The Federal Government plans to spend up to $1 billion on robot spy planes that have a track record of mishaps and crashes.

More than half of the seven prototypes of the unmanned Global Hawk - a central plank of the new $50 billion defence spending plan - have been damaged or destroyed through technical failure or misadventure.

The dismal record emerged as a Herald investigation found that a number of current defence projects including jet fighters, destroyers, helicopters and missiles - worth billions of dollars - have been hit by cost blow-outs or are behind schedule.

The Global Hawk disasters include one lost after a bolt was installed backwards during servicing, a potential $80 million mistake if it were to be repeated in Australia.

A second plane was lost after accidentally picking up a self-destruct signal that was designed to bring down a nearby missile.

A third was lost through engine failure, and a fourth was damaged after crashing when its US military handler made a typing error on a computer control pad.

Stephen Gumley, the new head of the Defence Materiel Organisation, which buys military hardware, has admitted that up to 24 major projects are overtime and over budget.

Dr Gumley said most of the problem projects were quite old but the Herald has found that many are current, including:  The Bushranger light armoured vehicle project, which has almost doubled in price to $329 million, and the vehicles have yet to enter full service.  Higher costs and likely delays for the $12 billion new-generation joint strike fighter.  A $1.5 billion blowout in the cost of air-warfare destroyers.  Multi-million-dollar cost increases on helicopter projects such as the Seahawk upgrade and the planned trooplift helicopter.  A 10-year delay in the installation of new air-to-ground missiles on F-111s, meaning that they will be fitted only four years before the planes are phased out.

Dr Gumley said: "The projects that have been taken on since DMO was formed [in 2000], virtually all of them are on time and on budget. But we do have a few problem ones which we are working to fix.

"The vast majority is due to technological discord. You have got to understand that military technology keeps moving on and this is something that perhaps isn't apparent to many people in the public."

The purchase of up to 12 Global Hawks is considered a certainty after one was tested at taxpayer expense in Australia in 2001.

The successful maritime surveillance tests cost about $15 million but most of the benefits flowed to the US military and the manufacturer of the planes, Northrop Grumman, the company has confirmed.

When the Federal Government first proposed buying robot planes just three years ago it said they would cost about $150 million.

But the Defence Minister, Robert Hill, announcing his 10-year Defence Capability Plan in February, said the program could now cost between $750 million and $1 billion.

The US General Accounting Office says that when the Global Hawks were conceived in 1994 they were expected to have an average "flyaway price" of $US10 million each.

Although the original concept has been heavily modified over the years, no explanation has been given for the sudden huge cost increases.

Senator Hill has praised the success of the robot aircraft in operations in Iraq and Afghanistan.

But Northrop Grumman admits that two of the craft were lost during these operations - more than a quarter of the prototype fleet. Neither was shot down but were affected by technical failures or poor maintenance.

"The losses of Global Hawk must be discussed within the context of their employment," the company said in a statement. "The Global Hawks employed in combat were Advanced Concept Technology Demonstrators [ACTD].

"The decision by senior US Air Force officials to employ the non-combat ready 'test' ACTD Global Hawk system recognised the fact that there were many risks to the system including no routine spare parts, no operationally trained crew members or maintenance personnel, and no proven and tested employment guides."

But the company said that these risks were far outweighed by the benefits of adding Global Hawk's capabilities to help shorten conflicts and save military and civilian lives.

Pentagon data on the number of crashes per hours flown shows that the Global Hawk had a crash rate 50 times higher than the F-16 fighter, a plane that frequently flies more dangerous missions and at lower altitudes, says a report in the August edition of New Scientist magazine.

A spokesman for Northrop Grumman said that it was unfair to compare a mature fighter aircraft with a pre-production prototype.

Only two general production versions of the Global Hawk have so far been built. They are in service with the US Air Force.