Community > Forum > Official Armadillo Aerospace Forum > Landing legs

Landing legs

Posted by: JamesHughes - Mon Oct 24, 2005 12:47 pm
Post new topic Reply to topic
 [ 79 posts ] 
Go to page 1, 2, 3, 4, 5, 6  Next
Landing legs 
Author Message
Space Station Commander
Space Station Commander
User avatar
Joined: Wed Aug 18, 2004 8:47 am
Posts: 521
Location: Science Park, Cambridge, UK
Post Landing legs   Posted on: Mon Oct 24, 2005 12:47 pm
Ever since I saw the original Armadillo test bed, I have been trying to figure out why it has the landing legs that it does. I mean, why not use splayed out legs like the Apollo lander? It seems obviously (well, to me), that a tall craft with a narrow leg base is bound to fall over at the first opportunity - hence the mishap at the X Prize cup.

Surely the added effort/cost - if any - of having spread out legs must be worth it for the occasional off centre landing, and also to get the legs further away from the exhaust plume on take off.

Anyone know why Armadillo has such a narrow footprint? Or have I missed something?

Hmm, just had a thought - splayed legs have to be stronger, and therefor heavier - but the cost of falling over must outweigh that I would have thought.

James


Back to top
Profile
Space Walker
Space Walker
User avatar
Joined: Thu Jun 17, 2004 3:01 am
Posts: 173
Location: Dallas, TX
Post    Posted on: Mon Oct 24, 2005 2:11 pm
The main reason has always been that these vehicles are not intended to be just hovering landers. Eventually, they need to go very fast for suborbital flights. Having legs protruding past the head-on profile of the vehicle would significantly increase drag, and we want to avoid that if possible. Having legs that extend out dynamically as the vehicle is readying for landing increases both weight and complexity. But, since tipping over on landing is obviously an issue, I suspect the final design will have to compromise on some of those issues.


Back to top
Profile WWW
Space Station Commander
Space Station Commander
User avatar
Joined: Wed Aug 18, 2004 8:47 am
Posts: 521
Location: Science Park, Cambridge, UK
Post    Posted on: Tue Oct 25, 2005 7:58 am
Thanks for the reply Matthew. Seems to me that the prototypes needs wide legs that can be permanent fixings, and as you get faster and higher some sort of retractable mechanism needs to be developed. I would have thought a spring activated self locking system would be more than adequate - just need a solenoid per leg to set the whole process off. Just off the top of my head, long legs pivot mounted half way up, a rod to push the leg out on another vertical rod with a spring. When spring is released, pivoted rod moved down vertical rod, as it moves down it also pushes out the main leg.

Hmm. That looks OK in my head but is very difficult to describe!!

I can teach my grandmother to sucks eggs too if necessary!!!!!

James


Back to top
Profile
Moon Mission Member
Moon Mission Member
avatar
Joined: Tue Feb 10, 2004 2:56 am
Posts: 1104
Location: Georgia Tech, Atlanta, GA
Post    Posted on: Wed Oct 26, 2005 4:34 pm
Remember: having fixed gear that's slightly unstable is always better than having retractable gear that won't extend. Especially when you don't have a nice fuselage to land on, like you do with a plane.

_________________
American Institute of Aeronautics and Astronautics
Daniel Guggenheim School of Aerospace Engineering

In Memoriam...
Apollo I - Soyuz I - Soyuz XI - STS-51L - STS-107


Back to top
Profile
Spaceflight Trainee
Spaceflight Trainee
User avatar
Joined: Tue Jul 06, 2004 9:37 pm
Posts: 44
Post    Posted on: Thu Oct 27, 2005 4:09 am
How about one that is still landable when not deployed, but extends when in a manner that gives a stable surface even if it fails in mid deployment.

For example rotating L brackets. The lower sections of the L would remain up against the current landing gear then rotate outward.


Back to top
Profile
Space Station Commander
Space Station Commander
avatar
Joined: Thu Oct 27, 2005 7:44 am
Posts: 707
Location: Haarlem, The Netherlands
Post    Posted on: Thu Oct 27, 2005 8:37 am
*delurk*

I know John is trying to remove all "things with springs" from the vehicle, but how about legs that splay out upon landing due to the weight of the vehicle, and are pulled into a vertical, low-drag configuration by springs when the vehicle is in the air?

Something like this:
Code:
    |     |
    |     |
    |\   /|
    | ^^^ |
  _/       \_

Flight position


    |     |
    |     |
   / \   / \
\_/   ^^^   \_/

Ground position
It seems this board breaks ASCII art. Oh well, a working image is a copy-paste away :).

The legs are attached to the fuselage by a hinge on the side (possibly sunk into the side of the vehicle to minimise drag) and are pulled towards the fuselage by springs. When the vehicle is in the air, the springs keep the legs in a vertical position.

