Talk-Polywell.org

Things always seem to come back to that "pick two of three" dilemma, don't they?

Buck Rodgers, whee are you?

Or rather, 4 legs that are permanently deployed would solve reliability issues. The fins on the old SF rockets, and the V2 are the solution. Of course this adds a new aerodynamic component to landing, especially if the rocket is flying backwards, like Falcon 9, etc. Leading with the fins is possibly very challenging from a guidance and stability perspective. Flying forward would solve this (like a dart). The top of rocket guide vanes may not be needed- the control is in the tail fins (perhaps some weight saving). As landing was approached, the rocket would need to do a pitch up maneuver and the lateral G loading and aerodynamic forces on a long rocket may be considerable....

A reusable rocket (stage) that looks like the old Science Fiction rockets is appealing.

Dan Tibbets

D Tibbets wrote:Buck Rodgers, whee are you?

Or rather, 4 legs that are permanently deployed would solve reliability issues. The fins on the old SF rockets, and the V2 are the solution. Of course this adds a new aerodynamic component to landing, especially if the rocket is flying backwards, like Falcon 9, etc. Leading with the fins is possibly very challenging from a guidance and stability perspective. Flying forward would solve this (like a dart). The top of rocket guide vanes may not be needed- the control is in the tail fins (perhaps some weight saving). As landing was approached, the rocket would need to do a pitch up maneuver and the lateral G loading and aerodynamic forces on a long rocket may be considerable....

A reusable rocket (stage) that looks like the old Science Fiction rockets is appealing.

Dan Tibbets

Fin-legs that are permanently deployed would add a lot of drag during launch. This is why even the much smaller grid fins are tugged closely to the fuselage during launch (with even a small aerodynamic shoe in front of them).

Another issue with flying with the top of the stage leading, is that the heaviest part of the stage is the engines, and stability issues mean that having the heavy part leading and the fins at the other end just works more easily, like an arrow. In addition, doing it with the engines leading means that the fuel will stay down at the intakes where it is needed for the reentry and landing burns.

I think SpaceX probably got it right....

The Politically Correct solution:

Alright, I'll concede that coming down nose first is not a very good option.

Tail fins may not add that much significant drag as the rocket is out of most of the atmosphere before it is going too fast. It didn't bother V2 rockets, and almost all military antiaircraft missiles have large tail fins and they go over Mach 3 at relatively low altitude. Also, the Falcon 9 already has tail fins- granted they are just small protuberances (but fat) sticking out from the rocket cylinder before deployment. Most antiaircraft missiles also have forward fins near the nose, and a few have probes extending beyond the nose of the missile, yet they are controllable.

The question , I think, is if the assumed weight savings, and reliability issues can be utilized vs the question of aerodynamic stability. The assent drag issues may be no more or even less than the current folded legs on the Falcon 9. The top guidance grid fins are probably needed (or small vernier rockets), but perhaps some weight saving might be possible here as well.

Note that the tail fins do not have to be massive structures like in the Science Fiction rockets, just large enough solid or grid legs to support the landing. The Blue Origins rocket may approach a workable configuration. With the lump on the top end, even if the landing legs were permanently deployed, the aerodynamic drag may still be greatest near the top end of the rocket and stability issues minimized during desent. For that matter, the Falcon 9 with the 18 foot wide payload fairing has a large forward structure that during assent, still allows for control/ stability of the rocket. The Saturn 5 first stage also had tail fins, admittedly relatively small, that still allowed for good payload functionality.

Dan Tibbets

D Tibbets wrote:Alright, I'll concede that coming down nose first is not a very good option.

Tail fins may not add that much significant drag as the rocket is out of most of the atmosphere before it is going too fast. It didn't bother V2 rockets, and almost all military antiaircraft missiles have large tail fins and they go over Mach 3 at relatively low altitude.

Dan Tibbets

Don't underestimate how much fuel drag costs.
the V2 was not going into orbit, neither does the Blue Origin New Shepard. But when you go to orbit, every little bit of drag matters. The first stage is actually spending quite a bit of time in the atmosphere and especially in the beginning, drag matters. As I said, that is why the grid fins have aerodynamic shoes (and the tips of the legs too), even though they are completely folded in. The F9 legs have so much drag, that they could serve as a means to slow down the rocket for landing. That was at least at some point the plan, that might have since then been abandoned (not enough control over how they extend is the presumed reason for that). It is presumed that Blue Origin will choose a different leg design for their future orbital vehicle.
About coming nose down first: It depends on the launch vehicle. the DC-X was going to come down nose first. They decided to do that because they wanted the extra cross range that (combined with the shape of it) gave them. A big part of the DC-X program was to demonstrate that this could be done with a VTOL rocket.

I'm just trying to stimulate open thinking. There are of course multiple plus and minus considerations. Things like launch tower clearances on takoff, atmospheric drag, reentry heating, etc. etc.

Having said this, I am wondering about the weight and drag of permanently deployed landing gear/ fins compared to the weight and drag of the current folded Falcon 9 landing gear. The fins and engine shrouds of the Saturn 5 certainly added drag, yet they were apparently the best solution for that rocket. The Solid boosters on Delta 4 certainly adds drag, but yet again they were the best approach for that rocket concept. The drag of the huge external fuel tank, solid boosters and wings of the Shuttle certainly had a lot of drag, yet they were chosen , perhaps unwisely in retrospect.

I am envisioning four solid fins, perhaps 6-12 inches wide and extending out to ~ the same distance as Falcon 9 deployed landing gear. There would be a pod on the end to house the shock absorbing landing pads may be nessisary. Aerodynamic control surfaces may or may not be an option on the fins. Canards on aircraft are examples of flight control being managed at least in part by forward placed flight control surfaces (rearward placed controls during assent).

Modern flight control software can apparently handle the task as evidenced by the F22 or Su35 maintaining control while falling backwards during post stall maneuvering.

Dan Tibbets

How long does the Falcon 9 first stage burn past Max Q? Not very long, I'm guessing, if it is like most launch platforms. Drag follows dynamic pressure (Q), so drag continues to increase through most of the first stage burn. This really does emphasize that added drag is not negligible. I would not be surprised to find that the added fuel required during launch would eat up the surplus needed for the landing.

When you get down to it, a stiffer set of springs on the latches probably only add ounces, and no additional drag.

The launch profile for F9 flight 20 (the one that landed on land) is here. It looks like the first stage initial burn was for 140 seconds, with stage sep at T+2:24 (4 sec after MECO). Max Q happened at about T+60 sec, with Mach 1 ten to fifteen seconds later.

Without proper data it is hard to express a design. Point to ponder include that the ship (craft?) sits on the tips of the landing gear. This would greatly increase the weight penalty in all designs.
I think that the reason that current design is used has the following advantages.
The lowest drag available on ascent with the highest on decent .(Anyone know when the gear actually deploy?)
Although it is not the lowest weight for the gear to have it is the lowest weight with the highest CG when all four legs deploy correctly.
A rough guess would put the CG at 20 degrees before toppling,
Absorbing the landing-impact energy and limiting loads induced into the first stage structure , remember this is a suicide landing the rocket is stopping the downward descent just in time for touchdown because there is no hovering because oddly one motor has too much thrust.(something they may want to look deeper into on the next versions)
There are designs that have some advantages but lacking in other considerations Fins for example would probably reduce the weight of the landing gear, but as the rocket motors are active correcting for guidance fins for stability would be redundant so why take the drag penalty? The main goal is weight to orbit while first stage recover is secondary.
It seems to me that they are on the right track for an all around rocket, robbing Peter to pay Paul to get the best overall system.

Gear deploys right before touchdown. It is in the final decell. They may be using the inertia delta to help ensure deployment.

ladajo wrote:Gear deploys right before touchdown. It is in the final decell. They may be using the inertia delta to help ensure deployment.

They were at some point planning to use the legs to help with deceleration. I don't know what changed but they are not doing it now. People assume that they could not master the deployment to be precise and controlled enough to control attitude at high speeds.

Slightly off topic but it is about building space ships

http://blog.nasm.si.edu/conservation/us ... on-begins/

Skipjack wrote:They were at some point planning to use the legs to help with deceleration. I don't know what changed but they are not doing it now. People assume that they could not master the deployment to be precise and controlled enough to control attitude at high speeds.

With legs closed you generally get a nice laminar flow of air around rocket that will not effect the descend route, opposed to the turbulence that would be generated between the legs and the rocket in case of early opening.
The contribution to the deceleration from the drag of the open legs is really not worth the hassle of the increased control complexity, especially in case of strong side wind.