As a direct result of concern over the escalating cost of fighter manufacture, the French government and air force instituted a program for chasseurs légers or 'light fighters' in 1926. This was unofficially known as the 'Jockey' program, and it envisaged the use of moderate guns, minimal equipment and small amounts of ammunition. Emphasis was placed on climb rate, endurance and a ceiling (high for the time) of 8000 metres. To meet this requirement, Morane-Saulnier designed the MoS-121, renamed the MS 121 in 1927. The prototype appeared in that year and was a single-seat parasol monoplane of mixed construction.
After flying for the first time in mid-1927, it proved underpowered and incapable of climbing easily and was discarded in favour of the Morane-Saulnier MS 221.
- Crew: 1
- Length: 6.72 m (22 ft 1 in)
- Wingspan: 9.80 m (32 ft 2 in)
- Height: 2.98 m (9 ft 9 in)
- Wing area: 16.00 m2 (172.22 ft2)
- Empty weight: 915 kg (2017 lb)
- Gross weight: 1230 kg (2712 lb)
- Powerplant: 1 × Hispano-Suiza 12Jb 12-cylinder water-cooled engine, 347 kW (465 hp)
- Maximum speed: 257 km/h (160 mph)
- Range: 1100 km ( miles)
- Service ceiling: 7800 m ( ft)
- 2 x forward-firing 7.7 mm Vickers gun
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- Green, William; Gordon Swanborough. The Complete Book of Fighters. Godalming, UK: Salamander Books. p. 415.