Finless rocket

[darkhelmet]

This could theoretically work for launch lugs in LPR too but would be more difficult on small diameter rockets and the mechanism would add weight. I'm not sure it would be worth it, then again a 30% reduction in drag could easily compensate for some additional weight.

"Pop lugs" were pretty common in NAR competition. Similar mechanisms were also used in some sounding rockets. The idea is to have a band of springy material that want to stay flat & straight (plastic on small rockets, spring steel on larger ones). Put two pieces of launch lug (or rail guide) on one end with a gap in the middle. Put one piece on the other end that lines up with the gap. Size the length of the strap such that the lugs fit together nicely when the band is wrapped tight. The rod through all 3 lug pieces will keep it from popping off until the rocket leaves the rods (or rail). The "pop lug" assembly will fly off, away from the pad (somewhere!).

 

[Shade]

Is there a way to launch a finless rocket? I am not talking about launching a finless rocket the normal way you launch a finned rocket because it simply wouldn't work.

I guess my question is, are there any innovations and ideas out there about how to do this?

Some of the ideas I have thought about probobly won't work...

- spin launch a rocket out of a tube using air pressure and then fire the motor almost imediately after the pressure tank is fired. The gyration will help create stability.
- somehow mount the motor towards the front of the rocket

Basically I want to know if there is a way to have ZERO fin drag and achieve much better performance than a rocket with fins.

Chi-Town are you going to build a finless one?

 

[Chicagonative17]

If I come up with a design that is probable. I was thinking about doing an R and D project on it for NARAM. But I need some pretty good ideas before I even start puting money into it.

I would LOVE to see an efficient finless rocket, one that is powered by only 1 motor and one that doesn't take a lot of prep time. If this is possible, rocketry would be revolutionized.

 

[adrian]

I would LOVE to see an efficient finless rocket, one that is powered by only 1 motor and one that doesn't take a lot of prep time. If this is possible, rocketry would be revolutionized.

A stick isn't exactly a fin, which means a firework rocket - or a scale model of the Congreve artillery rocket - could fit your specification. But the stick on the original Congreve was cumbersome and took a lot of prep time, so the Congreve was eventually replaced by the spin-stabilised Hale rocket. After that, it was the addition of fins which revolutionised rocketry.

 

[bobkrech]

It is difficult to get a small finless rocket to spin fast enough to be stabilized without fins.

Several folks have gimbeled the motor and used a servo system to vector the thrust to maintain vertical flight. That how the pros do it.

If you search TRF you can find the references.

Bob

 

[denverdoc]

I think we're both correct, but are looking at different constraints on the problem.

Once I get caught up on a few things, I'll see if I can locate the old papers that discuss various drag components.

Here's something to ponder.... if a lug (or button) contributes to drag and it is an asymmetrical feature (one side of the rocket), then it provides a torque that pitches the rocket. There will be an angle of attack where the corrective force of the fins/body balances the side torque of the lug. Even a small angle of attack adds significant drag. Depending on the masses and moments, there may be a dynamic condition where the rocket oscillates and rolls because of the asymmetrical disturbance. This further adds to the drag that was computed under static assumptions.

I understand what you are suggesting: that the rocket will fly somewhat sideways at equilibrium, and may pitch about seeking that equilibrium. In both cases additional drag is created. I would argue the following in response:
a) That the torque created by say a 2 inch launch lug stationed near the center of gravity will be relatively small. This applies to any force normal to the axial line, the torque for parallel axial forces is of course fixed at the body tube radius. Rail buttons that straddle the Cg would create offsetting torques. This is just good practice in any event to minimize the forces on the guide during contact with the rail/rod.

b) That at small angles of attack, the increase in drag forces is relatively small from 0 incidence.

c) That the relative magnitudes of the corrective force generated by a launch lug versus that of the lift created by the rocket flying at a small angle of attack can be appreciated from the following plot.

This is a rocksim plot of a stock PML Cirrus Dart flown on an H180. The Cd and the Normal force coefficient, Cna, are both dimensionless numbers that need to be multiplied by the term (1/2 pho*v^2*Aref) to calculate force. They are acting on the same longitudinal moment of inertia.

Even if we take the generous assumption that a lug would generate an additional 0.3 or so of Cd, you can see that this is a small number indeed compared to the minimum Cna.

From this, and the fact that its acting with only a body radius long lever arm, I don't believe the aoa at equilibrium (assuming zero roll rate) would amount to much. Moreover the increase in drag would be negligible as well.

Of course, all of this may be wrong--if so, I'm certain to know about it short order.

In the FWIW category, the lugged version ( 3 inches 1/4 dia) lost 150' of 5200'. This I know is likely an underestimate from the R&D reports you mentioned. The fins by the way came in at a total of 25% of the total Cd.