Rocksim question

[DynaSoar]

I assume Rocksim will adjust altitude computation based on launching from elevations other than sea level. That is, you can set the "0" altitude at some height above seal level and it will reduce the drag according to the reduced air pressure and give you the greater altitude acheived for a given rocket/motor.

But will it also calculated stability factors on this? I need to design a rocket for launching from high altitude, and I need to be able to tweak the design for launching as much for result.

Could someone please:
1: plug in a plain vanilla 3FNC
2: plug in a common motor
3: launch it from 0 elevation
4: record altitude, your selected speed to stability, and required launch rod length for that stability speed
5: reset altitude for 100,000 ft
6: launch and record the same things

If Rocksim won't do it, what will?

 

[teflonrocketry1]

See attached RockSim file. This all depends on the model RockSim uses for the earths atmosphere. I have attached a RockSim 5 file that can be opened using the Demo software. The differences in altitude attained from launching at 0 ft (sea level) and 100,000 ft is only 3 feet more!

Bruce S. Levison, NAR #69055

 

[DynaSoar]

Originally posted by teflonrocketry1
See attached RockSim file. This all depends on the model RockSim uses for the earths atmosphere. I have attached a RockSim 5 file that can be opened using the Demo software. The differences in altitude attained from launching at 0 ft (sea level) and 100,000 ft is only 3 feet more!

Bruce S. Levison, NAR #69055

Thanks, but no need to open it then. SpaceCAD adjusts altitude for starting altitude (my Vulcanite with Ellis H50 will go 3372' from sea level, but will go 9697' starting at 100,000'). However, in both cases is says the minimum rod length is 2.65' and that can't be right. Designing for stability in low pressure is one problem, designing the tower for it, a related but different problem.

 

[rocket72175]

Yes, RockSim accounts for base elevation.

The fact that the minimum rod length is the same at sea level and at 100000 ft is not surprising. The min rod length is determined when the rocket reaches some specified velocity (RockSim default is something like 15 fps). During lift off, the thrust and weight forces dominate over the air drag, so air density isn't a big factor.

However, a difference of only 3 ft of peak altitude does seem odd.

Ken

 

[teflonrocketry1]

When I reran the same simulation in RockSim 7 using an Estes Wizard on a C6-7 motor I get a maximum altitude of 1460 ft AGL from 0 ft and 9580 from 100000 ft. RockSim version 5 gives 1460 ft AGL and 9553 from 100000 ft. Must have been too late in the evening for me when I posted the last set of data! An updated RockSim version 5 file is attached.

The launch guide length for the higher altitude flight should be greater because the fins will have less restoring force in the thinner atmosphere and hence be less efficient. Your program probably chose the rod length based on a minimal speed and doesn't take into account the thinning atmosphere.

Bruce S. Levison, NAR #69055

 

[cls]

Teflon, the sim for the 100k' start shows

Altitude: 100000.000 Ft.
Relative humidity: 70.000 %
Temperature: 75.000 Deg. F
Pressure: 30.091 In. Hg.

not sure about RH up there but I am sure the pressure would be, oh maybe 1 millibar or so?

interestingly,1 millibar "only" gives 2900' altitude, while 100 millibar give 13,000' altitude. all sims eject long before apogee, need a C6-22 or C6-30 up there!

I think we've exceeded the limits of rocksim's atmospheric model

 

[DynaSoar]

Originally posted by teflonrocketry1
When I reran the same simulation in RockSim 7 using an Estes Wizard on a C6-7 motor I get a maximum altitude of 1460 ft AGL from 0 ft and 9580 from 100000 ft. RockSim version 5 gives 1460 ft AGL and 9553 from 100000 ft. Must have been too late in the evening for me when I posted the last set of data! An updated RockSim version 5 file is attached.

The launch guide length for the higher altitude flight should be greater because the fins will have less restoring force in the thinner atmosphere and hence be less efficient. Your program probably chose the rod length based on a minimal speed and doesn't take into account the thinning atmosphere.

Bruce S. Levison, NAR #69055

Zackly. It's not accounting for changes in cD and stability due to conditions (ie. air pressure).

AeroDRAG seems to be what I need. I don't care about complete CAD capability. I just want to model for drag and do the sims accurately.

 

[DynaSoar]

Originally posted by cls
Teflon, the sim for the 100k' start shows

Altitude: 100000.000 Ft.
Relative humidity: 70.000 %
Temperature: 75.000 Deg. F
Pressure: 30.091 In. Hg.

not sure about RH up there but I am sure the pressure would be, oh maybe 1 millibar or so?

interestingly,1 millibar "only" gives 2900' altitude, while 100 millibar give 13,000' altitude. all sims eject long before apogee, need a C6-22 or C6-30 up there!

I think we've exceeded the limits of rocksim's atmospheric model

Yup, there's something broke there. Temp would be -75 or something. Hey, it was never meant for this, and temp shouldn't matter to this sim.

I've been getting some decent high altitude results from SpaceCAD. I've managed to hit 1 million feet with three stages each with a Loki Research 50000 Ns P motor. But I *still* can't get the speed to adequate stability starting at 100K'. The rocket isn't so much the problem, beyond tweaking. I need to know how long to make the tower.