Aeronautics Class 11 Review Handout PDF

A Beginner’s Guide to Aeronautics Session 11: Hypersonics In our previous class session, we began exploring high speed aircraft. At high speeds, the temperature of the flow around the aircraft is so great that the chemistry and molecules of the gas must be considered. This is a special branch of aer...

A Beginner’s Guide to Aeronautics Session 11: Hypersonics In our previous class session, we began exploring high speed aircraft. At high speeds, the temperature of the flow around the aircraft is so great that the chemistry and molecules of the gas must be considered. This is a special branch of aeronautics called Compressible Aerodynamics. Flight faster than Mach 1 is called Supersonic, up to Mach 5. When aircraft fly at Mach 5 (5 times faster than the speed of sound) or above, they are called Hypersonic Aircraft. So, any aircraft flying at Mach 5 and above is considered Hypersonic. How fast is Mach 5? Mach 5 = approx 3,800 miles (6,116 km) per hour! AT LOW HYPERSONIC SPEEDS (Mach 5 up to 10) – molecular bonds vibrate which changes the magnitude of the forces generated by the air on the aircraft. AT HIGH HYPERSONIC SPEEDS (Mach 10 and above) – the molecules break apart producing an electrically charged super-hot plasma around the aircraft! Mach 10 = approx 7600 miles (12,300 km) per hour! Hypersonic aircraft have to be built to withstand extremely high heat and turbulence of the air flow. As shown above, there are 3 main missions or use cases for Hypersonic flight: 1) Re-entry from space orbit 2) Hypersonic cruise 3) Hi-speed accelerator (also known as a reusable booster for spacecraft) The first use case involves slowing an extremely high-speed vehicle going from space to Earth, while the other two involve extremely powerful propulsion system to reach hypersonic speeds. Since these situations are very different, we will look at each of these cases separately. Use Case #1: RE-ENTRY FROM ORBIT or Hypersonic Re-Entry As a spacecraft re-enters the earth's atmosphere, it is traveling much MUCH faster than the speed of sound. Typical low earth orbit reentry speeds are near 17,500 mph and the Mach number is nearly twenty five, M < 25. Almost 25 times the speed of sound! Due to this very high speed causing compression of the airflow, the temperature of the flow is so great that the chemical bonds of the molecules of the air are BROKEN. The molecules break apart producing an electrically charged plasma around the aircraft. What is Plasma? Plasma is a super-heated cloud of protons, neutrons and electrons where the electrons have been ripped from their respective molecules and atoms. Plasma is often called “the fourth state of matter,” along with solid, liquid and gas. This image (right) shows plasma at the forward part of the Orion capsule as it re-enters Earth’s atmosphere. NASA's Orion capsule captured this footage of its reentry to Earth's atmosphere on Dec. 11, 2022, at the end of the Artemis 1 moon mission. (Image credit: NASA) Spacecraft use ablative heat shields designed to burn away slowly during reentry. These heat resistant tiles are used to protect the spacecraft and its occupants. The Mach number decreases from 25 to 10 as the vehicle constantly decelerates. Crewed space vehicles must be slowed to subsonic speeds before the parachutes can be deployed. Use Case #2: Hypersonic Cruise Use case #2: Hypersonic cruising aircraft and cruise missiles fly at the lower limits of hypersonic, from Mach 5 to 10. In this image (right), an early X-15 hypersonic aircraft is pictured. The X-15 used a rocket propulsion system to achieve sustained Mach 6 flight. Today, modern hypersonic aircraft are powered by air breathing ramjet and scramjet propulsion systems, which are more efficient than rockets. “Air breathing” jet engines are all jet engines that use atmospheric air. This air is taken in, compressed, heated and expanded back to atmospheric pressure through a propelling nozzle - this expansion generates Thrust. Ramjet Design Scramjet Design Ramjets and Scramjets use the force of the hypersonic aircraft ramming through the air to provide needed compression of the airflow. Ramjets & scramjet have to be launched into flight by another jet aircraft. Why? Because ramjet or scramjet engines only function at supersonic speeds, they must “hitch a ride” on another regular jet to get off the ground and up to speed. This has been a major limiting factor in the development of hypersonic aircraft. But a new company, Hermeus, is trying to change that. They are developing a new “Ramburner” engine that can function as a regular jet engine for take-off, and also as a hypersonic engine once the aircraft reaches hypersonic speeds. The image to the right is a concept illustration for a new supersonic passenger plane being developed by the company Hermeus. Use Case #3: Airbreathing Hypersonic Accelerator For use case #3, the hypersonic accelerator must continually produce excess thrust (thrust greater than drag) in order to accelerate. Unlike use case #2, it is not flown in a steady cruise condition. A hypersonic accelerator can be used to launch a vehicle into orbit in one single stage. Or, it may be the first stage of a two stage booster used to launch a vehicle into orbit. For example, the Ram Accelerator is a launcher that uses chemical energy to accelerate vehicles to hypersonic speeds. Unlike the Ramjet Engine, fuel is added at the beginning of the propulsion process and the gas undergoes stronger compression. As a result Ram Accelerators generate very strong thrust. They may be less expensive than rockets to get payloads into orbit. Rockets are expensive because rockets carry both fuel AND its own oxygen. Rockets are “Non-Air Breathing.” This leads to the problem of “The Rocket Equation” discussed earlier in the course. The heavier the rocket & payload, the heavier the fuel + oxygen and the higher the cost! Rockets are expensive, costing $2,500 to $25,000 per kilogram from Earth to low Earth orbit Activity: Draw lines to connect the Mach numbers to the correct category. Mach 2 Mach 11 Supersonic Mach 20 Mach 8 Hypersonic Mach 3 March 16 (Answers below) Recommended Homework for this week: - Complete the Class Review Handout - Explain to a family member: What is hypersonic flight? Categorizing Activity Answers: Mach 2 Supersonic, Mach 11 Hypersonic, Mach 20 Hypersonic, Mach 8 Hypersonic, Mach 3 Supersonic, Mach 16 Hypersonic