Rocketry Design Team - Rail Exit Velocity Help

[VirginiaTechRocketry]

Due to quite a few mess ups, my rocketry design team is struggling to meet rail exit velocity requirements for a competition. What are some ideas you guys have to help improve rail exit velocity? We are currently adding fillets to the fins to help reduce drag.

 

[Maxwelljets]

Fillets won't help you with that problem. Additionally, fillets are almost always a structural item. Any aerodynamic benefits they have are secondary to their main purpose of making sure your fins stay on the vehicle in flight.

Rail speeds are slow enough that drag will not affect them at all. The difference in drag force at such low speeds is essentially negligible compared to the gravitational force on the rocket.

There are 3 ways you can increase rail exit speed:
1. Decrease rocket mass, thus increasing acceleration and rail exit velocity.
2. Change to a higher thrust motor, thus increasing acceleration and rail exit velocity.
3. Increase the length of the rail, thereby increasing the distance over which you can accelerate.

That's it. You can choose between accelerating faster and increasing acceleration distance. There are no other ways.

 

[anbhtblr]

How about some info on the rocket, the rail, the attachment of the rocket to the rail...

 

[VirginiaTechRocketry]

Fillets won't help you with that problem. Additionally, fillets are almost always a structural item. Any aerodynamic benefits they have are secondary to their main purpose of making sure your fins stay on the vehicle in flight.

Rail speeds are slow enough that drag will not affect them at all. The difference in drag force at such low speeds is essentially negligible compared to the gravitational force on the rocket.

There are 3 ways you can increase rail exit speed:
1. Decrease rocket mass, thus increasing acceleration and rail exit velocity.
2. Change to a higher thrust motor, thus increasing acceleration and rail exit velocity.
3. Increase the length of the rail, thereby increasing the distance over which you can accelerate.

That's it. You can choose between accelerating faster and increasing acceleration distance. There are no other ways.

I should've clarified more. Do to competition rules, we have a set motor (we can change if absolutely necessary, but lose points) and we are currently using the longest rail available. Our options are to find easy ways to reduce drag or mass (this team is made up entirely of people who have never launched a rocket before now, even if it seems absurdly simple it'll probably help us)

 

[VirginiaTechRocketry]

How about some info on the rocket, the rail, the attachment of the rocket to the rail...

We are using a 1515 rail, 144 in long. The competition involves deploying a drone, which is where a lot of problems come in. We are also using a L585 motor which gets us a rail exit of 53.1 ft/s (actual launch data) with a competition minimum allowed of 52 ft/s. However, we expect the rocket weight to increase as the drone is built, lowering the rail exit velocity. The entire team is new to rocketry and was wondering if there were any simple ways to increase rail exit velocity (obviously reducing mass is a major goal). Our current recovery plan is to tangle every component together (including the drone inside of a blue tube) under a single drogue and a single main. We believe this may be a source of unnecessary mass, but are unsure how to fix it to decrease mass (parts can come down separately without being connected)

 

[Maxwelljets]

I should've clarified more. Do to competition rules, we have a set motor (we can change if absolutely necessary, but lose points) and we are currently using the longest rail available. Our options are to find easy ways to reduce drag or mass (this team is made up entirely of people who have never launched a rocket before now, even if it seems absurdly simple it'll probably help us)

Right. My point is that for the purposes of rail exit speed, you're not going fast enough for drag to matter at all.

Unfortunately, without knowing far more details about you design (as in, you giving us a guided video tour of the rocket levels of detail), it's impossible to give specific examples of where your design is overbuilt. Things that may or may not be doable:
-switch to thinner/more packing efficient parachutes and cut off any excess tube length
-replace metal components with composite, where applicable
-properly size (or eliminate) your quick links. 90% of rockets I've seen (or even built myself) use quick links that are way overkill. Consider switching to thing knots at the attachment points, just make certain that whoever ties the knots really knows what they're doing.

These solutions are really last-ditch, but there aren't really any other things I can think of that can be done after the rocket is built (which I assume yours is). Again, many more details will help here.

 

[J. Gallagher]

This sounds like USLI.

We were in a similar boat recently, and dropped three pounds from the rocket by swapping to a lighter nose cone from PML. But yes, more details about the rocket would help give relevant advice as to where you could be losing weight -- as others have said, drag is irrelevant to rail exit velocity since it occurs over such a short distance. The increased weight from fillets will hurt much more than the improved aerodynamics.

Also, remember that you don't actually have the full 12' rail to build up launch speed. It's measured to the point where the first button exits the rail.

 

[John Taylor]

Other than everything already mentioned, insure the rail is absolutely clean and dry lubed. Make sure your guides are perfectly straight and smooth.

 

[richP]

You can definitely save some serious weight by switching your quicklinks with knots.
Nylon to kevlar of a smaller diameter.
Steel to aluminum all-thread.
Lose the paint, or very sparingly at most.
Lightweight chute (pricey, but can save a good amount of weight).
Smallest LiPos that'll work with your gear.

Fillets will actually decrease your rail velocity.

 

[jmmome]

Mass is your enemy. All the recommendations as listed before are great. Weigh some of the individual components or component groups (like QuickLinks) that are possibilities to change to lighter material or a smaller size. Roll the dice on downsizing items like QuickLinks.

Not sure that eyebolts were mentioned, but try to downsize them to save weight.

I think you mentioned Blue Tube, which by nature is a heavier material. Can any of the other rocket body tube parts be switched to a cardboard tube & still hold up to your stresses?

Agree with securing the lightest chutes that your group can afford, as well as the smallest diameters that will give you a landing that meets your success criteria. Feel free to PM me if you want a couple of parachute vendor recommendations.