History of Hybrids

On the morning of January 6, 2024, a new spaceliner soared high above the New Mexico desert. This very unorthodox looking airplane reached its peak operating altitude. The engines struggled to keep the aircraft afloat in the thin atmosphere. After a brief countdown, the airplane split into two as the middle fuselage dropped abruptly, and the pilot of the smaller craft made a sharp dive downwards, distancing itself from the other aircraft. Just 5 seconds later, a dark puff of smoke erupted from the engine, followed by a massive plume of fire blasting out from the rear of the aircraft as the pilot pulled back on the stick and ascended into the dark sky. In a mission dubbed "Galactic 06", this event was the 8th flight of the spacecraft carrying passengers. This is Richard Branson's revolutionary new space tourism vehicle designed to take paying passengers above Earth's atmosphere for a moment in space. Branson’s new spaceliner, the ‘SpaceShipTwo’, utilizes a new class of rocket engine called the Liquid-Solid Hybrid. With decreased costs and heightened reliability, the hybrid engine makes spaceflight more accessible to smaller companies with aspirations of sending crew and cargo to low-earth orbit. Combining this technology with today’s access to 3D printing technology poses a revolutionary leap in propulsion engineering that can accelerate the aerospace industry, providing cheaper and safer options for sending crew and cargo into space

On January 28, 1986, space shuttle Challenger lifted off on a cold morning above Cape Canaveral, Florida. Seventy-three seconds into the flight, the vehicle broke up, causing the death of all seven crew members. After the initial explosion, two white plumes shot away from the blast, creating a Y-shaped cloud of white smoke in the sky. These were the remnants of the solid rocket boosters still lit and flying away from the broken-up orbiter. While these boosters landed far into the ocean, causing no damage to the ground, they serve as a spectacular reminder of the dangers associated with propulsive engineering. After the incident, the cause of the accident was revealed to be the solid rocket boosters; they were designed to operate in the hot Florida climate, but on this extraordinarily cold January morning, the solid rocket boosters O-ring seals malfunctioned, causing a fatal rupture of the motor casing. Many lessons were learned from Challenger, but solid motors were still used for 84 more shuttle missions, highlighting their importance to the space industry. In modern times, solid motors are seldom used, especially in human spaceflight applications. Their volatility and uncontrollability plague propulsion engineers, compelling them to develop new methods of space propulsion—one that can combine the reliability of solid motors and the controllability of liquids: a hybrid of the two.

Hybrid rocket engines represent a new and radical form of propulsion making its way into the space industry. A hybrid engine is shockingly simple, it takes a simple, inert fuel source such as plastic or rubber, and sprays a commercially available form of oxidizer over top of it. The area where they are making the most impact is in the educational setting, with high school rocketry teams experimenting with small, simple hybrid rockets constructed using their school’s 3D printers. The versatility of hybrids lies in their ability to transform any compound that can melt with heat into rocket fuel, including 3D printable plastic, candle wax, tire rubber, and other more complex compounds. Notably, hybrids are intrinsically safe as the fuel, on its own, cannot sustain a flame; thrust is only produced when combined with a separate source of oxidizer, typically nitrous oxide. The separation of fuel and oxidizer enables the engine to throttle the thrust it generates. This feature played a significant role in Richard Branson's space company, Virgin Galactic, opting for a hybrid engine in their space tourism vehicle, SpaceShipTwo. Hybrid rocket engines appeal to clients seeking a once-in-a-lifetime opportunity to briefly visit space. Unlike traditional rocket engines that often employ corrosive and toxic chemicals, hybrids ensure safety, offering a reliable means of propulsion that is accessible without the uses of expensive specialized tools or machines. One of the critical drawbacks of hybrid engines is the heavy components used in their construction. This causes hybrids to be used in only specialized circumstances, where mass is not a crucial factor to the vehicle's performance. Hybrid engines are making strides in the commercial space industry, and are improving the accessibility of space to our population.

Rocket engines are a distinctive form of propulsion that generates thrust through a continuous combustion process, following the same principles of a car engine. The key components required for combustion are a fuel and an oxidizer. In a car engine, gasoline serves as the fuel, interacting with intake air, which acts as the oxidizer, to initiate a small combustion event. A rocket engine follows the same principle, except in the vacuum of space there is no air to act as an oxidizer, so the rocket must carry its oxidizer onboard with it. Combining the fuel and oxidizer is a volatile process, with just a single spark being able to create a massive explosion, which is why rocket propulsion can be so dangerous. For the propulsive engineers designing modern rocket engines, they take every effort to separate the fuel and oxidizer to avoid any unexpected explosive events, which comes with many significant engineering challenges.

Hybrid rocket engines represent a new approach to utilizing existing technology, seamlessly integrating the simplicity and reliability of solid rocket motors with the controllability and efficiency of liquid engines. The primary benefit of these types of rocket engines lies in the ability to use common materials as fuel sources for shockingly powerful engines. Hybrid engines leverage modern technology such as 3D printing to make them accessible in an educational setting, enabling students to experiment with hybrid motors in a safe and dependable manner. The chief concern with hybrid motors is their high mass properties, rendering them less attractive for extended space voyages where vehicle mass is a critical factor. Additionally, hybrids present a challenging integration process. While making notable strides, hybrids represent a new form of propulsion not yet fully adapted to the aerospace industry, yet it is making strides in smaller scale hobbyist and educational industries. Despite these drawbacks, hybrid engines embody the culmination of years of technological development, offering a unique blend of accessibility, safety, and operational efficiency.