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[信息] 美国核火箭发动机项目

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发表于 2018-5-5 14:09:18 | 显示全部楼层 |阅读模式
这是美国核火箭发动机项目的故事,冷战时期的一个交叉点。如果您不熟悉特定冲量的概念以及它与排气速度和推力的关系,建议查看维基百科有关它的文章,火箭发动机设计时这是一个非常重要的概念。

美国核火箭发动机项目

美国核火箭发动机项目


美国核火箭发动机项目

美国核火箭发动机项目

 楼主| 发表于 2018-5-5 14:11:33 | 显示全部楼层
原文如下:
Before this gets started, if you’re not familiar with the concept of specific impulse and how it relates to exhaust velocity and thrust, I suggest looking at the Wikipedia article about it as it’s a pretty important concept when talking about rocket engine designs. Very simply, it’s a measure of fuel efficiency. A chemical rocket engine like we’re all familiar with has a very high thrust but a pretty weak Isp. This makes it like a big supercharged V8 engine, great for climbing hills and drag racing, but it gets pathetic gas mileage. An engine with high Isp and low thrust is like a little 4 cylinder engine, it’s not going to win any races but it goes a lot further than the V8 on the same amount of fuel. With rockets, this makes chemical engines really good at taking off from Earth’s surface (climbing hills), but they eat fuel at a rapid rate. Nuclear thermal rockets generally have lower thrusts than chemical engines but they can be much more efficient. Generally it’s a tradeoff between Isp and thrust, there are designs that have both high exhaust velocities and high thrusts but they’re uncommon and they have other problems.

Basic operation of a NTR using the hot bleed cycle.

So, what exactly is a nuclear thermal rocket? It’s pretty simple really. You take a nuclear fission reactor and bolt it onto an exhaust nozzle. Instead of running water through the reactor and into a turbine generator, you run hydrogen through it and out the nozzle. It’s basically the same process that produces thrust in a chemical rocket, except the energy for heating the working fluid is coming from nuclear fission reactions in the reactor rather than chemical reactions between a fuel and an oxidizer. Solid core NTRs can't run as hot as chemical rockets because it would melt the reactor, but because the molecular weight of the hydrogen exhaust is so low compared to the H2O exhaust of LH2/LOX chemical rockets the exhaust velocity is higher. Also, since the energy density of nuclear fuel is much higher than that of chemical fuels, the efficiency and specific impulse of the engine is significantly better. How much better? Well, the solid-core NTRs had specific impulses of about 1000 seconds, more than twice that of chemical designs like the Space Shuttle main engine. As an aside, using hydrogen as a propellant in an NTR gives the best exhaust velocity (due to its low molecular weight), but there’s no reason you can’t use other propellants since its just being used as reaction mass. Ammonia, methane, carbon dioxide, nitrogen, or just plain water, you can run practically any non-corrosive fluid through the reactor to produce thrust. This is really handy if you want to, say, fly a mission to the rings of Saturn. Once you get out there you can find a nice chunk of ice to melt and siphon into your propellant tanks for the trip back, you don’t need to haul massive amounts of fuel around for the whole mission if you can top up the tanks when you get to your destination.

Project Rover

The history of the US nuclear rocket programme begins in 1955 with project Rover. The principal R&D work was carried out at the Los Alamos laboratory, with the goal of producing reactor fuel systems that would operate with hydrogen at temperatures above 2200K. Initiated by the US Atomic Energy Commission and the US Air Force, the original goal was to create a nuclear engine for missile applications. However, in 1958 the newly formed NASA took over responsibility for the project and the proposed engine was to be used in long-haul space missions to the moon and Mars. Progress was unexpectedly rapid, it proved to be much easier to reduce the reactor size and weight than was initially thought. Phase one of Rover produced the Kiwi graphite-core reactors, named for the flightless bird. First tested in 1959, Kiwi proved that a small lightweight reactor operating at high temperatures and cooled by hydrogen was essentially viable. The Rover project went on to be very successful and produced a number of small, powerful reactors.


美国核火箭发动机项目

美国核火箭发动机项目

Reactor designs produced under the Rover programme.

美国核火箭发动机项目

美国核火箭发动机项目

Kiwi-A reactor.

