Friday, October 28, 2011

Are two N motors enough for space?

It is always shocking to see how much altitude the pros (NASA, NACA, others) have gotten with motors in the M to R range. Yet within our hobby we routinely see R motors fail to break 100,000 feet, and even performance O motors struggle to pass 50,000 feet. The Super Loki Dart, something like a fast O motor, can scrape the bottom of space! There are many factors here including maximized mass fractions, thin steel cases, and plenty of aerodynamic testing beforehand in the pro rockets. Today, the technology is on hand to allow even hobby rockets to achieve similar performance. We have the computer technology to design airframes and simulate complex flights, the composite technology to build extremely light airframes, and the onboard electronics to allow for complex flight profiles. Plus, we can now stick tiny HD cameras onboard and use GPS to recover after the flight.

There are many out there in the hobby who do incredible work with composites and high-altitude rockets. Adrian Adamson not only sells great altimeters, but also builds really nice high altitude rockets. Particularly some 38mm airframes that reach 17,000 feet on an I motor or 22,000 feet on a J motor. (And 13,000 feet on a J570 for my Cirrus Dart seemed like a lot!) Adrian has a great forum discussion going on about how 2 or 3 N motors could possibly, just maybe achieve space.

"Is it possible to get to space (100 km, 328,000 feet) using just commercial motors?

The CTI N5800 is a good motor choice for this because of its high Isp and high mass fraction. I used Rocksim to whip up a quick rocket model that I put into RASAero. With radios and recovery all inside a 6:1 Von Karman nosecone, and about 2 calibers stability, you get a 4" rocket that is 77" long, with 53" of body tube. I'm guessing about 10 lbs empty weight.

In RASAero, it sims to about 87,000 feet when launched from the floor of the Black Rock desert. The trouble is that its peaks at Mach 4, and it would take either extraordinary composite technology or an aluminum, heavier airframe to survive that when launching from the ground at Black Rock. But what if it's igniting from much higher? What ignition altitude would allow a 100 km flight?

RASAero says it would need to be launched from in the neighborhood of 50,000-60,000 feet to achieve the space threshold, if it were launched from there from a standstill. This exercise also shows that empty mass is critical for the final altitude. Going from 10 lbs empty weight to 8 lbs empty weight increases the sustainer delta altitude from 266,000 to 303,000 feet. That's on top of the 60,000 foot launch altitude.

So what would it take to get this 42 lb liftoff weight rocket to 50kft or 60 kft? For a quick and dirty check, I doubled the mass to 84 lbs and used another N5800 motor, launched from the ground. Sadly, the rocket only gets to 36,000 feet, so it would be short of the space threshold goal. So we would need to add a booster stage to augment the first N5800. If I use an N10,000, and use the 2 upper stage liftoff weight + 6 lbs as the empty weight for the 3-stage rocket, I get a total liftofff weight of 111 lbs, and the N10,000 kicks it off the pad with 23 Gs and a top speed of 670 feet/second, or Mach 0.6. This is actually a good ignition velocity for both stages, so now I'll do a RASAero multi-stage trick: Add an instant impulse to the front of the motor file so that when the motor lights, it's already going around 600 feet/second. Let's see if that's enough to get it over the hump. I'll give that a try and come back for the next post."

Read more at the Rocketry Planet.

No comments: