The Grasshopper Can Fly Sideways 127
Phoghat writes "I'm of a 'certain age' and as a child grew up watching shows like "Rocky Jones, Space Ranger and others popular at the dawn of the space age. They always showed rocket ships sitting on their tails and blasting off, and landing, straight up. The shuttle went up that way but had to land like a plane, and anything else was considered impossible or impractical. Now, the Space X's rocket Grasshopper can not only do that, but has demonstrated sideways flight also."
Yes, it is impractical (Score:5, Informative)
XKCD just covered this! [xkcd.com] Good timing for the question.
TL;DR: Heat shields aren't going away because they are efficient.
Re:Gravity pulls toward the Earth (Score:4, Informative)
Actually, pretty quickly after takeoff, a rocket's inclination is changed to 25ish degrees. If you just go straight up, you're just going to fall back to earth and never achieve orbit.
Re:Yes, it is impractical (Score:5, Informative)
TL;DR: Heat shields aren't going away because they are efficient.
And the rocket equation is not. People need to be aware what they are looking at in these videos. This is not a spacecraft coming back to the Earth to land after it did some awesome mission. It is a depleted lower stage of a rocket, where the upper stage(s) has separated and continued on. Now the light lower stage has just enough fuel to fly home (because it is so light after burning up most of its fuel). It is a really, really clever idea for reusable lower stages. But it does not allow rockets to reenter the Earth's atmosphere at orbital velocities, slow down, and land. A phrase scientists and engineers use when they talk about the rocket equation is tyranny [nasa.gov]. Tyranny is right. It took a rocket the size of a skyscraper and weighing as much as a diesel submarine to go to the Moon and back. Without the heat shield, the rocket would have to be the size of an aircraft carrier.
Re:Watching the video (Score:5, Informative)
Actually the article misses the point. This isnt the reentry vehicle. This is the launcher. The first stage of a multistage vehicle, and it never leaves hte atmosphere. The idea is to create completely reusable launchers and thus lower cost. Now the upper stages could also benefit from this series of experiements and developmental work; this craft is testing multiple things, and a reentry vehicle that simply lands vertically back home has a few advantages (no really big landing field at really high speed like the shuttle, no uncontrolled parachute descent like current capsules).
But the main thrust (pun) of it is reusable launcher stages, with a side benefit of also being able to apply the tech to upper stages and the reentry vehicle as well. So its not a SSTO (single stage to orbit) vehicle like the old DC-X mcdonnel douglas was toying with.
Thrust vector control (Score:5, Informative)
We used to call it "thrust vector control". I worked in the Morton-Thiokol TVC lab for a while. The video shows a really excellent example of the technique, which is not new or controversial.
You can do TVC with hydraulics (heavy, but parts are easy to source and last longer) but you'll get better impulse numbers [wikipedia.org] for the vehicle as a whole if you can divert some proportion of the pressure from the combustion chamber into mechanical actuators that change the direction the nozzles are physically pointing. With multi-nozzled rocket motors (regardless of whether they have multiple combustion chambers or not) you can point some thrust down and some to the side (which appears to be happening in the video) and get this kind of behavior.
Similar things can be done with moving vanes in the exhaust plume, but those will erode even faster than the mechanism described above, and will be far slower to change the thrust vector. Erosion of parts that have high pressure hot gasses flowing through them is a huge issue in rocketry, although fairly well understood at this point. External aerodynamic vanes like the space shuttle's wings will obviously work too, and won't erode much (during liftoff) but they are also slow and clumsy.
When I say the technique's not new, I do not mean to denigrate the achievement. I can confidently state that it's really, really hard to do it as well as is being shown in this video. I would love to be able to work with these guys, because they are clearly just full of the right stuff.
Another alternative system to TVC is separately fueled ACMs - Attitude Control Motors - such as vernier thrusters [wikipedia.org] or the solid fuel ACMs on hypersonic crusie missiles. When you use gimballed nozzles to achieve TVC, though, you can potentially have the entire force of the main thrusters available for attitude control, and the fuel delivery system can be much more concentrated and simple.
Graphical overview of the common methods of TVC here [nasa.gov]