美国核火箭发动机项目

美国核火箭发动机项目

Kiwi cutaway illustration.
 楼主| 发表于 2018-5-5 14:14:02 | 显示全部楼层

美国核火箭发动机项目

美国核火箭发动机项目

Kiwi-TNT, destructive test of the reactor carried out by deliberately taking it supercritical as quickly as possible. Kiwi-TNT was conducted to test a worst case scenario of a reactor explosion on the launchpad. The reactor exploded 156 milliseconds after the control drums were rotated to the fully ‘open’ position. The explosion released an intensely radioactive cloud of debris. Anyone within 400 feet would have received a fatal dose of radiation. January 12, 1965.

美国核火箭发动机项目

美国核火箭发动机项目

Shattered remains of graphite fuel elements from destructive reactor test.

Since the reactors operated at much higher temperatures than more traditional reactors they were considerably more powerful, and with each new reactor the power density increased. The Phoebus-2A reactor test in 1968 ran for more than 12 minutes at 4000 megawatts, at the time it was the most powerful reactor ever built. In 1960 the NERVA programme was initiated and in 1963 it was tasked with taking the graphite-based reactor developed under Rover and creating a functioning nuclear rocket engine.

NERVA

美国核火箭发动机项目

美国核火箭发动机项目

NERVA mockup.

美国核火箭发动机项目

美国核火箭发动机项目


美国核火箭发动机项目

美国核火箭发动机项目

NERVA illustrations and technical drawings. Click for full images.
 楼主| 发表于 2018-5-5 14:16:04 | 显示全部楼层
Nuclear Engine for Rocket Vehicle Applications. Unlike the Rover reactors , NERVA was a prototype for an actual flight system, with Westinghouse providing the reactor and Aerojet building the engine systems. As built, NERVA would produce approximately 75,000lbs (333 kN) of thrust and a specific impulse of 825 seconds. The operating temperature was 2300K and the reactor operating lifetime goal was one hour. NERVA was intended for use in space, for two reasons: 1. its thrust to weight ratio was low due to narrow channels in the reactor that limited the mass flow rate, and 2. environmental concerns. No one’s going to notice a little more radiation out in space, but using NERVA for liftoff would have been unpopular with anyone living downwind of the launch site.

美国核火箭发动机项目

美国核火箭发动机项目

NERVA reactor cutaway illustration.

美国核火箭发动机项目

美国核火箭发动机项目

The flight engine configuration was 22 feet tall from the upper thrust structure that mates to the hydrogen tank, to the exhaust bell nozzle. The spherical tanks beneath the upper thrust structure contain actuating gas for the engine’s pneumatic systems. Just beneath those is the gimbal assembly that allows the rocket to be steered. The turbopump machinery is located in the lower thrust structure. The aluminium pressure vessel contains the reactor and beryllium radiation shield. There’s no radiation backscatter in space so the internal shadow shield combined with the hydrogen propellant tank was considered to be enough radiation protection for the payload. The mushroom-looking things on top of the pressure vessel are the actuators for the reactor control drums. The exhaust nozzle mounted beneath the reactor is cooled by the hydrogen propellant flow.

美国核火箭发动机项目

美国核火箭发动机项目

 楼主| 发表于 2018-5-5 14:17:53 | 显示全部楼层

美国核火箭发动机项目

美国核火箭发动机项目


美国核火箭发动机项目

美国核火箭发动机项目

Radiation flux from an operating NERVA engine.

美国核火箭发动机项目

美国核火箭发动机项目

NERVA cold flow test. The rocket engine assembly contained no fissionable material for this test. Startup operations and operation procedures were the main goals of the test. December 1, 1967.
 楼主| 发表于 2018-5-5 14:19:25 | 显示全部楼层

美国核火箭发动机项目

美国核火箭发动机项目

Same test as above, this is the first NERVA test engine, the XECF. CF for cold flow. December 1, 1967

美国核火箭发动机项目

美国核火箭发动机项目

NERVA engine undergoing test firing at Jackass Flats. Note the lack of any containment of the exhaust plume.

