“We’re not trying to bash Iron Dome,” says weapons scientist Richard Lloyd, author of a research paper that calls into question the effectiveness of Israel’s now famous rocket-defense system. “We’re trying to help Iron Dome. The way to help Iron Dome is to identify some of its problems.”

Representatives for the Israel Defense Forces claim that Iron Dome has been about 90 percent effective in knocking down Hamas missiles fired from Gaza. Lloyd and a handful of other outside experts, including Theodore Postol of Massachusetts Institute of Technology, have been challenging the IDF’s assertions on Iron Dome’s success rate since at least 2012.

On Monday, Lloyd e-mailed me a copy of a 28-page analysis that’s the most detailed critique yet of the holes in the Iron Dome system—holes so big that, if he’s right, would justify calling it Iron Sieve. He says his paper is based entirely on open-source documents and observations and was cleared for public release by the Pentagon in late May.

Lloyd wrote the paper for Tesla Laboratories, a defense contractor in Arlington, Va., for which he is a consultant, and plans to post it on the Internet “sometime.” He spent more than two decades at Raytheon (RTN) and was a past engineering fellow at Raytheon Integrated Defense Systems.

The strength of Lloyd’s critique is that ordinary people can evaluate the effectiveness of Iron Dome just by looking up in the sky at the contrails of the antimissile system’s Tamir interceptors. If it’s working properly, the interceptor missiles shoot upward and meet incoming Hamas rockets as they fall to the ground at a steep angle. The contrails are short and go straight skyward. If it’s not working, the contrails form loop-de-loops as the interceptors chase after the rockets and catch them from the side or behind.

Hamas rockets that are hit from the side or behind may be knocked out of the air, but their warheads usually aren’t detonated, Lloyd says, so they can still explode when they hit the ground and cause serious damage.

Postol, a professor of science, technology, and national security policy at MIT who is familiar with Lloyd’s work, compares the Iron Dome interceptors to an outfielder who misjudges a fly ball and has to backpedal furiously to get under it. “The radar system should be able to track incoming rockets with reasonable precision. That should not be a big problem, but there’s something going on,” Postol says.

One problem is that there aren’t enough Iron Dome batteries to cover the country, Lloyd says, so interceptors have to race long distances and can’t always get in position in time to meet rockets head-on. The recent addition of an eighth battery should be helping at least a little.

Jonathan Mosery, a spokesman for Israel’s Ministry of Defense, stood by the government’s statements about the accuracy of Iron Dome and said he would seek a more specific reaction to the claims in Lloyd’s paper.

Hamas warheads aren’t easy to blow up in midair. The explosive used—TNT—is fairly inert and encased in a thick steel tube. The Tamir interceptors are supposed to blow up when they get close to the rockets and shred them with small steel rods. But the rods spray in all directions, so most don’t even touch the rocket. Many of those that hit may deflect off. The rods don’t have much chance if the timing or direction of the interceptor is even slightly off, according to Lloyd’s calculations.

If Iron Dome isn’t effective, then why have so few Israelis died from rocket fire? Both Lloyd and Postol attribute that mainly to the low firepower of the rockets and to Israel’s excellent early-warning system and network of shelters.

Lloyd and Postol both have potential conflicts of interest that critics might argue color their analysis. Lloyd has developed a concept for a different warhead for the Tamir interceptors, and he thinks his design would be more successful. Postol, for his part, has been a long-term critic of all kinds of missile-defense systems. He’s in a heated conflict with his employer, MIT, which he says has tried to suppress him because it wants contracts from Raytheon, the world’s biggest missile maker andnow a supplier of parts for Iron Dome.