At the bottom of each leg there is a small curl, possibly with a fin on top for extra streamlining (I didn't draw the fin). When the vehicle touches down, the legs slide outward on this curl along the (concrete, I presume) landing pad until they hit their maximum angle. This motion is dampened by the springs (and possibly bending of the legs, depending on material and strength of the legs), so you get some shock absorbtion on landing as well. For low-speed hovering or boosted-hop demonstrations in fields, the curls could be a bit wider to avoid sinking into the mud. A bigfoot configuration if you will :).

The strength of the springs should be large enough to keep the legs aligned with the fuselage on descent, against the aerodynamic pressure, but small enough to ensure that the nearly empty vehicle will easily push them apart upon landing. As noted in the most recent update, springs aren't that accurate when it comes to the force needed to open or close them, so this will probably only work correctly if there is a lot of room between these two limits. As an alternative, the legs could be designed to splay during the descent phase, using very weak springs. That would cause extra drag below the CG however, and may cause the vehicle to flip over and descend upside-down. This is actually a part of the bigger problem of descent stability.

There seem to be several failure modes. A leg may get stuck, which would land the craft at an angle, but does not have to tip it over on landing if the leg length, position and maximum angle is well-chosen. The spring may fail, which lessens the dampening on landing, but if the stop still works the landing would still be successful, just a little bumpy. In the ascent phase, aerodynamic pressures will probably (IANAAE) keep the leg flush with the fuselage in case of spring failure, although it's not impossible that turbulent airflow will cause problems.The biggest potential problem is probably when a loose leg flares out during the descent and causes asymmetric drag. Depending on how (un)stable the vehicle is during descent, this may cause loss of attitude control.

On the other hand, there are no actuators that can break, no sensors to fail, and it does reduce the risk of tipping over. A useful compromise perhaps?


Back to top
Profile
Space Station Commander
Space Station Commander
User avatar
Joined: Wed Aug 18, 2004 8:47 am
Posts: 521
Location: Science Park, Cambridge, UK
Post    Posted on: Thu Oct 27, 2005 8:40 am
spacecowboy wrote:
Remember: having fixed gear that's slightly unstable is always better than having retractable gear that won't extend. Especially when you don't have a nice fuselage to land on, like you do with a plane.


So have extendable landing gear that will extend. The sort of idea I tried to put across above needs one activation solenoid to relase a strong spring. Not much to go wrong there. Or put another way, if you cannot get one solenoid to work everytime, then you shouldn't be making rockets!! There are a few mechanical details to get right, but they are not much different to standard locking undercarriage on planes (and they have a very low failure rate)

The Apollo lander (and I believe all non-air bag landers) has splayed legs for a reason.....

James


Back to top
Profile
Space Station Commander
Space Station Commander
avatar
Joined: Thu Oct 27, 2005 7:44 am
Posts: 707
Location: Haarlem, The Netherlands
Post    Posted on: Thu Oct 27, 2005 11:30 am
What if the solenoid fired accidentally during flight, say, due to a power surge, wiring problem, or a glitch in the flight computer? Of course, you can say that these things shouldn't happen in the first place, but if I were to fly into space, I'd rather do it in a rocket that can still land safely with a couple of broken or malfunctioning parts. Also, remember the regulatory problems Armadillo had with putting a parachute on their vehicles? That was also just a solenoid, but the regulators still weren't happy with the prospect of having it fire accidentally at high altitude, and the vehicle drifting away on the winds into inhabited areas.

I think your idea is the most logical active system, but I think a passive system would be inherently safer.

I thought of another solution. Attach the legs to the fuselage using a hinge that can slide up and down an inch or two. Make it so that the legs can not splay outwards when the hinge is in the lowest position. Use a spring to keep the hinge in the lowest position as long as the vehicle is in the air. Then when it lands, the weight of the vehicle will be transferred to the legs, pushing the hinge upwards against the spring, thus releasing the legs and allowing them to splay. Before liftoff, either the vehicle would have to be supported some other way than on its legs, or the legs would have to be kept vertical by supports in or on the launch pad. Possible, but not a very nice solution.

The main problem however is that this system is non-linear, because a very small change in force on the legs greatly affects their behaviour. A landing vehicle that hits the ground with one leg first will put some of its weight on it.The hinge goes up, the leg splays outward, and suddenly doesn't support the vehicle anymore. That would require a pretty smart landing routine in the flight control software to compensate for that. My previous solution doesn't have this as much: there is only a small nonlinearity when the legs hit their maximum outward angle stop, but by that time they're all on the ground anyway.


Back to top
Profile
Space Walker
Space Walker
avatar
Joined: Wed Jul 30, 2003 8:23 am
Posts: 195
Location: Lincoln, England
Post    Posted on: Thu Oct 27, 2005 1:32 pm
Could the struts and braces of the landing gear be made into flight surfaces, and/or shaped for speed?

