Moscow - It looks like a bonanza for the Russian space industries -- the planned retirement of the US space shuttle fleet in about three years would make Russia the principal carrier of crews and cargo to the international space station, sharply raising its revenues.

But some Russian cosmonauts and space experts are worried. They fear the lead will be short-lived and will slow development of what's really needed -- a replacement for the veteran Soyuz spacecraft, the reliable but plodding workhorse of the nation's space program for 40 years.

A Soyuz blasted off over the weekend from the Baikonur cosmodrome carrying two cosmonauts to the international space station, along with Charles Simonyi, a US software billionaire who paid $20-25 million for a 13-day trip to the station and back.

Russia currently builds two Soyuz spacecraft a year for manned launches, and four unmanned Progress cargo ships. The fleet is expected to expand to four Soyuz and seven Progress vehicles starting in 2010. Unlike the United States' three space shuttles, they can only be used once.

The government has been slow to earmark money for a next-generation spacecraft, and some experts fear rising demand for rides in the Soyuz could further slow funding.

"Building more ships will divert resources from other projects," said Igor Marinin, the editor of Novosti Kosmonavtiki, the leading Russian space magazine.

On top of that, the cash to be earned from space tourists encourages authorities to send smaller Russian crews into orbit, Marinin said.

Cosmonaut Pavel Vinogradov, who returned from a stint on the international space station last fall, said Russia already has reduced itself to being a "space cabbie" -- although space limo might be a better description, since Simonyi's ride costs him up to $1,300 a minute.

Vinogradov warned that without a next-generation spacecraft, Russia could fall hopelessly behind the United States, the European Union and others.

"The Americans will build their new spacecraft ... and we will be left behind with our old ship which no one will need," he said in a recently published interview.

The United States is working on the Orion capsule which should be ready for manned flight around 2015.

Russia's space industries are still reeling from the post-Soviet economic meltdown, when they couldn't afford modern equipment and suffered an exodus of workers and engineers.

Russia's space program survived mostly on commercial launches of foreign satellites and income from ferrying foreign astronauts and space tourists.

Government spending on the space program has since increased thanks to Russia's economic boom, but the industry still lacks the money to design replacements for its boosters and spacecraft.

The Soyuz and the Progress date from the mid-1960s, and Russian space officials stress their excellent safety record and updated control systems, saying the only thing they share with the original Soyuz craft is their exterior design.

But a Soyuz can only carry only 110 pounds of cargo back to Earth along with a crew, sharply limiting its ability to transport scientific experiments or equipment.

The Progress hauls about 2.75 tons of cargo to the space station, less than a fifth of the space shuttle's capacity. The Soyuz's three cosmonauts must stay seated during the two-day trip to the space station. During re-entry to Earth's atmosphere the craft decelerates rapidly, exerting severe G-forces. Parachutes drop the craft on the Central Asian steppes, making for a rough touchdown.

RKK Energiya, which builds the Soyuz and Progress ships, has long proposed building a new, reusable spacecraft called the Clipper.

Energiya says it would make space travel more comfortable and cut the cost of delivering crews to orbit by two-thirds. The Clipper, with six seats compared to three on Soyuz, could be used for a moon mission, as well as ferrying space station crews.

Energiya chief Nikolai Sevastyanov said building five Clippers would cost about $1.5 billion.

So far, however, Energiya has only built a plywood and plastic model.

"The Americans, the Europeans, the Japanese all are developing space technologies of the future, while Russia is just marking time," Marinin said.