Because the rest didn't "explode" in the usual sense. SN1 lower tank failed a liquid nitrogen pressure test in a rather spectacular way. SN2 was a short one t

Because the rest didn't "explode" in the usual sense. SN1 lower tank failed a liquid nitrogen pressure test in a rather spectacular way. SN2 was a short one tank version that passed its pressure test. SN3 collapsed under a screwed up pressure test when the lower tank was emptied before the upper tank. Lower tank collapsed under the weight and the upper tank popped when it hit the ground. SN4 didn't really fail any tests. It was the fault of the disconnect. SN5 was supposed to be moved to the test pad today but there may be a delay.