added citations

citations.bib

@@ -0,0 +1,23 @@
+
+@book{shackelford_introduction_2015,
+	address = {Boston},
+	edition = {Eighth edition},
+	title = {Introduction to materials science for engineers},
+	isbn = {9780133826654},
+	publisher = {Pearson},
+	author = {Shackelford, James F.},
+	year = {2015},
+	keywords = {Materials},
+}
+
+@book{curtis_orbital,
+	title = {Orbital mechanics for engineering students},
+	isbn = {9780128240250 9780323853453 9780081021330},
+	language = {English},
+	author = {Curtis, Howard D},
+	year = {2020},
+	note = {OCLC: 1235349370},
+	publisher = {Butterworth-Heinemann Publications},
+	edition = {Fourth edition}
+
+}