw it in the shape editor;
• Import DXF... a half-section from a drawing, used as drawn, its bore being the gas path;
• Start from AeroTech... to begin from any AeroTech nozzle as traced from its drawing. Save as custom nozzle with an AeroTech nozzle selected copies its traced shape exactly.

Four numbers are read off the shape and can be typed to move the points they come from: Throat diameter (the narrowest point of the bore; typing it moves the throat land), Exit diameter (the bore at the aft face), Length (forward face to exit face; typing it moves the exit face) and Outside diameter (the widest the outside gets, a flanged nozzle's flange). A line under them gives the throat land, the mouth, the expansion ratio, both cone half-angles, and whether the nozzle has a flange.

No flange: the nozzle sits in the case or its carrier, held as a nozzle in the bore is. A flange wider than its body: it is AeroTech-style, held by an aft closure.

Picking a custom nozzle sets the Motor tab's throat, exit, throat length and both cone half-angles from its shape, so the simulation uses the nozzle you drew. Your nozzles are kept in your own hardware library, and a motor keeps a copy of the nozzle it is set up with, so a shared .bsx draws it on someone else's computer, and editing the library never changes a saved motor.

Custom closures

Editing a forward closure that sits in the bore. The snap ring bears on its shoulder and the boss passes through the ring; the dashed line marks the tube's end. Tube ends at 0 means the whole part is in the tube.

On the Case tab, each end has a Closure part row. Its dropdown offers the automatic choice, the maker's closures for this tube and your own. Edit closures... opens the editor for that end; Save as custom... starts a closure of your own from a copy of the one the case uses. The aft end's editor is Custom Aft Closures and Nozzle Carriers, the forward end's Custom Forward Closures.

A closure is a revolved half-section, used as drawn: it is a solid part, so unlike a revolved grain there is no distance from the axis to set. A new one starts as a four-point rectangle at the case bore (solid for a forward closure, a ring for an aft one). Then:

• draw it in the shape editor; or
• Import DXF... a half-section, as a revolved grain's profile is imported: you pick the axis and the rotation, but no distance from the axis. Flip ends turns a drawing that came in the other way round, and Reset to rectangle starts again.

Two numbers describe the whole part:

• Diameter - the part's widest point. It starts at the case bore; set it a little under for a slip fit. Changing it moves every point at the part's current diameter in the tube.
• Length - end to end. It follows the points. Typing a length moves the inner end, so the outer face and where the tube ends stay put.

Where it meets the motor is worked out from the shape, drawn as dashed lines, and can be typed over (a typed value then stays put; Recalculate from shape clears them):

• Tube ends at - where the tube's end meets the part: the farthest point outside the bore. 0 means the whole part is in the tube.
• Grains start at (forward closure) - its inner face, where the reload starts.
• Nozzle bears at, Seat diameter and Pass bore (aft closure) - the lip a nozzle's flange bears on (or, on a bolted end, a flangeless nozzle's exit face), the widest nozzle the seat takes, and the bore the nozzle's body passes through.

A forward closure replaces the plain bulkhead, and the grains start at its inner face. One with nothing wider than the bore sits behind the snap ring, and the editor draws that ring and the tube past it. An aft closure holds a flanged nozzle, and on a radially bolted aft end it is also the carrier for a nozzle that sits in the bore: the nozzle's exit on its lip, and the bolts into it.

The case keeps a copy of the closures it uses, so a shared .bsx draws them. A closure is drawing and case fit only. It is not part of the strength check, which uses the Case tab's retention numbers alone.

The shape editor

The nozzle and closure editors share one drawing tool:

• Drag a point to move it. It snaps square to its neighbors and onto the axis and the case bore. Hold Alt to place it freely, and Shift to keep the move straight.
• Double-click an edge to add a point there. Right-click a point, or select it and press Delete, to remove it (a shape keeps at least three).
• A selected point's Z and R can be typed for exact values.
• Mouse wheel zooms, dragging empty space pans, and Fit (or Home) shows the whole part. Undo and Redo (Ctrl+Z, Ctrl+Y) step through your changes.
• The part is shown whole, with the lower half mirrored, and the case tube ghosted round it. A shape that crosses itself is drawn in red and can't be saved until it is fixed.

Washers

A snap ring joint's aft end can have a washer under the snap ring, picked on the Case tab. Edit washers... keeps your list: each washer has an outside diameter (0 means the case bore), an inside diameter and a thickness. The washer sits between the ring and the nozzle, so the nozzle moves forward by its thickness and the tube needs that much more room. Of the built-in cases, only Loki's offer a washer. Like the closures it changes the drawing and the case fit, not the strength check, and the motor keeps a copy of it.

Colors

Settings → Motor Colors... sets the color each part is drawn in, in the 3-D view and the Along axis section: the case tube and plain bulkhead, closures and nozzle carriers, the nozzle, snap rings, the washer, bolts, the propellant's non-burning and inhibited surfaces, its burning surfaces, the section view's inhibitor band, and the 3-D background. Each row has a swatch that opens a color picker, a #RRGGBB box and Reset; Reset all puts every color back. Changes show as you make them; Save keeps them for future runs and Cancel puts them back.

See also

• Case Strength Check - the Case tab's fields and the strength formulas, which use the numbers entered, not the drawing
• 3-D Burn-Back View - the whole motor in 3-D
• BurnSim Fields - the Motor tab, the throat picker and the ullage fields
• Revolved Profile Grain - the same DXF import, for grains