美国核火箭发动机项目

美国核火箭发动机项目

NRX-EST engine on the test stand.
 楼主| 发表于 2018-5-5 14:20:40 | 显示全部楼层
Between 1959 and 1972, 23 Rover/NERVA reactor tests were carried out at Jackass Flats in Nevada, about 160km west of Las Vegas. For most of these tests there was no attempt to contain the exhaust plume. The NERVA engine proved to be very reliable, the XE-prime (last engine in the NERVA series) was tested with flight hardware under simulated vacuum conditions and operated for a total of three hours forty eight minutes, with 28 restart cycles. The engine burn times were limited by the size of the hydrogen tanks at Jackass flats, not the capabilities of the engine. The longest continuous burn time was 90 minutes, and it was extrapolated that reactor lifetime could exceed 3 hours of operational time. XE-prime demonstrated that a nuclear rocket engine was suitable for space flight and could operate with twice the specific impulse of chemical rockets. NASA deemed NERVA was ready to begin flight tests.

美国核火箭发动机项目

美国核火箭发动机项目


美国核火箭发动机项目

美国核火箭发动机项目


美国核火箭发动机项目

美国核火箭发动机项目

Configurations of the NERVA reactor core. The narrow propellant passages limit the engine’s mass flow rate and hence its maximum thrust.
 楼主| 发表于 2018-5-5 14:21:59 | 显示全部楼层
RIFT
The NERVA engine was to be flight rated by mounting it as a third stage on the Saturn V, this configuration was known as the S-N (Saturn Nuclear) stage. Originally planned in 1962, the Reactor In Flight Test was to be mounted on a Saturn V with a dummy S-II second stage, so the nuclear rocket engine was supposed to fire while still suborbital, with the reactor splashing down in the Atlantic. Note that this was a flight test, orbital testing was to be carried out if everything panned out with the suborbital tests. NASA mission planners proposed using NERVA for a manned mission to Mars, the impressive capabilities of the nuclear engine made such missions feasible. It was also proposed for lunar missions, using a nuclear third stage the Saturn C-5 could carry three times the payload of the chemical version. Ultimately the Mars mission was to be NERVA’s downfall, RIFT was delayed continually after 1966 and public interest in human spaceflight was waning after the space race was won. Congress was not willing to commit to decades of expensive development for a manned Mars mission, and RIFT was never authorized. The Saturn production line was shut down in 1970 and with no Saturn-N to perform flight tests, NERVA was effectively dead. Nixon shut down the NERVA programme in 1972 after expenditures totaling $1.4 billion.

美国核火箭发动机项目

美国核火箭发动机项目

Proposed nuclear configuration of the Saturn C-5.

美国核火箭发动机项目

美国核火箭发动机项目


美国核火箭发动机项目

美国核火箭发动机项目


 楼主| 发表于 2018-5-5 14:23:20 | 显示全部楼层

美国核火箭发动机项目

美国核火箭发动机项目

RIFT illustrations.

美国核火箭发动机项目

美国核火箭发动机项目

NERVA as the engine for a Saturn V upper stage.

美国核火箭发动机项目

美国核火箭发动机项目

Lockheed proposals for the applications for the NERVA engine.

 楼主| 发表于 2018-5-5 14:24:03 | 显示全部楼层
The original NERVA reactor was based on Kiwi as developed under project Rover; though the later reactors were more powerful by the time they were developed the Apollo programme had already been largely defunded and the money for NERVA was running out. But in the early 1960s, enthusiasm for manned space exploration was still high and many NASA mission planners saw NERVA’s impressive performance statistics and incorporated it into their proposals. Extended lunar exploration and manned Mars expeditions were the two main focuses of attention. Chemical rockets are unworkable for deep space missions with large payloads, you need huge amounts of fuel to boost the large payloads necessary for manned expeditions, then you need more fuel to boost that fuel and the vehicle size spirals out of control. NERVA was much more efficient than chemical designs and could run for long periods with multiple shutdown/restart cycles, which made it perfect for deep space missions. Modular propulsion units were designed, with an engine and propellant tank package multiple modules could be combined to build a vehicle for a specific mission.

美国核火箭发动机项目

美国核火箭发动机项目


NERVA propulsion module illustration from a NASA report to congress.

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