L11 - Reusable Launch Vehicles Flashcards
What is required from a reusable vehicle?
- Fully reusable vehicle: No expensive throw-way bits
- Adequate margins: Cater for mass growth in development
- Safe against component failure: Full abort capability
- Use often: Capable of handling a wide range of missions
- Easy to turn around: Much reduced servicing effort
- Engine has to: have high specific impulse (LOx/LH2), High trhust at launch, reliable, reusable with minimum maintenance
What are the economic issues associated with a RLV proposal?
A reusable launch vehicle has a very big up-front (development) cost compared to expendable rockets, but in the long term it will provide savings in costs.
What are the engine technology issues associated with a RLV proposal?
- Design for operation sea-level to vacuum→ complex!
- Reuse engine life means de-rating engine
- Specific impulse implies closed-cycle engines→ very challenging
- Cryogenic engine moisture condensation problems
- Engine monitoring for safe abort & Reliability requires extensive testing
What are the thermal protection issues associated with a RLV proposal?
should be Robust, fully reusable, Minimum inspection requirements, Fail operational BUT Low density LH2 tank reduces thermal loading. Should also have low mass per unit area, TPS integrity with structure expansion/contraction, Damage resistant, Confidence in inspection (low cost/unit area). It should also accommodate for aerodynamic control surfaces & undercarriage.
What are the structural challenges associated with a RLV proposal?
Minimum mass, Integrated LH2 tank (it’s big!) and must provide servicing with it on-board, must be fail safe for temperature extremes between shell & tank (hot & cryogenic structure), must accommodate inspection of tank integrity in situ, must have payload bay integration & handling.
What are the recovery issues associated with a RLV proposal?
A ballistic shape will need: heat shields, parachute/decelerator, ground impact attenuator, ground handling after recovery and will have poor cross range. An aerodynamic shape will have wings/control surfaces, aero balance through wide Mach range, undercarriage and has a good cross range.
Describe the various ways to get to orbit with reusable launches.
Single Stage to Orbit (SSTO) vehicles have a low payload fraction, which means that they tend to be large and expensive, but are reusable. Two stage vehicles improve the performance, but require the development of two separate vehicles…unless… use Siamese (two identical stages, one is the booster and one is the orbiter; separation occurs towards the end of the regime where the booster can glide back without additional propulsion)), Air launch (use an already existing aircraft to bring rocket to a certain altitude; increased drag losses because vehicles doesn’t climb vertically, but reduced gravity losses and get extra velocity by aircraft. You can also reach a wider range of inclinations) or recoverable booster.
What benefit have airbreathing engines?
They provide a higher specific impulse.
What penalties come with using airbreathing engines?
1- Airbreathing engines weight more per unit thrust
2- Airbreathing engines performance drops as speed increases
What are the three main types of airbreathing engines?
Turbo Ramjet
Precooled Cycles
Scramjets
Describe Turbo Ramjets.
turbo jets at low speeds, then ram cycle at high speeds. They cause problems cus you need to ‘stop’ the air from entering when you reach Mach 5 in order for the engine to switch from Airbreathing mode to rocket mode & are too heavy to reach orbit when Airbreathing must finish
Describe Precooled Cycles.
Has same ‘top speed’ limitation of turbo ramjet, but is much lighter. Is less fuel efficient then Turbo Ramjet.
Describe Scramjets.
Air is never slowed below supersonic speeds, but they need to be accelerated to supersonic speeds by other means! –> Not practical!
What is the problem with all airbreathing engines?
None work over a wide range of mach numbers.
Describe the qualities of the Skylon plane briefly.
Skylon is developing a combined cycle engine (SABRE) which is Airbreathing up to Mach 5.5, then it becomes a rocket engine. SABRE is not a particularly efficient Airbreathing engine (low Isp; but still better than a pure rocket), but it’s a good rocket engine (high Trhust/Weight; more than twice of conventional airbreathing engines).
