NASATest Goes Wrong

[Eric Mc]

Although NASA seems to make landing a rover on Mars look easy, landing a rocket powered lander on earth seems to be very hard -

http://www.youtube.com/watch?v=-hvlG2JtMts

[jurbie]

Seems to me that take off is the problem and there hasn't been much progress since the tethered tests.

http://www.youtube.com/watch?v=kTmlDmlVbFc&fea...

[Eric Mc]

I saw that too. Balancing a device on the tip of a rocket exhaust is not easy.

[Simpo Two]

So how did all the lunar modules manage to take off from the Moon? Or did the lower gravity make the difference?


Edited to add 'how'...

Edited by Simpo Two on Wednesday 15th August 22:48

[BonzoG]

Gravity on the moon is less than a fifth of gravity here - if you look at any of the videos of the LM "blasting off" from the surface, it seems to get fired almost immediately onto a ballistic escape.

In contrast, this looks like a ridiculously complicated balancing act - heavy lander on top of a single suitably powerful rocket, not being fired upwards through the atmosphere but rather being (or attempting to be ) balanced with relative precision at a constant height.

Ballsy!

[Simpo Two]

Perhaps they should take inspiration from the Harrier, or even the Curiosity skycrane, both of which work.

[mybrainhurts]

You get a nice barbecue going, then two fire trucks turn up and ruin it...

[Eric Mc]

This isn't a test of the principle of rocket powered hovering flight. We have worked out how to do this decades ago.

This is a test of a number of new technologies that could be used by autonomous landers on the moon or elsewhere using a new type of rocket motor using methane fuel. That has never been done before. Methane has a number of advantages over tradiotional rocket fuels. Methane is fairly plentiful, It is relatively easy to store. And, in theory, it could be manufactured on the moon from materials found in the lunar regolith.

Obviously, the test didn't go quite as well as it should but the idea is sound.

[BonzoG]

Simpo Two said:

Perhaps they should take inspiration from the Harrier, or even the Curiosity skycrane, both of which work.

Both of those vector thrust from multiple outlets - 4 nozzles on the harrier, 4 individual engines on the Mars Science Lab. Probably the most stable configuration if you want to avoid reliance on outboard attitude thrusters (although the Harrier still has them on the wingtips, I guess because the main nozzles are so close together laterally).

This thing has one single engine, with tiny outboard thrusters for attitude control. I'm as bad at metaphors as I am at physics, but I'd liken it to a seal trying to balance a ball on its nose. Whilst juggling, on a tightrope. They must have good reason for the concept, though.

[Eric Mc]

It is a test of a new type of propulsion system and its fuel together with the autonomous stabilisation and hazard avoidance system.

[Simpo Two]

There are two different things here - the fuel and the layout of the thrusters. If you want to try new fuels it is probably best to use a thruster layout that works, so you can actually test the fuels - ie beyond seeing how long they burn for after a crash!

Testing new stuff is great but once you have the wheel round, it doesn't sensible to add flat pieces to see if square is better.

[Eric Mc]

It's acalled all-up testing - something that was first used in Apollo in order to speed development along. It can have its drawbacks though

[robmlufc]

BonzoG said:

Both of those vector thrust from multiple outlets - 4 nozzles on the harrier, 4 individual engines on the Mars Science Lab. Probably the most stable configuration if you want to avoid reliance on outboard attitude thrusters (although the Harrier still has them on the wingtips, I guess because the main nozzles are so close together laterally).

The nozzles on the Harrier just control up and down in the hover, the reaction control valves are always needed for roll/pitch/yaw control and stability. Its still the same as a seal trying to balance a ball on its nose, or like trying to balance a small ball on a big ball

I presume that MSL could throttle each individual engine independantly to maintain the hover. This single vectored engine looks very ambitious!

Edited by robmlufc on Thursday 16th August 09:34

[Eric Mc]

Keeping a Harrier in the hover - especially the early GR1s and GR3s, was no easy task. Indeed, hovering flight using jet or rocket thrust alone is very, very tricky and there have been many accidents involving vehicles (manned and unmanned) that depend on such lift.

[robmlufc]

Eric Mc said:

Keeping a Harrier in the hover - especially the early GR1s and GR3s, was no easy task. Indeed, hovering flight using jet or rocket thrust alone is very, very tricky and there have been many accidents involving vehicles (manned and unmanned) that depend on such lift.

Yup, makes the MSL landing that bit more impressive.

[Otispunkmeyer]

jurbie said:

Seems to me that take off is the problem and there hasn't been much progress since the tethered tests.

http://www.youtube.com/watch?v=kTmlDmlVbFc&fea...

They did get it right though?

http://www.youtube.com/watch?v=AA1e33lRNs4&fea...

There's been plenty of this type of thing in the past though. Not sure why it wasn't done first time.

[anonymous-user]

I would imagine that there was a fairly high level control system failure, as the control system makes no attempt to correct the crafts trajectory. These days, it really isn't difficult to write control code for a 3 degrees of freedom and 3 axis unstable craft. In fact, the average college kid can do it if you look at all the autonomous and semi autonomous quad/hex rotor craft amatures have built.

I would guess that something major in the control code simply didn't execute when it should have done so........


(there are also unforunately numerous instances of human error causing such crashes, from gyro's installed upside down, to actuators in backwards and even erronious control code where +ve numbers equal left, when +ve should equal right!! etc ;-(

[Simpo Two]

Otispunkmeyer said:

They did get it right though?

http://www.youtube.com/watch?v=AA1e33lRNs4&fea...

There's been plenty of this type of thing in the past though. Not sure why it wasn't done first time.

That's more like it - you can see it gimballing like mad to stay upright/in place. Congenitally unstable but held up by computers.


This isn't bad either - take off aorund 2'15"; cameraman needs a bit of help though!

http://www.youtube.com/watch?v=JXwsRJzI8vE&fea...

[Otispunkmeyer]

Simpo Two said:

Otispunkmeyer said:

They did get it right though?

http://www.youtube.com/watch?v=AA1e33lRNs4&fea...

There's been plenty of this type of thing in the past though. Not sure why it wasn't done first time.

That's more like it - you can see it gimballing like mad to stay upright/in place. Congenitally unstable but held up by computers.


This isn't bad either - take off aorund 2'15"; cameraman needs a bit of help though!

http://www.youtube.com/watch?v=JXwsRJzI8vE&fea...

Suppose in the past they'll of had more than one motor... it is pretty cool just having the one motor and then some fast actuators to continually alter the aim of the nozzle to keep it up right.

[Eric Mc]

The Lunar Module and Viking all landed using just one main motor - although the Lunar Module had quads of small thrusters to move the vehicle about its various axes.

The difference between these older designs and this on is essentially the rocket fuel being used.