<Rockets

**A model rocket simulation spreadsheet**

(Don't worry, the thing is in English, despite the Finnish name.)

- Old version:
- raketin_vakavuus.xls (Mikro$oft XLS)

- New and improved:
- raketin_vakavuus2_testiversio.ods (LibreOffice / OpenOffice.org ODS—recommended)
- raketin_vakavuus2_testiversio.sxc (OpenOffice.org SXC)
- raketin_vakavuus2_testiversio.xls (Mikro$oft XLS—not tested)

Gray cells are for your **input values**Yellow cells are the main **calculated results**Pale yellow cells are intermediate or less important results, which may or may not be of minor interest to the more nerdy types Light blue cells are for parameters which you *probably*won't need to or want to touchWhite cells are used by raketin_vakavuus for its own calculations, you will better maintain your sanity by ignoring these.

The table is divided into sections:

**Rocket definition:**This is where you enter the data for all the parts of the rocket, from nose cone to fins to internal payloads**Calculated stability:**Once you have defined the structure of your rocket, its critical stability data are displayed here**Wind stability:**You decide how much stability you want to maintain, and this section will calculate the maximum wind speed that you can safely fly your rocket in**Fin calculation:**Define all elements of your rocket in the**rocket definition**section*except*the fins. Then define here how much stability you want, and it will tell you how big your fins must be.**Added mass calculation:**If you're happy with your rocket's appearances but it's just not stable, then define here how much stability you want, and it will tell you how much mass to add to the nose-cone.

All elements are given a mass. The *nose*-, *tube*- and
*widen*-elements are given their length, whereas *fins* and
*mass*es are given their position. The position is measured
relative to the "current position", i.e. to the bottom of the last
body-element (*nose*, *tube*, *widen*) defined. If
located e.g. on a tube already defined, then that relative position is
negative, otherwise positive (i.e. down is positive, up
negative). Other parameters for the elements are as follows:

*nose*: diameter, shape (this can be*ogive*,*cone*,*parab*or*ellip*) and hollowness (enter 1 if hollow)*tube*: no other parameters. Diameter is inherited from preceding*nose*- or*widen*-elements.*widen*: new diameter (i.e. diameter of bottom end). Old (top end) diameter is inherited from preceding*nose*- or*widen*-elements.*fins*: number of fins*n*, and geometry parameters*S*,*Cr*,*Ct*and*Xt*(see figure). Note: Fin calculations need to know the diameter of the body tube the fins are attached to. This is inherited from preceding elements, so you must*first*define the tube section,*then*the fins (i.e. the fins' position will be negative in almost all cases).*mass*: no other parameters.

Other parameters in this section are the angle of attack (*alpha*) and
*K* for Galejs stability calculations [2] and for the projection
method, the ratio of total fin area to projected fin area (*k*).

In this section, you provide the number of fins *n*, the diameter of the
tube where they are attached (*Dt*, unlike in rocket definition, this is
not inherited from anywhere!), the areal density of the fin material in
grams per square centimeter (don't use 0 even for very light material,
instead use 0.00001 or such, to avoid a division by zero), and the fin's
geometry—these values will be scaled equally up or down, retaining the
fin's overall shape. Like above, the required stability can be entered
as cm or calibers, the larger of the two will be used. The position of
the fins is given in cm from the top of the rocket, measured to the
center of gravity of the wings, *not* their top edge as before!

For Galejs calculations, the *alpha* and *K*
values entered in the **Rocket definition** -section will be used.

The projection method has been solved analytically, whereas Barrowman and
Galejs approximations are performed in three iterations. The first two
results are visible in pale-yellow so you can see if the results are
converging correctly. The final dimensions *S*, *Cr*, *Ct*
and *Xt* of the fins
are calculated, as well as the position (from the top of the rocket) of
the fin's top edge (*Xf*). Fin areas and their mass contribution to the
rocket are also presented.

- James S. Barrowman, Judith A. Barrowman,
*The Theoretical Prediction of the Center of Pressure*,

http://www.apogeerockets.com/education/downloads/Barrowman_report.pdf - Robert Galejs,
*Wind Instability—What Barrowman Left Out*,

http://www.cmass.org/member/Robert.Galejs/sentinel39-galejs.pdf

Antti J. Niskanen <uuki@iki.fi>