Space Vehicles Go Green – Reusable Launch Vehicle Concepts

In launch vehicles these days, it’s all about reusability. SpaceX recently tried to reignite its first stage booster, which is the initial step toward landing it vertically on the ground. If this first stage is landed successfully, it can be used again and again. DARPA is calling for proposals for a “reusable hypersonic vehicle with costs, operation and reliability similar to traditional aircraft.” Companies will propose their ideas on Monday October 7th.  Across the Atlantic, Reaction Engines is developing the SABRE engine, a radical engine for the Skylon spaceplane. Each design has the same intention, lowering the cost of access to space by reusing vehicles over and over again.

SpaceX has taken the most traditional approach to launch vehicles, using a vertical launch system and powerful rockets to get payloads into orbit. However, this system has not been reusable in the past. To add reusability to their system, SpaceX intends to have the massive first stage of the rocket land itself on the launch pad. The company is currently exploring the vertical landing technique through the Grasshopper research project, which completed a 325 meter leap test on June 14. While SpaceX’s system is not fully reusable, they are working toward that goal.

On the opposite extreme, Reaction Engines in the U.K. is developing a fully reusable spaceplane known as Skylon. Unlike SpaceX, which uses multiple stages of rockets to get into space (dropping used ones, such as the first stage, along the way), Reaction Engines wants their spaceplane to get into orbit using a single stage. To achieve this lofty goal, Reaction Engines is developing SABRE (see picture), a revolutionary rocket engine that uses atmospheric air as rocket oxidizer for a portion of the trip. Reaction Engines explains that SABRE sucks in atmospheric air through an inlet and then cools it to nearly liquid, basically generating its own oxidizer. However, once the Skylon exceeds Mach 5, the engine morphs into a typical rocket engine, using its own oxidizer stored onboard. This design allows Skylon to carry less liquid oxygen than a typical launch vehicle, saving space and weight for other essentials. With this radical engine, Skylon could be the start of a whole new family of launch vehicles.

In contrast, DARPA’s XS-1 spaceplane design concept seeks to be a melding of the traditional and radical. The radical part of the design is a suborbital spaceplane that can launch just like an aircraft, from the ground. This distinction is important because SpaceShipOne, a successful suborbital spaceplane, launched from a flying “mothership” airplane in 2004. The XS-1 concept, according to the proposer’s day announcement, is a reusable suborbital space plane with a (more traditional) secondary stage that would launch payloads into their required orbit. DARPA’s goal is ten flights in ten days, which would be a radical departure from current launch timeframes.

Although the goal of reusability is shared across these programs, the designs are completely different. They each also have their own timeframe. SpaceX is already conducting preliminary tests. The other two space planes are still in the concept phase. While parts of SABRE, such as the precooler system, have been tested, the Skylon system is years from launching. The XS-1 design is still flexible, so there’s no telling when it may be ready to fly. This flexibility shows how DARPA recognizes that they are many different ways to make reusable launch vehicles. The plethora of designs in development right now shows that reusable space vehicles are likely to be the next way to access space.

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