Complex sentence: Because of the low ballistic coefficient, Skylon would end up slowing down at higher altitudes where the air is thinner, meaning the skin of the vehicle would only reach 1100 Kelvin (K).
Split into simpler sentences: Because of the low ballistic coefficient, Skylon would be slowed down at higher altitudes where the air is thinner. As a result the skin of the vehicle would only reach 1100 Kelvin (K).