Senators from both parties criticized the administration yesterday for what they said was a lack of leadership in restoring the shattered space program.

At a hearing focusing on quality slippage and other failings of the National Aeronautics and Space Administration, they took particular aim at the Reagan administration's failure to provide Congress with estimates on how much the recovery will cost.

Calling the lack of meaningful budget information "inexcusable," Sen. Donald W. Riegle (D-Mich.) suggested that someone in the administration is forcing NASA to be "deliberately vague" about its funding needs.

He told NASA officials present, "Somebody in NASA is going to have to speak out, . . . to be strong enough and steady enough to tell us what the truth is, and not in whispered sessions in the hallway but in open sessions like this."

The administration has not yet requested funds for a shuttle to replace Challenger, which blew apart on Jan. 28, killing its crew of seven. It reportedly is considering requiring NASA to find the money in its existing budget.

Rear Adm. Richard H. Truly, new head of the space shuttle program, said he has recommended that the next shuttle launch be scheduled for late July 1987, or six months later than original agency estimates. He also foresees a flight rate of 6 or 7 the first year, 9 to 11 the second and a maximum of 15 annually with the three existing shuttles, rather than as many as 18 a year as earlier projected. Building a replacement for Challenger would take at least 3 1/2 years.

NASA had given the subcommittee a preliminary budget based on the earlier, optimistic projections.

The resumption of flights could be complicated by the redesign and testing of the joints on the shuttle's solid rocket boosters, which apparently caused the shuttle accident. The presidential shuttle commission intends to demand that independent scientists supervise a total redesign, because of a suspicion that NASA may be settling for a quick fix not much different from the old design, according to a report by the Orlando Sentinel.

Sen. Ernest F. Hollings (D-S.C.) urged Truly to give serious consideration to a proposal from a contractor in Florida to replace the "unsolid" solid rocket booster that fits together in segments with a real "solid" rocket all in one piece, eliminating the joints and seals that caused the accident. (The term "solid" refers to the propellant.)

Accusing NASA of trying to "tippy-toe" around an obvious solution, he added, "This country is not going to stand for any more joint problems, I can tell you that right now."

The hearing of the Senate space subcommittee also produced attacks on NASA's quality control apparatus, which agency figures indicate has been reduced by 71 percent agency-wide in the last 15 years.

Sen. Albert Gore Jr. (D-Tenn.), who said he obtained the figures from a NASA employe after agency officials denied they existed, told the hearing, "Some say NASA is just having an extraordinary run of bad luck. I wish I could believe that explanation. But I think the evidence points overwhelmingly to a serious breakdown in quality assurance."

Gore suggested NASA's successes have made it a victim of "technological hubris" and said the numbers showed "the devastating decay of what was once the finest quality control program anywhere."

Truly, a former astronaut, said, "Most major accidents are caused by a chain of events. Quality may very well be in that chain."

But he added, "As incredible as it seems, I have no indication today that these are not three totally coincidental accidents that have no direct connection."

Truly said he is not familiar with the details of the quality assurance situation, which is run out of a separate office under NASA's chief engineer.

Also yesterday, Sen. Slade Gorton (R-Wash.) asked Truly about an aerospace newsletter report that improper welds and cracks had been discovered in the space shuttle main engines and that the problems would have grounded the vehicles even without the Challenger accident.

Truly said he was aware of a problem with cracks in turbine blades in the engine fuel pumps that were discovered just before the accident. He added that the problem is under review along with other critical shuttle items. "I don't know if it would have grounded the program," he said.