hypersonic trivia.

 

对于这里的一个奇怪现象，我觉得令人费解。这就是所谓的 victim playing, 无论扯什么都能扯到美国反华上面。然而 Everyone is entitled to their own opinions, but not their own facts. Physical Law doesn't change regardless whatever people may choose to believe. 我觉得对于 Hypersonic Vehicles 这个 topic 应该可以说说，不知道中文有什么有关这个的科普书籍，不过英文的有很多，诸如 Aviation Week, NASA website or English Wiki. 

我不想涉及政治层面上的东西，只想讲讲一些 Basic Hypersonic. Of course, I will talk about the Falcon HTV-2, but not so much about WU-14 since I don't have any creditable information about. 我不期望能改变什么人的看法，尤其是基于政治立场的看法，我想的是从技术的角度去欣赏这些 technical achievement, and understand the challenges people were facing. Btw, I write this as my spare time, and I don't proof read it, so bear with me for the mistakes.

In other post, I talked about the meaning of Mach number, and for hypersonic, we are talking M > 5. To reach hypersonic speed, a vehicle needs either with rocket, or ramjet/scramjet propulsion. The X-15 was the first hypersonic aircraft built. It was rocket propelled to exo-atmospheric ballistic trajectory, but instead falling back like a ICBM warhead, X-15 bounced off the atmosphere using its wing to any terrestrial target, then re-enter the atmosphere  and glided to land. Later, the Space Shuttle orbiter used the same approach  to return to earth from the orbit. However, there is a significant difference between the Space Shuttle orbiter and X-15, that is the angle of attack. Considering an winged aircraft, one side of the wings is flat, and the other curved. The flat side forms the chord line of the wing (this is not the definition in the text book, but to give an idea to the readers). The angle of attack is the angle between the chord line and the flow. 

Space Shuttle, or most of the spaceships like Apollo, or Russians or Chinese, even ICBM warheads are by design a "blunt object" and they re-enter the atmosphere with a high angle of attack. During reentry, the shuttle orbiter keeps its bottom toward the atmosphere, almost perpendicular to the atmosphere, . The mission profile of Space Shuttle is shown in the following figure. The belly of the orbiter is curved, and if there is a big sphere, you can put the orbiter there and it fit. 

Apollo, Gemini and the Russians capsules, which also serves the basis for Chinese capsules, all have similar shape shown in the following picture marked Manned Capsule concept. ICBM warheads basically take the shape of

Missile Nose Cones. The reason for such a configuration is that during re-entry, about 98% of the energy is dissipated into shockwave. 

With high attack angle, these vehicles experience significant drag which reduces the velocity quickly, and they don't have to sustain a long hypersonic flight. For X-15, the goal is different, and X-15 wants to achieve longer hypersonic flight. It will have to pull up and bounce off to the edge of atmosphere again, and glides with hypersonic speed. At this point, the shockwave is closer to the picture marked as the initial concept. In this case, the energy is absorbed at the nose. Also, with a low angle of attack, meaning that flying like an airplane, it is very difficult to deal with because we can simply use Newtonian flow to predict the aerodynamic characteristic.  

Almost all of orbital vehicles reenters the atmosphere at about M25. The speed of sound at the edge of the atmosphere is not easy to predict, but it is slower than the speed of sound at sea-level. It is possible for a reentry vehicle going faster than the orbital speed at Low Earth Orbit.  The key is that the cosmic speed is tangential speed. As seen in the Shuttle mission profile figure, when the shuttle de-orbit, it decelerates, and left the orbit, say 200km about the surface. Now the gravity pulls it toward the surface, at 9.2 m/s^2 and the gravity is getting stronger and stronger without atmosphere to slow it down. By the time the orbiter reaches the 100km, it has already gained a significant velocity. For sub-orbital vehicles, like ICBM, the re-enter speed is slower. 

Since most of the re-entry vehicles is designed to be slow down by drag, after going through the hypersonic descending, they slow down to desired sped. As for The shuttle, it changes pitch (pitch, nose up and down, yaw, nose left and right, and roll) to almost horizontal when the speed drops below supersonic. The reason is obvious, it will be extremely difficult to control, because there are two flows when the shuttle pitches down, the flow begins to attach on the back of the orbiter, in hypersonic speed, it creates a lot of uncertainty how the two flows, the one below and the one just starting attaching to the back react.

For Falcon HTV-2, it is the first of its kind, because the goal is not to brake to much, but to maintain in high hypersonic flight. This is extremely difficult. Some said it could not be in that speed because of the plasma  generated around the vehicle will stop it from communicating. That is not exactly true. The plasma forms around where the vehicles hitting the air, not surround the whole thing. At hypersonic speed, the shockwave forms at the tip of V shape seen in the INITIAL concept picture. The HTV can still communicate with ground through satellite rely on the its back. The Falcon HTV-2 flight profile is shown in the following figure 

 

This means the Falcon HTV-2 maintain high hypersonic throughout out the flight, and that has never been done before. 

The first test flight, after 9 min, the test vehicle started to yaw, and lost control. Since HTV has low angle of attack, it is very difficult to test in wind tunnel and scale up. Lockheed engineer did a very good job in designing this. After the first flight, they learned the lesson, and adjusted the center of mass. At the second flight, the adjustment worked, the vehicle was stable. However, to maintain a long hypersonic flight, the C-CAT aeroshell cracked due to the extreme heat and that created shock and the HTV started to tumble. More work needs to be done on the materials. For such a high speed flight. Most of the Russians and Chinese Reentry Vehicles are protected by Ablative head shield which disintegrates in high heat so that it dissipates the heat. The shuttle orbiter uses ceramic tile. In either case, these material doesn't fit HTV's goal. New materials is required. The follow up budget for the Falcon HTV-2 is to perfect this new material. As the two flights have given the aerodynamics, material and avionic. Further test isn't required.

The design goal for Falcon project is to develop a vehicle that is capable of travailing  over 9000 nautical miles in 2 hours, and strike every where. This is a very difficult task to achieve. From the flight profile above we can see the insert point (the launch vehicle separate from the HTV) isn't ideal, and the HTV had to dive and pull up in extreme velocity. This increase the difficulty significantly comparing to other flight profiles. The blue trajectory is the HTV-2 boost profile.
 

There is no details about the mission profile of WU-14 test, nor any technical information. All we see is the media picking HTV-2 data. However, The velocity of WU-14 is about M10, and as many experts suspected, this is likely a weapon used to deny-access to aircraft carriers rather than global strike. Hence it doesn't need to fly very far. Based on this information, it is very likely that WU-14 is similar to the AMaRV developed in the late 70s by then McDonnell Douglas, now Boeing. Which likely has fins, but more closely resembles the missile nose cone than HTV-2.

所有跟帖： 

• 事实是, 英文的都是纸上谈兵, 没成过, 中文的少见, 中国人玩实的, 成了. -无国无家- ♂ (0 bytes) () 02/14/2014 postreply 11:40:49