Me-163 Fuel Burn Bug

[Steel]

Gents,
  As a engineer major and a rocket science buff I have serious issues with the fuel burn rate of the Me-163. The 163 is fueled by hypergolic propellants which ignite on contact. No ignition is needed and no external supply of air is needed. Specifically the rocket is capable of providing thrust in a vacuum. Having said that the Me-163 should provide the same amount of thrust independent of airspeed. The specific impulse (read thrust/fuel burn in lbs) is steady state at a given altitude. With any given speed the fuel burn rate will remain constant. This is not so with the Me-163 as modeled in game. Above 500 KIAS the fuel burn rate is around 3000 GPH and below 350 that number soars to 4900 GPM. No other rocket device in the real world does this and is undoubtedly impossible with a common rocket nozzle. The only way this would be accurate is if you introduced atmosphere to the combustion device. This would generically be called a SCRAM jet which until recently hasn't even left the drawing boards. The only way the Me-163's burn rate is to change with altitude. Depending on the specific design of the rocket nozzle contour the change may be positive or negative. Even sea level to 50,000 feet the increase/decrease in thrust should be measured in single digits in terms of percent. The one way my argument is void is to have the plane throttle itself back to prevent overspeed. Given that this is not the case in any other plane I doubt its likely here. I do not have specific documents regarding the Me-163 but I can back up my arguments with amounts to "laws of rocket propulsion".

Edit: To be specific I feel the fuel burn rate should maintain a constant value for a given throttle setting over a variety of speeds.

Steel

[Anaxogoras]

I thought you were going to point out that the 163 can break the sound barrier!

[Spyder]

I hope I read that right, but you want to make the fuel burn rate the same for any throttle setting? I don't fly the 163 very often, but only because it has 5 mins of fuel, without throttling back, its a one way trip.

[SectorNine50]

I hope I read that right, but you want to make the fuel burn rate the same for any throttle setting? I don't fly the 163 very often, but only because it has 5 mins of fuel, without throttling back, its a one way trip.

He's saying that speed and altitude shouldn't affect the thrust of the rocket on the 163.

[Steel]

I am saying the speed shouldn't effect the thrust or fuel burn rate. Altitude can increase or decrease the thrust/efficiency a few percent depending on the nozzle exit pressure.

[Steel]

HTC have anything to add or feedback.....extremely curious!

Steel <S>

[BaldEagl]

I'm just curious how anything can produce thrust in a vacuum.  A vacuum is the absence of any particles and the only way to produce thrust is to propel particles, or more specifically, to push against them.  This then would make sense in that at higher levels of the atmosphere, the particle density decreases and so does thrust at any given throttle setting.  This is partially offset by the increased ability of the aircraft to move through the thinner air.

[MiloMorai]

BaldEagl, how did we send rockets to distant places in space then?

For those interested, here is a link to the Walter rocket motor used in the Me163, http://www.walter-rockets.i12.com/

[Steel]

BaldEagl,
      A great mechanical analogy would be a infinitely expanding cone inside of a bell. The cone expands itself creating a outward and forward force. Of course the energy to expand the cone has to come from somewhere. That somewhere is the combustion process and the potential energy stored within the propellants. If you stop and ponder the forces of our cone you start to see a few things. The first is that if you contain the energy within the cone the process is independent of the environment surrounding it. You need to propel particles to produce thrust but you do not need push against them. I will try to explain why thrust changes with altitude without getting to complicated. Party because I don't want bore anyone and partly because I lack the writing skills to do so. When you talk about the combustion pressure and propellant flow there are a few parameters that stand out. The biggest when considering thrust in general is chamber pressure and exit pressure. Both exit and chamber pressure of a nozzle are usually set parameters in most engines. It defines the profile of the thrust curve over the range of altitudes. Much like aircraft engines are built to work within certain altitude ranges so are rocket nozzles. If you build a nozzle for peak performance at sea level pressure the high altitude suffers. The reverse is true as well but to a lesser extent. This is why performance is related to altitude and can effect fuel efficiency. I don't know if this really explains the matter to you but I tried anyway. In the end the opposite of what you said is true. The less pressure the more efficent the engine becomes. In dealing with the Me-163 the amount of change would be in the low single digits in terms of percentage. If anyone wants to see the math I can work something up with the right information.

Steel

[Old Sport]

BE - check out Newton's basic laws.

For every force-action there is an opposite and equal force-reaction.

and

f=ma

Force (of thrust) = mass (of fuel being burned in the chamber) x acceleration (of the burning mass of gasses, from the chamber to the exhaust nozzle, which is explosively fast)

The gasses, that are virtually exploding, can only escape through the nozzle. So a force-reaction on the plane, thrust, is felt simply because the gasses are forced in the opposite direction out the nozzle.

It doesn't depend on an atmosphere to push against, though there is a slight effect as Steel points out. As Milo says, rocket thrust works in space.

The thrust for the Walther motor posted at the link by Milo is between 450-3750 lbs

3,750 lbs of thrust is a lot of power for the little 163.

< S >

[BaldEagl]

OK.  Got it.

[SmokinLoon]

I hope I read that right, but you want to make the fuel burn rate the same for any throttle setting? I don't fly the 163 very often, but only because it has 5 mins of fuel, without throttling back, its a one way trip.

Is this another arguement for not applying actual and proper modeling to an simulated aircraft for the sake of game play???

I strongly believe that if someone can present documented evidence to counter the flight models or aircraft models AH2 has in their sim-game... it should be investigated and considered to be applied.  I have no knowledge of rocket science in the lest bit except not to take flame near its fuel... so I cat vouch either way towards the O/P's claim, however I do know there are multitudes of aircraft in AH2 that are either shackled or boosted by their incorrect flight model.

[Karnak]

They have changed their models based on supplied data in the past.

[Steel]

Karnak,
     You will never see any data about speed vs engine efficency for the Me-163. The was never any reason to make data because its simply not a factor. Right now its a catch 22 because I dont know how to prove without a doubt with WW2 data. Other than math formulas based off hard data from the rocket engine itself I dont know what to do. Some input from HTC would be nice...

Steel

[Bosco123]

The 163 never had any throttle control, it was either full, or dead. So, we could not possibly know what the fuel burn rate was for the 163, when it never had any throttle control to begin with.
<S>