Overview

Liquid engine sizing is the process that relates the chemical processes of combustion to physical parameters. It is separate, but related, to engine design which will consider materials, heat transfer, and many other physical constraints.

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In reality, the process is straightforward, not simple, but straightforward. Figure 2 shows the process RPL used to size its CPLC engine.

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Selecting Initial Parameters

Engine sizing involves a large number of variables both quantitative and qualitative. However, nearly all of these can be contained by the selection of a propellant combination and two of the following three variables: chamber pressure

See the following pages for more detail on:

1. The propellant combination is one of the most significant factors in engine sizing. Nitrous Oxide, N2O is our oxidizer, and Isopropyl Alcohol, IPA is our fuel. IPA was selected as fuel due to its low cost and accessibility.

2. Three main variables constrain engine sizing,

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   body P_{c}

   , thrust

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   body F_{t}

   , and

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   body \dot{m}

   . We used a Simulink simulation to decide on an

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   body F_{t}

   of

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   body 2500N

   and metallurgy and COTS valve/fitting pressure ratings to arrive at a

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   body P_{c}

   of

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   body 2500 \mathit{kPa}

   or

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   body 362.6\mathit{Psi}

   .

3. Using NASA CEA. We input

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   body P_{c}

   ,

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   body P_{e}

   , and

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   body \mathit{MR}

   for our N2O oxidizer and IPA fuel. CEA allows you to simulate various mass fractions. We chose 4.0 as it gives the best ISP performance, this burns hot but not as hot as a more OX-rich mix. CEA outputs the

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   body {M}

   ,

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   body {\gamma}

   (equivalent to

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   body \frac {C_{p}} {C_{v}}

   ),

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   body \frac {A_{e}} {A_{t}}

   ,

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   body {V_{e}}

   , and

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   body \textrm{ISP}

   .

4. The following parameters are fed into a Matlab script:

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   body \dot{m}

   ,

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   body \mathit{MR}

   ,

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   body P_{c}

   ,

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   body P_{e}

   , and

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   body L^\star

   (L* is somewhat arbitrary and selected) as well as chemical properties. m_dot is adjusted! We chose an L* of 1 m, and m_dot 1.1 kg/s. That yields V_c. We pick 0.15 m L which gives a D_c of 7.73cm and exit diameter of 6.3 cm and a throat diameter of 2.99 cm.

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