Nike-X required at least one interceptor missile to attack each incoming warhead. As the USSR's missile fleet grew, the cost of implementing Nike-X began to grow as well. Looking for lower-cost options, a number of studies carried out between 1965 and 1967 examined a variety of scenarios where a limited number of interceptors might still be militarily useful. Among these, the I-67 concept suggested building a lightweight defense against very limited attacks. When the Chinese exploded their first H-bomb in 1967, I-67 was promoted as a defense against a Chinese attack, and this system became Sentinel in October. Nike-X development, in its original form, ended.