Re: [Jsbsim-devel] Rocket Thrust to Weight Ratio Problem

[Jon S. B. <jon...@co...>]

Good catch, Ron. Looks like a nozzle exit area problem. The area of the nozzle should be:

A = pi*r^2

A = pi*(56.9/24)^2 (in feet, where 56.9 is the diameter in inches of a single nozzle)

A = 17.65 FT2

At sea level this would give a thrust of (two nozzles):

933400 - 2117 * 17.65 * 2 = 858600 lbf

That's almost right on the money.

Jon


> -----Original Message-----
> From: Ron Jensen [mailto:wi...@je...]
> Sent: Friday, August 28, 2015 8:25 AM
> To: Development issues
> Subject: Re: [Jsbsim-devel] Rocket Thrust to Weight Ratio Problem
> 
> In the nozzle file you had:
>   <area unit="FT2">  335.45  </area>
> 
> A couple of quick calculations:
> The difference between Thrust[vac] and Thrust[SL] divided by sea level
> pressure:
> 
>   (930000-860000)lbf/(2116lbf/ft2) = 33.08 ft2
> 
> Diameter of a single nozzle used to calculate area:
>  2*(56.9in/2)^2 * pi = 35.32 ft2
> 
> Your nozzle area is off by a factor of 10. Using 33.545 (assuming you
> misplaced the decimal) and removing the lift and side forces from the
> aerodynamics section gave me:
> 
> Burn Time:         227 seconds
> Maximum Thrust: 931528 lbf
> Velocity:         12.5 mach
> Altitude:      660,000 ft
> 
> 
> Using your initial number of 335.45 propulsion/engine/thrust-lbs only
> reads 220,000.
> 
> 
> 
> On Thursday 27 August 2015 15:42:59 Uriah George wrote:
> > Hi JSB Devs,
> >
> > I've been working on several FDMs for various rockets, and I am
> having an
> > issue with the thrust to weight ratio. My numbers are very close to
> > accurate, however the rocket does not perform as expected and seems
> unable
> > to lift off without excessive thrust.
> >
> > In the case of the Atlas V 501 Launch Vehicle, the total launch
> weight
> > (initial mass with propellant) is 761,370 lbs, and the thrust for the
> > RD-180 engine is 931,528 lbf, which equates to a thrust to weight
> ratio of
> > approximately 1.22. I crosschecked the numbers, and everything is
> nominal,
> > in fact the thrust is even too high at sea level, which should be
> more like
> > 860,000 lbf. With this thrust to weight ratio the rocket should lift
> off,
> > however it doesn't budge.
> >
> > After some serious head scratching and coffee drinking, I decided to
> double
> > the thrust by adding a second engine feeding off the same tanks, just
> to
> > see what happens. Now the thrust to weight ratio is much higher,
> above 2.5
> > initially, and yet still it doesn't lift off. It will eventually lift
> off,
> > but the thrust to weight ratio must be above 3.8, after a burn time
> of
> > nearly 55 seconds, and this with two engines.
> >
> > I checked to verify the hold-down force is off, and logged some other
> > properties to check if my thrust vector was inverted, or if there is
> some
> > other issue that wouldn't be readily apparent, but I can't find
> anything
> > obvious.
> >
> > I'm testing in Outerra for reference, but I don't think there should
> be any
> > difference. The Jupiter 246 (J246) example has an initial thrust to
> weight
> > ratio of 1.7 and lifts off without a problem, so why can't the Atlas
> V 501
> > lift off until 3.8?
> >
> > I've uploaded the FDM for the Atlas V 501 Launch Vehicle
> Configuration to
> > MediaFire, which can be downloaded here:
> > http://www.mediafire.com/download/d10lgac7qdfv5ai/atlas_v_501_FDM.zip
> >
> > Note, the attached FDM includes the original single engine
> configuration.
> >
> > Any help would be greatly appreciated! Thanks.
> >
> > Best regards,
> > Uriah
> 
> 
> 
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