Reusable Launch Vehicles (RLV)
Context:
Reusable technology has reduced the cost of access to space per kg by a factor of 5-20 compared to traditional expendable rockets, while significantly increasing launch cadence.
Reusability shifts the industry from a "disposable" model (where rockets are discarded like single-use items) to a "transportation" model (like aviation)
What are Reusable launch Vehicles:
Reusable launch vehicles (RLVs) are rockets that can be used multiple times for space missions, unlike traditional expendable rockets where each stage is discarded after use.
RLVs recover and reuse the first stage. After detaching from the upper stages, the first stage uses engines or parachutes to descend and land back on Earth.
It can then be refurbished for future launches, significantly reducing costs.
The Physics of Launch:
Tsiolkovsky Rocket Equation: Space travel faces a "weight problem" defined by this equation:
A rocket needs fuel to lift its own fuel.
Consequently, over 90% of a rocket's mass is propellant and tanks, leaving less than 4% for the actual payload (satellite).
To beat this equation, rockets use "staging" (discarding empty fuel tanks and engines mid-flight to shed dead weight and improve the propellant-to-mass fraction)
Mechanisms of Reusability:
Recovery Methods:
Retro-Propulsion:
This is used by SpaceX's Falcon 9, where the stage fires engines to slow down and land vertically.
It uses a mix of aerodynamic drag and engine firing to dissipate kinetic energy.
Winged Re-entry:
A "mini-shuttle" concept that lands on a runway (similar to ISRO's RLV plans).
Challenges:
The reuse limit is determined by structural fatigue caused by extreme temperature swings (cryogenic fuel vs. combustion heat) and immense g-forces.
Refurbishment must be cheaper than building new.
As flight counts increase, the cost of inspection and testing eventually outweighs the savings.
SpaceX has reused a single first stage over 30 times.
ISRO’s Reusable Launch Vehicles:
ISRO is working on two primary recovery models:
RLV (Reusable Launch Vehicle) which is a winged spacecraft that re-enters and lands on a runway.
Vertical Recovery that is recovering the spent first stage using aerodynamic drag and retro-propulsion (similar to Falcon 9).
Reusable Launch Vehicle – Technology Demonstrator (RLV-TD)
The configuration of RLV-TD is similar to that of an aircraft and combines the complexity of both launch vehicles and aircraft.
RLV-TD has various technologies, namely, hypersonic flight, autonomous landing and powered cruise flight.
In future, this vehicle will be scaled up to become the first stage of India’s reusable two stage orbital launch vehicle.
This technology demonstrator was boosted to Mach no: 5 by a conventional solid booster (HS9) designed for low burn rate.