Reusable launch system

A reusable launch system (RLS, or reusable launch vehicle, RLV) is a launch system which is capable of launching a payload into space more than once. This contrasts with expendable launch systems, where each launch vehicle is launched once and then discarded.

No completely reusable orbital launch system is currently in use. The closest example was the partially reusable Space Shuttle. The orbiter, which included the Space Shuttle main engines, and the two solid rocket boosters, were reused after several months of refitting work for each launch. The external tank and launch vehicle load frame were discarded after each flight. However, several at least partially reusable systems are currently under development, such as the Falcon 9.

Orbital RLVs are thought to provide the possibility of low cost and highly reliable access to space. However, reusability implies weight penalties such as non-ablative reentry shielding and possibly a stronger structure to survive multiple uses, and given the lack of experience with these vehicles, the actual costs and reliability are yet to be seen.

Hash

Hash, Hashes, Hash mark or Hashing may refer to:

Hash (food)

Hash is a dish consisting of diced or chopped meat, potatoes, and spices that are mixed together and then cooked either alone or with other ingredients such as onions. The name is derived from the French verb hacher (to chop).

Corned beef hash became especially popular in some countries including in Britain and France during and after World War II as rationing limited the availability of fresh meat.

In many locations, hash is served primarily as a breakfast food on restaurant menus and as home cuisine, often served with eggs and toast (or biscuits), and occasionally fried potatoes (hash browns, home fries, etc.). The dish may also use corned beef or roast beef.

Hash has recently made a comeback as more than just a dish for leftovers or breakfasts of last resort. High-end restaurants now offer sophisticated hashes and the first cookbook dedicated exclusively to a wide variety of hashes was self-published in 2012.

United States

The meat packing company Hormel claims that it introduced corned beef hash and roast beef hash to the U.S. as early as 1950, but "hash" of many forms was part of the American diet since at least the 18th century, as is attested by the availability of numerous recipes and the existence of many "hash houses" named after the dish. In the United States, September 27 is "National Corned Beef Hash Day."

Hash function

A hash function is any function that can be used to map data of arbitrary size to data of fixed size. The values returned by a hash function are called hash values, hash codes, hash sums, or simply hashes. One use is a data structure called a hash table, widely used in computer software for rapid data lookup. Hash functions accelerate table or database lookup by detecting duplicated records in a large file. An example is finding similar stretches in DNA sequences. They are also useful in cryptography. A cryptographic hash function allows one to easily verify that some input data maps to a given hash value, but if the input data is unknown, it is deliberately difficult to reconstruct it (or equivalent alternatives) by knowing the stored hash value. This is used for assuring integrity of transmitted data, and is the building block for HMACs, which provide message authentication.

Hash functions are related to (and often confused with) checksums, check digits, fingerprints, randomization functions, error-correcting codes, and ciphers. Although these concepts overlap to some extent, each has its own uses and requirements and is designed and optimized differently. The Hash Keeper database maintained by the American National Drug Intelligence Center, for instance, is more aptly described as a catalogue of file fingerprints than of hash values.