Monday, November 23, 2015

Manned Mercury Mission - Part 01 - Introduction

There's been quite a lot written about manned Mars missions. Missions to Mars show up in Hollywood movies, news articles, and are even planned for the 1980s  1990s 2000 2020s 2030s. It looks like the logical next step after the moon; it's pretty close, has a nice red color, liquid water (sometimes), and might even have had life once! It's no wonder that so many people are fascinated by it. Still, I don't plan on doing a rehash of a bunch of stuff that's already been written by quite knowledgeable people, so I'm not going to be writing about manned Mars missions here.

Less well known are the plans for manned Venus missions. Venus is the closest planet to Earth, but that's about all it has going for it. Temperatures on the surface are hot enough to melt lead, and the pressure would crush a man instantly. The atmosphere is composed mostly of carbon dioxide and sulfuric acid; definitely not a life-friendly environment. Despite this, there were planned for manned flybys as far back as the 1970s. The American plan would have used Apollo hardware, while the Soviets would have used an uprated N1 (the N1F) with a heavily modified Soyuz as the crew module. Neither plan came to fruition (due to budgets cuts / changing priorities in the American case, and the failure of the N1 for the Soviets). Instead; we (mostly the Soviets) sent dozens of unmanned probes; mapping the surface from orbit, investigating the atmosphere, and parachuting in landers that lasted for minutes at a time. Despite the inhospitability of the planet, you'll occasionally hear about plans for missions to Venus; some of them even include deploying blimps in the upper atmosphere (there are a some layers of the Venusian upper atmosphere where the temperatures are survivable).

Far less has been written about manned missions to Mercury. That planet is farther away from Earth than Venus or Mars. Also, it's a lifeless, barren rock; with no atmosphere to speak of, the sunward side is roasted, while the night side experiences cryogenic temperatures for weeks at a time. Mercury's magnetic field is horribly weak compared to Earth's, meaning that the surface is exposed to obscene amounts of radiation (this is also why Mercury can't retain an atmosphere).  Far less is known about Mercury than other nearby planets; the first probe (Mariner 10) did flybys in the 1970s. The MESSENGER orbiter expanded mankind's knowledge of Mercury greatly, but compared to the Moon, Mars, or Venus, there's still many unanswered questions.

While a manned mission to Mercury would be highly difficult, it would certainly be possible with future technology. In fact, it could probably be done by 2050, assuming sufficient funding and will were available. While that's unlikely to happen (indeed, there are far better targets for exploration), it's still an interesting problem to look at.

Monday, November 9, 2015

via Astronautix

The history of American efforts to develop nuclear thermal rockets is relatively well known. Similar Soviet efforts have remained far more obscure. However, during the Cold War, the Soviet Union developed and tested an advanced nuclear thermal rocket engine, designated the RD-0410. Unfortunately, relatively little English-language information about the RD-0410 can be found (at least in easily available sources).

Similar to the American NERVA program, development of Soviet nuclear rocketry began in the mid-1950s. Serious research began in 1955, with development of a rocket beginning in 1956 (the people working on this project included such notable people as Kurchatov, Keldysh, and Korolev). Initially, the Soviets planned to use the nuclear rocket to power an intercontinental ballistic missile, or possible a cruise missile. However, it was quickly realized that chemical rockets were good enough for suborbital flights. As a result, by the 1960s, it was decided to develop the engine for usage in space.

The engine was developed by the KBKHA bureau, which had also developed engines such as the RD-0105 (used on some derivatives of the R-7). The goal was to develop an engine with a specific impulse of roughly 800-900 seconds, double what can be achieved with normal chemical rockets. Doing this would require creating a nuclear reactor that was both very light, and capable of withstanding very high temperatures around 3000 Kelvin. I have seen a few references to a program to develop a 2,000 isp engine, but this would require temperatures (over 15,000K) well in excess of what was possible in the 1950s (or even today) for a solid core design.

The test site selected for the Soviet nuclear engine was Semipalatinsk in Kazakhstan, a remote location similar to Jackass Flats in Nevada. The Soviets had already tested numerous atomic weapons (including their first in 1949 there), so the place was no stranger to nuclear activity. It appears that tests of the engine were conducted in a mine shaft approximately 150 meters deep, unlike the American NERVA, which was tested aboveground. Most likely, this was due to concerns over radiation should the engine malfunction. At some point, the engine acquired the designation RD-0410, it is less commonly known by its GRAU designation 11B91. That the engine received a GRAU designation means that it was almost certainly considered for military applications.

The American NERVA had a thrust of approximately 330 kilonewtons. This was much more than the RD-0410, which had about 35 kilonewtons. This was both by design, and due to political/monetary considerations. The Soviet government had somewhat lost interest in the project once it had become apparent that the nuclear engine was not usable as an ICBM upper stage. More importantly, by developing a lower power engine, the reactor assembly as a whole would be smaller. The RD-0410, including propellant, was planned to mass roughly 15 tons when completed; putting it well within the payload capabilities of Soviet launchers like Proton. The actual engine itself weighed only about two tons. In contrast, the American NERVA was much heavier, and could only be launched by a Saturn V or similar vehicle. 

There were other important differences between NERVA and RD-0410. The NERVA’s fuel elements were hexagonal in cross section, with several holes drilled in them for hydrogen to pass through. Hundreds of these elements (each about an inch wide) made up the NERVA’s reactor.
NERVA Fuel Elements

 It has been difficult to find exact information about the geometry of the RD-0410’s fuel rods, however, it appears that they had a complex shape. The fuel rods were twisted, and had a complex cross section, shaped like the petals of a flower. This was intended to lock the fuel rods together, and prevent fuel from falling out of the reactor if a few rods cracked or became dislodged. The fuel elements were made of uranium carbide, in order to better withstand the high temperatures of the core.  

Development and testing of the RD-0410 proceeded slowly. By 1973, America’s NERVA had already been test fired, then cancelled before actually flying. However, large scale tests of the RD-0410’s components did not begin until 1978. The test reactor was first started on March 27, 1978, and ran for 70 seconds. Gradually, the reactor was run for longer, and at higher temperatures. By 1981, the RD-0410 was running for an hour, its design duration. A specific impulse of 910 seconds was achieved; this was superior to that which was obtained with NERVA. The American Timberwind/SNTP project from the late 1980s planned to achieve similar efficiency with much higher thrust to weight, but it encountered numerous technical problems and did not reach the test stage. 

All accounts of the RD-0410 state that it’s testing at Semipalatinsk went very well. Originally, it was planned that the engine would fly in 1985 (likely replacing the Block D 4th stage on Proton). However, as the Soviet Union imploded during the 1980s, development slowed, then halted. Other Soviet nuclear rockets were planned, such as the RD-0411; a high thrust (~400 kN) engine that would have been used on a Mars mission, and an engine designated 11B97, which would have had the capability of either nuclear thermal or electric propulsion. However, like all other nuclear rocket programs, none of them came to be.

via Astronautix, a concept for a Soviet Mars spacecraft, that likely would have used RD-0411