diff --git a/Function/calculate_semi_major_axis.m b/Function/calculate_semi_major_axis.m
new file mode 100644
index 0000000000000000000000000000000000000000..9257f26cc46c2d86d0e261db49597c57aba61dc3
--- /dev/null
+++ b/Function/calculate_semi_major_axis.m
@@ -0,0 +1,4 @@
+function a = calculate_semi_major_axis(rp, e)
+    % Calculate the semi-major axis from periapsis and eccentricity
+    a = rp / (1 - e);
+end
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