WASHINGTON — Engineers at NASA’s Stennis Space Center in Mississippi are preparing to test-fire a pair of experimental engines built for the X-33 reusable launch vehicle prototype.

Tonight’s planned 90-second hot-fire test would be the last in a series of three engine firings NASA officials approved for the linear aerospike engines in July.

At the time, NASA officials planned to finish out the test series with a 100-second, full-throttle firing, but they have since decided that a shorter-duration test at just 85 percent power would be sufficient to accomplish testing objectives, said Stennis spokesman Paul Foerman.

The $3.8 million test program is focused on the performance of the engines’ electromechanical actuators and is funded under NASA’s Space Launch Initiative, a $4.8 billion effort to develop reusable launch vehicle technologies.

NASA canceled the X-33 program in March.