_________________
Sean Girling

Snowmen fall from Heaven unassembled.


Back to top
Profile
Space Station Commander
Space Station Commander
User avatar
Joined: Wed Aug 18, 2004 8:47 am
Posts: 521
Location: Science Park, Cambridge, UK
Post    Posted on: Thu Oct 27, 2005 3:10 pm
Lourens wrote:
What if the solenoid fired accidentally during flight, say, due to a power surge, wiring problem, or a glitch in the flight computer?.


But glitches could happen in any of the systems, if they could happen in that one. I'd prefer a landing leg failure to a 'failed to shut off the rocket engine when the capsule turned upside down' failure.

What I'm saying is that there are already control systems for the engines in place that cannot be allowed to fail - the same care should be taken over landing legs or any mechanical system on the craft.

James


Back to top
Profile
Space Station Commander
Space Station Commander
avatar
Joined: Thu Oct 27, 2005 7:44 am
Posts: 707
Location: Haarlem, The Netherlands
Post    Posted on: Thu Oct 27, 2005 5:48 pm
So that would mean a second solenoid for reliability, with a separate control line to the flight computers...

Maybe I'm being way too careful here? But why use a complex solution when a simple one will work?

Incidentally, I've looked up a few things on dynamic pressure during reentry. For the shuttle, which is aerobreaking from orbit rather than just coming down from 100 km, it doesn't exceed 3 * 10^4 Pa. That's 3 N/cm^2, meaning that if a leg has a footprint (bottom view) of say 30 cm^2, the aerodynamic force on the legs will be equivalent to a weight of about 10 kg (22 lbs). Of course, for a suborbital vehicle it will be (much) lower, so let's take this as an upper limit. I've seen a vehicle weight of 2400 lbs (about 1000 kg) being thrown about for the X-Prize vehicle Armadillo were going to build, which means each leg would carry about 250 kg of the weight of the vehicle upon landing. That's a ratio of at least 25:1 for a non-locked system to work with. I'd say that that would be doable.


Back to top
Profile
Space Station Member
Space Station Member
avatar
Joined: Tue Dec 07, 2004 6:50 am
Posts: 265
Location: UK
Post    Posted on: Thu Oct 27, 2005 7:08 pm
Dynamic pressure for any kind of reentry is a function of ballistic coefficient and L/D. Given a vehicle of similar ballistic coefficient a nearly vertical reentry involves much greater pressure so even with much lower energy for a 100km flight the dynamic pressure (but not heating) might easily be as great.


Back to top
Profile
Moon Mission Member
Moon Mission Member
avatar
Joined: Tue Feb 10, 2004 2:56 am
Posts: 1104
Location: Georgia Tech, Atlanta, GA
Post    Posted on: Thu Oct 27, 2005 8:57 pm
Y'know, it's amazing how much discussion can come out of one simple observation.... Great job, folks. I'm impressed.

I do kinda like a cross between Sean's solution and Lourens' second solution (the sliding legs), though. See where I'm driving at?

_________________
American Institute of Aeronautics and Astronautics
Daniel Guggenheim School of Aerospace Engineering

In Memoriam...
Apollo I - Soyuz I - Soyuz XI - STS-51L - STS-107


Back to top
Profile
Moon Mission Member
Moon Mission Member
User avatar
Joined: Tue Oct 05, 2004 5:38 pm
Posts: 1361
Location: Austin, Texas
Post    Posted on: Fri Oct 28, 2005 12:13 am
Keep it simple, at least at first.

The simplest fix is just don't land in the mud. Isn't that what airports and spaceports are for, so you don't have to land in the mud?

Or have two sets of legs. Bolt on the wide legs for hover tests and boosted hops where drag isn't really an issue. For high flights bolt on the narrow legs and be careful to land on the pavement.


Back to top
Profile WWW
Space Station Commander
Space Station Commander
User avatar
Joined: Mon May 31, 2004 9:47 pm
Posts: 816
Location: Yerushalayim (Jerusalem) - capital of Israel!
Post Legs of landing   Posted on: Fri Oct 28, 2005 7:22 am
1. Make the legs attached to the top of the vehicle, for support
2. Have the legs swing into grooves in tho vehicle, for aerodynamics.
3. Make the legs servo- activated
4 Think self operating tripod. They make pretty reliable tripods. For that matter, Armadillo might be able to pay someone else to make the landing legs, or find them ready made.
5. Why not have both at the same time, if reliability is the worry?

_________________
“Once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return.”
-Anonymous


Back to top
Profile WWW
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 79 posts ] 
Go to page 1, 2, 3, 4, 5, 6  Next

Who is online 

Users browsing this forum: No registered users and 6 guests


cron
© 2014 The International Space Fellowship, developed by Gabitasoft Interactive. All Rights Reserved.  Privacy Policy | Terms of Use