Friday, August 22, 2008

Ares Program's Shocking Development

Apparently the engineering team working on the ARES CEV are considering adding weights and shock absorbers between the first (SRB) and second stage in order to dampen hefty vibrations that might otherwise blur the vision of the crew or cause parts to fall off. The vibrational load is in the 5 to 6 g range. It's hard to honestly evaluate the impact of this new development, but one wonders if a liquid fueled first stage wouldn't have addressed this concern in the first place. An interesting blog to which I was recently introduced covered this story before I did, and it's owner wonders if this is a case of NASA carrying over lessons learned from the Apollo era. It may well be, although the solution to eliminate first stage pogo-ing in the Saturn didn't so much involve shock absorbers as it involved accumulators in the fuel and oxydizer lines.

POGO-ing is a longitudinal oscillation that involves pressure waves in the rocket fuel (not unrelated a water hammer) coupling into the engines, causing surges and reductions in engine thrust. A pressure wave in the fuel will occur just by suddenly applying thrust to the bottom of the rocket, which as one can guess occurs during every lift-off. Once the thrust spikes from the pressure wave, the fuel gets bounced back up the fuel tank. Then it comes slamming down on the bottom of the tank again, causing the next thrust spike -- repeat ad nauseum.

The Apollo solution to pogo-ing wasn't all that different from the common plumbing solution for water hammers, which involves adding a gas filled chamber -- closed at the top, attached to the fuel or water line at the bottom -- called an accumulator. When the surge travels down the line from the fuel tank, it encounters this detour into the gas-filled chamber. Since fluids don't compress, but gases readily do, the surge in the line is able to rush into the chamber, compressing the gas within as it goes, rather than surging into the engines. This way, the gas acts as a shock absorber for the fuel, eliminating or greatly reducing pogo-ing.

By the way, to the best of my knowledge there is no such solution for solid rocket motors, and besides which pogo-ing is a liquid fueled engine phenomena. Adding weight and shock absorbers to a rocket seems contrary to the basic direction of rocket improvement, so I am still left wondering, "what gives?"

Thursday, August 07, 2008

SpaceX Suffers Stage Separation Anxiety but Problem Solved


Sorry, but I couldn't help the title. In short, the third flight of the Falcon 1 failed because the time between first stage engine cutoff and second stage separation was a tad too short. The third flight incorporated fixes to the problems seen in the second flight as well as a change to the first stage engine (from the ablatively cooled Merlin 1 to the higher performance, regenatively cooled Merlin 1C).

The ablative engine uses an engine liner -- made of something akin to fiberglass and not so different from a capsule heat sheild in function -- to keep the engine from melting. The liner keeps the engine cool by slowly burning away as the engine runs. The burning bits leave the engine, carrying the heat with them, which prevents the unburnt part of the liner from heating up. This method is often used, such as with the Apollo Lunar Excursion Module's descent and ascent engines. It's great stuff and makes for a simple engine design. Fuel and oxydizer are mixed at the top, burnt, and thrust shoots out the bottom. But the thrust performance changes over the time of flight due to erosion/burning of the liner's surface.

Regenatively cooled engines run their fuel and/or oxydizer through a cooling jacket surrounding the inside surface of the engine. This method keeps the inside surface of the engine from melting, removing the necessity of an engine liner, and also heats up the fuel/oxydizer for more efficient energy transfer. This is a more complex engine design, but it has the advantage that the engine performance remains constant throughout the flight.

Now comes the problem. When an ablative lined engine shuts down, it shuts down, the fuel/oxydizer valves are shut and the engine shuts down at once. Not so with a regenatively cooled engine, when it shuts down, there's still a bit of fuel and oxydizer left in the cooling jacket that continues to burn. This creates a bit of excess thrust which takes a bit of time to fall off. This extra bit of time needs to be added to the delay between first stage cutoff and second stage separation. Time was added, but just not quite enough. In the video you can watch the stages separate then clunk back together. The statement from Elon Musk, head of SpaceX, talks about this issue. He also indicated that since the problem is well understood, the next flight of Falcon 1 can proceed within the next month.

This is the first rocket company I've seen that has been so open and honest with the public at large. The video footage from each launch is invaluable and allows not only SpaceX, but the public to witness and maybe even assist the development of SpaceX's technology. Great stuff.

Wednesday, August 06, 2008

SpaceX: Third Time not the Charm

Image Courtesy Falcon 1 Flight 2

Sadly, SpaceX wasn't able to make orbit last Saturday. Communications with the rocket ended a few minutes into the launch around the time of second stage separation. Apparently, second stage separation did not occur. This link includes video footage from the launch.

Potential Perchlorate Source on Mars

I recently read articles about perchlorate being found on Mars. Some are trying to figure out if the Phoenix lander's pyrotechnics are involved, but I found an interesting link about natural perchlorate formation on Earth that might explain the Martian source. This abstract discusses perchlorate detected in the Black Rock Desert and in Death Valley, which apparently forms when titanium dioxide, salt and sunlight and/or UV light are combined. Seems to me that since Mars is a vast desert with apparently a fair amount of salt and no appreciable atmosphere to stop the UV, then this mechanism could easily be responsible.

By the way, I found this information by Googling "perchlorate formation" and the third link down was an article about natural perchlorate formation. Also I tried googling "desert perchlorate" and came up with the photo of the gents sampling perchlorate in a hole in the desert. Go check it out.

Monday, August 04, 2008

How to Eat Fried Worms

Whilst browsing about, I came upon this interesting recipe Since the little devils would do double time as excess vegetative matter break-downers, using them in space colonization might be a plus. Although, I have no idea what they taste like. Anyone with an idea, please comment.