Slashdot Mirror
The Grasshopper Can Fly Sideways
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."
XKCD just covered this! Good timing for the question.
TL;DR: Heat shields aren't going away because they are efficient.
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.
Heat shields are the efficient way to slow from orbital speeds for reentry (e.g. the Shuttle), but conveniently for recovery the first stage isn't orbital. Grasshopper is basically a modified Falcon 9 first stage, and the goal of the testing is recovery of the first stage of Falcon 9-R, which is much easier than reentry from orbit..
We're not talking single stage to orbit here, and recovery of the second stage would certainly involve a heat shield. The first stage is a different animal. SpaceX seems to be intending to use a boost-back trajectory concept. I look forward to seeing how that works. (The controlled water "landing" attempt will be something to see, too, of course.)
bad comparison. the LM actually operated in reverse. it landed at a site, then took off. that is very different from taking off and then landing back at that exact same site. furthermore, the part that took off was a totally seperate piece with its own rocket engine, so technically it was two craft (or two stages) performing two seperate operations, not one craft performing both. the grasshopper is also far far larger than the LM, and exercising greater degree of control and precision in a heaver gravity and different atmosphere.
and while you alude to the crew capsules landing without fuel, the current crop of LAUNCHERS in use, are disposable single use entities, which means you apparently missed the entire point of this experimental rocket is to validate the concept of a reusable launcher, which would dramatically reduce costs.
short version: shutup
The guy who said the election was rigged won the presidency with the second-most votes.
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.
The guy who said the election was rigged won the presidency with the second-most votes.
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 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 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
The way I see it, it's not so much that they can launch a vehicle vertically and then move it horizontally. The impressive part is that they do it with an actual rocket that is 106 feet tall, and that they have launched it 7 times with 0 failures. And this is all in prelude to their 9-engine 160-foot tall rocket that they will test at altitudes of up to 300,000 feet. When you have that working in your backyard, you let us know and we'll be happy to pat you on the back. Or, if you're as competent at designing rocket control systems as you seem to think, go ahead and work for them. I'm sure Elon Musk pays his people well.
"Our two-party system is like a bowl of shit looking at itself in a mirror." - Lewis Black