Battle continues over LOHAN's mighty rod
Vulture 2 launch system a load of ballockets?
To recap, the current line of attack is to hang the aircraft under the fantastical flying truss on a titanium rod which passes through two Teflon inserts in the vehicle's fuselage. Here's a sketch we made earlier of the concept:
Note that we've already decided to reduce the size of the aluminium plate behind the aircraft to the absolute minimum, at our beloved readers' behest, since as shown it could cause serious problems with the rocket exhaust.
It's the launch rod, though, which has caused most brouhaha. The consensus is that we need to adopt a launch rail system, as Ron B explains:
For heavy rockets, those in the biz use a square Aluminium C rail. That rail could replace the apex of your beam. So, the flat side of the beam faces up, and the apex points down. Then a pair of Delrin rail buttons are screwed to the top of the plane, and they slot into the open end of the rail. The bottom end of the rail just needs a thru-bolt to stop the buttons from sliding all the way through. That also eliminates the need for a plate and bumper at the back of the plane.
However, the same commenter then notes:
Too much spin or side wind on the wings in the lower altitudes will batter the craft and try to rip the launch bearings off of their mounts.
Quite so. While the titanium rod set-up allows the Vulture 2 to "rock" slightly on the ascent phase of the mission, thereby avoiding the Teflon inserts icing to the rod, the same motion could indeed snap launch buttons clean off.
For that reason, we've already examined and rejected said buttons. Obviously, we need a system to prevent the Vulture 2 taking a serious battering on its way to altitude, and we reckon either a padded U-bracket system cradling the wing trailing edges; or the cushioned Teflon strips running along the underside of the longitudinal truss members, as seen above; or a combination of both, will provide a viable solution.
Let's assume that will work. Will we still have a problem with the Telfon inserts icing to the rod? We intend to have the internal diameter of the inserts slightly larger than the diameter of the rod, so the surfaces can't freeze together and jam.
LeeE says this could backfire:
The first reason, and one that applies more at lower altitudes, is that by leaving a slight gap between the insert and the rod you're leaving space for the ingress of wind-blown dust between the insert and the rod and once the dust is in there it won't get blown out again. "Wind-blown dust?" I hear you say. Yes, the stuff that water droplets, which we see as clouds, nucleate around, which brings us to the more serious problem with leaving a gap between the insert and the rod and which applies more at higher altitudes.
I assume that you're going to launch on a cloudless day, so you can at least watch LOHAN for a good part of its journey, and lack of clouds will imply low dust levels: all well and good. However, this doesn't necessarily mean that there won't be much moisture in the upper atmosphere; it just might mean that there's nothing for the moisture to nucleate around/condense upon. The trouble is that when LOHAN drifts up through this air then the water will be able to condense upon it and in this case, contrary to the first mental image that may be conjured up, a moist LOHAN is not going to be a good thing.
Sisk chips in with:
The icing and dust problems could both be solved by filling the hole with the lube mentioned in the article before sliding the shaft in. With the tube thus filled with both slippery lube and a firm rod there will be no way for dust to blow in.
Indeed. The lube in question is Molykote 33, which will also be available for greasing the Vulture 2 control servos, launch rod and anything else which needs to be more slippery than a lard-coated hog at -60°C.
That won't make any difference, according to Poor Coco, who weighs in with a further objection to the rod system:
Well, the real problem is that the plane is dynamically *unstable* until stabilization moments are created by fast airflow; prior to that point any disruption that causes a minor change in the direction of the nose will lead to continually increasing deviations leading to tumbling and a total waste of the engine impulse. So, yes, it will require a destabilizing force – but those forces are EVERYWHERE, from asymmetry between the line of thrust and the CG of the plane; from turbulence in the exhaust gases; from launch rod friction; the list goes on and on. Unless non-aerodynamic stabilizers are used – and a launch rod WILL NOT WORK I will repeat that a million times – there is no realistic chance of an interesting powered flight.
One Anonymous Coward believes the problem might be even more serious than a wild launch trajectory:
Take a look at the setup in profile – the image on the right side of the picture. If that system were stiff, when the rocket fires the plane could slide up right off the rod. But because it's hanging from a tether, it's not stiff. When the rocket fires, the force of the thrust will create a torque that makes the entire structure want to rotate about its center of gravity, which will probably end up someplace inside the truss.
The truss is stiff, but that torque will cause a rotation about the one point whose rotation about the axis pointing straight out of the picture is not constrained... which is the where the three tether lines intersect. The entire structure (truss, rod, plane) will probably rotate clockwise... in which case the structure may start to rotate before the plane slides off the rod, resulting in the plane coming off at a significantly different angle than intended... or not coming off at all and just spinning around.
OK, the only way to settle this is to build the truss, or another model truss, suspend it above the ground and see what happens when we fire a rocket motor strapped underneath.
While we're at it, we can try out this suggestion from Ron B:
Hang a drogue (sea anchor style or just a paper streamer) off of the bottom end of the beam so that it always points into the wind, and then you shouldn't have trouble with the plane twisting and binding in the channel. Any pilot knows you take off into the wind.
Fair enough, although we don't expect much turbulence at the intended launch altitude. Have a look at conditions just prior to the Paper Aircraft Released Into Space (PARIS) Vulture 1 drop at 89,591ft:
Finally, we come to the matter of LOHAN's mighty thruster. Poor Coco opines:
As far as the boosters go, I seriously doubt a boost-glider engine will suffice. They are designed to supply low thrust and long burn durations, which is the opposite of LOHAN’s initial requirements because she MUST GET MOVING FAST, at least off-the-line, or she will never get momentum at all because she’ll be tumbling; in that case we should skip the rockets entirely and we have a glorified PARIS. Therefore short-duration boosters with beefy gyros which are dropped off on burnout seem like an ideal design.
I'm very much against the idea about strapping more boosters to either LOHAN or the launch rig (Or complicating the launch rod any further than a simple rod and runner). Every bit of complexity you add means another possible point of failure. And every possible point of failure means a bigger chance of something going wrong. Better adhere to KISS. Possibly get a bigger rocket with a special design combustion chamber (yes a solid fuel rocket has one too) to provide high initial thrust and then drop to sustained boost. Adding drop away boosters just gives Murphy's Law too much chance to come into effect.
It does. You'll have to take our word for it that the Vulture 2 will pack enough poke to get off the launch rod at a fair rate of knots. Furthermore, the elite Southampton University postgrad team, who are as we speak honing the spaceplane design, are fully aware of just how control surfaces will work at at speed and altitude, the change of aircraft centre of gravity as the rocket motor burns, and a plethora of other operational factors which need to be addressed.
As part of the design process, they do take on board your comments and suggestions, so keep 'em coming. One thing, though: be nice to each other. El Reg is not YouTube. ®
Further LOHAN resources:
- New to LOHAN? Try this mission summary for enlightenment.
- You can find full LOHAN coverage right here.
- Join the expert LOHAN debate down at Reg forums.
- All the LOHAN and Paper Aircraft Released Into Space (PARIS) vids live on YouTube.
- For our SPB photo archive, proceed directly to Flickr.
- We sometimes indulge in light consensual tweeting, as you can see here.