Saturday, 15 October 2005

Rockets, Radiation, Reliability and Recycling

From Science@NASA :
Research is underway into a new generation of liquid-fueled rocket designs that could double performance over today's designs while also improving reliability.
You might assume that, by now, every conceivable refinement in liquid-fueled rocket designs must have been made. You'd be wrong. It turns out there's room for improvement.

Integrated Powerhead DemonstratorLed by the US Air Force, a group consisting of NASA, the Department of Defense, and several industry partners are working on better engine designs. Their program is called Integrated High Payoff Rocket Propulsion Technologies, and they are looking at many possible improvements. One of the most promising so far is a new scheme for fuel flow:

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The basic idea behind a liquid-fueled rocket is rather simple. A fuel and an oxidizer, both in liquid form, are fed into a combustion chamber and ignited. For example, the shuttle uses liquid hydrogen as its fuel and liquid oxygen as the oxidizer. The hot gases produced by the combustion escape rapidly through the cone-shaped nozzle, thus producing thrust.

The details, of course, are much more complicated. For one, both the liquid fuel and the oxidizer must be fed into the chamber very rapidly and under great pressure. The shuttle's main engines would drain a swimming pool full of fuel in only 25 seconds!

This gushing torrent of fuel is driven by a turbopump. To power the turbopump, a small amount of fuel is "preburned", thus generating hot gases that drive the turbopump, which in turn pumps the rest of the fuel into the main combustion chamber. A similar process is used to pump the oxidizer.

Today's liquid-fueled rockets send only a small amount of fuel and oxidizer through the preburners. The bulk flows directly to the main combustion chamber, skipping the preburners entirely.

One of many innovations being tested by the Air Force and NASA is to send all of the fuel and oxidizer through their respective preburners. Only a small amount is consumed there--just enough to run the turbos; the rest flows through to the combustion chamber.

This "full-flow staged cycle" design has an important advantage: with more mass passing through the turbine that drives the turbopump, the turbopump is driven harder, thus reaching higher pressures. Higher pressures equal greater performance from the rocket.

Such a design has never been used in a liquid-fueled rocket in the U.S. before, according to Gary Genge at NASA's Marshall Space Flight Center. Genge is the Deputy Project Manager for the Integrated Powerhead Demonstrator (IPD)--a test-engine for these concepts.

Right: A rendering of the Integrated Powerhead Demonstrator, showing its innovative plumbing for routing fuel and oxidizer to the combustion chamber.

"These designs we're exploring could boost performance in many ways," says Genge. "We're hoping for better fuel efficiency, higher thrust-to-weight ratio, improved reliability--all at a lower cost."

"At this phase of the project, however, we're just trying to get this alternate flow pattern working correctly," he notes.
Breath-holding contra-indicated then. But it's just a matter of time before someone gets the idea working.

Also from Science@NASA :
Most household trash bags are made of a polymer called polyethylene. Variants of that molecule turn out to be excellent at shielding the most dangerous forms of space radiation. Scientists have long known this. The trouble has been trying to build a spaceship out of the flimsy stuff.

But now NASA scientists have invented a groundbreaking, polyethylene-based material called RXF1 that's even stronger and lighter than aluminum. "This new material is a first in the sense that it combines superior structural properties with superior shielding properties," says Nasser Barghouty, Project Scientist for NASA's Space Radiation Shielding Project at the Marshall Space Flight Center.
Barghouty is one of the skeptics: "Going to Mars now with an aluminum spaceship is undoable," he believes.

Plastic is an appealing alternative: Compared to aluminum, polyethylene is 50% better at shielding solar flares and 15% better for cosmic rays.

Left: Cosmic rays crash into matter, producing secondary particles. [More]

The advantage of plastic-like materials is that they produce far less "secondary radiation" than heavier materials like aluminum or lead. Secondary radiation comes from the shielding material itself. When particles of space radiation smash into atoms within the shield, they trigger tiny nuclear reactions. Those reactions produce a shower of nuclear byproducts -- neutrons and other particles -- that enter the spacecraft. It's a bit like trying to protect yourself from a flying bowling ball by erecting a wall of pins. You avoid the ball but get pelted by pins. "Secondaries" can be worse for astronauts' health than the original space radiation!

