Let's talk about boosted darts!

[lr64]

I seem to remember reading about some propellant using bismuth that didn't have the highest Isp, but provided a lot of oomph relative to its volume. Perhaps that would be good for a powered upper stage.

Seems to me that, however achieved, one could get the highest altitude for the smallest amount of propellant with just high enough thrust to jump off the pad and get a subsonic cruise, followed by modest thrust to maintain it, followed by high thrust once the air was thin. I'm imagining a booster with a single grain shaped like the one in Estes motors, i.e. a core that extends only partly into the grain, so it starts more normally and then becomes an end-burner. There could be some high powered propellant on top of that, or a second stage. Alternatively, a small, nozzleless booster, then an end burning second stage, then a high powered sustainer.

 

[pugachu]

At LDRS the record committee gave the BoD a proposal for a boosted dart altitude class.
The FAA has a density limit for HPR and even the "real" Loki dart isn’t even close.
A boosted dart rocket must follow the same safety code as all other rockets. Darts have not been banned, but they are pretty rare.

Mark

Is the FAA density limit based on mass/frontal area, or mass/volume? I can imagine really skinny rockets having a relatively high density, but any rocket with a recovery system should be a lot less dense than one designed to come down ballistic.

 

[Alan15578]

I seem to remember reading about some propellant using bismuth that didn't have the highest Isp, but provided a lot of oomph relative to its volume. Perhaps that would be good for a powered upper stage.

Seems to me that, however achieved, one could get the highest altitude for the smallest amount of propellant with just high enough thrust to jump off the pad and get a subsonic cruise, followed by modest thrust to maintain it, followed by high thrust once the air was thin. I'm imagining a booster with a single grain shaped like the one in Estes motors, i.e. a core that extends only partly into the grain, so it starts more normally and then becomes an end-burner. There could be some high powered propellant on top of that, or a second stage. Alternatively, a small, nozzleless booster, then an end burning second stage, then a high powered sustainer.

You are getting close to the optimal thrust. For MR it is approximately: max thrust until the optimal speed is reached followed by near constant thrust of about twice weight until burnout, followed by coast to apogee. For HPR when air density decreases significantly during the ascent, the "near constant thrust" increases significantly and can reach maximum trust. Of course in the real world you cannot get variable thrust through a fixed nozzle at near constant ISP. This brings you back to staging and clustering in an effort to achieve variable thrust and higher altitudes. It is also interesting to recall the old NAR Design Efficiency event, which was altitude/total impulse, which was effectively just 1/2A altitude.

 

[MClark]

Is the FAA density limit based on mass/frontal area, or mass/volume? I can imagine really skinny rockets having a relatively high density, but any rocket with a recovery system should be a lot less dense than one designed to come down ballistic.

Optimal weight for the dart is nowhere near the FAA limit. People talk of using super heavy metal for ballast but it is not required. The K motor dart I am working on is an aluminum cone and it was hollowed some to be optimized.
Even the Super Loki/Viper3A dart is a hollow steel nose. Steel is more for heat resistance than weight.

You are getting close to the optimal thrust. For MR it is approximately: max thrust until the optimal speed is reached followed by near constant thrust of about twice weight until burnout, followed by coast to apogee. For HPR when air density decreases significantly during the ascent, the "near constant thrust" increases significantly and can reach maximum trust. Of course in the real world you cannot get variable thrust through a fixed nozzle at near constant ISP. This brings you back to staging and clustering in an effort to achieve variable thrust and higher altitudes. It is also interesting to recall the old NAR Design Efficiency event, which was altitude/total impulse, which was effectively just 1/2A altitude.

Aerotech has motors like this.
One example.

https://www.thrustcurve.org/motors/AeroTech/K62N/