Ironically, heavier elements like lead, which people often assume to be the best radiation shielding, produce much more secondary radiation than lighter elements like carbon and hydrogen. That's why polyethylene makes good shielding: it is composed entirely of lightweight carbon and hydrogen atoms, which minimizes secondaries.
Parenthetically, Water is an excellent radiation shield.. but heavy, and structurally not so wonderful. On the other hand, raw sewage is mainly water. Have a cylinder composed of independant cells of drinking water, fill them with liquid waste when empty, and you have a decent storm shelter should a solar flare erupt. Cosmic rays are another matter.
Despite their shielding power, ordinary trash bags obviously won't do for building a spaceship. So Barghouty and his colleagues have been trying to beef-up polyethylene for aerospace work.

That's how Shielding Project researcher Raj Kaul, working together with Barghouty, came to invent RXF1. RXF1 is remarkably strong and light: it has 3 times the tensile strength of aluminum, yet is 2.6 times lighter -- impressive even by aerospace standards.

"Since it is a ballistic shield, it also deflects micrometeorites," says Kaul, who had previously worked with similar materials in developing helicopter armor. "Since it's a fabric, it can be draped around molds and shaped into specific spacecraft components." And because it's derived from polyethylene, it's an excellent radiation shield as well.
Some "galactic cosmic rays are so energetic that no reasonable amount of shielding can stop them," cautions Frank Cucinotta, NASA's Chief Radiation Health Officer. "All materials have this problem, including polyethylene."

Cucinotta and colleagues have done computer simulations to compare the cancer risk of going to Mars in an aluminum ship vs. a polyethylene ship. Surprisingly, "there was no significant difference," he says. This conclusion depends on a biological model which estimates how human tissue is affected by space radiation--and therein lies the rub. After decades of spaceflight, scientists still don't fully understand how the human body reacts to cosmic rays. If their model is correct, however, there could be little practical benefit to the extra shielding polyethylene provides. This is a matter of ongoing research.

Because of the many uncertainties, dose limits for astronauts on a Mars mission have not been set, notes Barghouty. But assuming that those dose limits are similar to limits set for Shuttle and Space Station flights, he believes RXF1 could hypothetically provide adequate shielding for a 30 month mission to Mars.
There's still a lot we need to find out before going to Mars. The more I see of it, the more I think the Chinese have got the right idea. First, set up a space construction facility for assembling interplanetary craft of arbitrary size. Then go about colonising the Moon, with research applicable to deeper space missions. Then onwards and outwards.

From Space Daily :
China's achievement in sending taikonauts Cols. Fei Junlong and Nie Haisheng into orbit on the manned Shenzhou-6 space craft for a four or five day mission is the second step in a long campaign of amazing vision mapped out by the genius founder of its space program half a century ago.

Within the next 10 to 15 years, China is determined to become the dominant space power and build first its own massive, permanent, orbiting space station as a stepping stone into the Solar System and then even build and man a long-term base on the Moon.

Can they do it? Yes.

The first thing to recognize about the Chinese vision is that it is serious and backed by a profound political commitment. Former President Jiang Zemin was the first Chinese leader determined to make his nation a major space power, and eventually the dominant one. And his successor, current President Hu Jintao, shares that commitment. But the program they have backed had been created decades earlier by a U.S.-educated genius called Tsien Hsue-Shen.

Tsien was a member of the first heroic generation of U.S. rocket scientists and space visionaries. He was a founder of the Jet Propulsion Laboratory in Pasadena, Calif. He was even included on the team of U.S. scientists who went to investigate Nazi Germany's groundbreaking V-2 rocket program after World War II. Researcher Mark Wade, writing about China's space program on the Web site described him as "one of the senior scientists advising the U.S. military on postwar development of rocket technology."

But in 1950 Tsien, according to Wade, was accused of being a Communist Party member and his security clearance was revoked. He was then held under virtual house arrest for five years before being allowed to return to China in 1955. He then led China's space program for decades.
Of course, there's many a slip, and all may not be "nominal" on Shenzhou 6 at the moment. I wish the Taikonauts good luck, a safe return, and happy landings. No doubt, as with the near-disasters and minor glitches that both the US and USSR had in the 60's and 70's, we'll hear the complete story sooner or later. Maybe in 25 years.

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