Does Rocketry Work beyond Earth's atmosphere?

[simonshack]

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Dear forum members,

Before unlocking this fine, intellectually interesting thread, I would like to invite the valiant contributors to this vacuum discussion to express their thoughts and views on my latest post at our Space Shuttle thread:

HOW DID THE SHUTTLES ACHIEVE ESCAPE VELOCITY?
http://www.cluesforum.info/viewtopic.ph ... 6#p2385666


Of course, since the Space Shuttle Program is credited for having rocketed a whole bunch of junk into orbit - for 30-odd-years - including the most (in)famous satellites, orbiting telescopes, the entire "I$$" and whatnot, it would seem logical to determine whether the Shuttle could technically reach the vacuum of space in the first place.

[simonshack]

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I am unlocking this thread for further discussion. This time around, I will tentatively approach this subject with, shall we say, a more constructive outlook - lest this forum be accused of being overly pessimistic about mankind's ingenuity and its ability to overcome the physical hurdles of our universe. Firstly, however, let me point out a few historical aspects of what we have come to know as 'space travel'.

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A BRIEF HISTORICAL LOOK AT
SPACE TRAVEL

As we have seen, the manner in which our planet's space agencies purport to propel their space vessels in the vacuum of space appear highly dubious and based on thin air. As it is, modern space travel turns out to have been dreamed up back in the early 1900's by two men, one Russian and one American, whose speculative theories seemingly predicted with wondrous prescience the intricacies of reaching, entering and moving around in the void of space: Mr Tsiolkovsky and Mr Goddard.



Mr Goddard, whose experimental rockets achieved a peak altitude of 1,6miles, wrote a monograph in 1919 titled "A Method of Reaching Extreme Altitudes". He was soundly derided by his peers at the time for asserting that rockets would work just fine in a vacuum space, yet the Smithsonian Institute (now the American 'sacred temple of science') decided to fund his plucky efforts. His claim was based on an experiment in which he fired a gun inside a vacuum tube - and observed the gun's recoil rearward motion. Therefore, he concluded, rockets could also be pushed by 'recoil force' in a vacuum. Today, Goddard is hailed as the American father of space travel - although his innumerable rocket experiments were but a string of unmitigated failures.



Mr Tsiolkovsky's predictions, formulated out of purely theoretical calculations, turned out to be even more ... astonishingly accurate (that is, if NASA is to be trusted) : in 1903, he published "The Exploration of Cosmic Space by Means of Reaction Devices" in which it was clearly stated that "the horizontal speed required for a minimal orbit around the Earth is 8km/s and that this could be achieved by means of a multistage rocket fueled by liquid oxygen and liquid hydrogen." Sure enough, today NASA claims to propel its modern, 21st-century space vessels with multistage liquid oxygen and hydrogen rockets - and that they leave our atmoshere at that precise velocity of 8km/s! It is anyone's guess how Mr Tsiolkovsky, for all his genius and hard work, could have theoretically determined - with no experimental verification whatsoever - the basic imperatives of space travel with such utmost exactitude. In any event, this tale of extraordinary foresight is what our planet's rocket scientists are taught and asked to believe in - without question - much like a religious dogma.


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As we well know by now, NASA appeals to Newton's laws when asked how their rockets are propelled in the near-vacuum of space. They say that "if you float in LEO (lower earth orbit) and throw a pumpkin as heavy as yourself, you and the pumpkin will be propelled at the same speed in opposite directions." This, of course, assuming that Earth's gravity doesn't pull both of you down first - which would certainly be the case in LEO... But NASA doesn't like to complicate things with such trifle. Well, they say that if you only can reach the speed of 8km/s (as Tsiolkovsky predicted back in 1903), you or indeed any object will just keep 'freefalling' around Earth - no more fuel needed (but for small thruster adjustments).


So if we were to apply this principle to a rocket, would this not be the ONLY way to achieve this ?
( Think of the pumpkin as being Mass A - and yourself as being the rocket body and Mass B )



Thanks for your thoughts on the above diagram. I hope you will agree that - if it were technically feasible to expel Mass A in one 'push' as shown, it would indeed be possible to propel a spacecraft in the vacuum of space - sometime in the future!
(That is, if the "8km/s" speed needed for orbiting is true, of course)

[hoi.polloi]

Simon, what you are talking about is achieving orbit rather than operation in a vacuum. I locked the thread since the discussion became more about getting things up to the vacuum rather than the science within the vacuum.

Why is this pertinent? Because, presuming we are already In the vacuum in a successful orbit, time no longer matters in the equation, since the ship is essentially moving and accelerating "for free". Hence, if the 50,000 kg mass of gas were ejected a little at a time, the tiny opposite-and-equal reaction on the ship will build up and it doesn't have to come out as a chunk, right? Presuming stable orbital pattern between gravity and momentum, right?

My only question regarding this would be to subtract Boethius' 2km/s speed of depressurization from the net velocity of the forced gas — given our assumed limit is indeed 2km/s. With a fuel speed of 4km/s, minus speed of 'free expansion', and you should still have a 2km/s value of exploding this fuel. Boethius, please explain the 2km/s speed better if you can, in your opinion. You have said 'instantly' before but what does that mean? Must the 2km/s speed be multiplied by however many molecules-wide the hole is offering depressurization? Are we talking about a much faster rate of "free expansion" for every stream of gas molecules squeezing out?

Since staying in orbit is a rather delicate operation however, let's stray 'off topic' a little here and talk about actually achieving orbit as you seem to be interested in. But only for a little while, please, before we give "Achieving Orbit" its own thread. Okay?

I have only one question for this, since we've already talked about the unlikelihood of rocketry succeeding to get to this speed. Given the ship were 50-megagrams and suddenly all at once "ejected" a 200-megagram pumpkin at 2km/s, would this launch the 50Mg ship at the requisite 8km/s? The language would have to change here since it's more like the 200Mg pumpkin is "ejecting" the ship, it now being the smaller body.

[hoi.polloi]

Fascinatingly mainstream web site which fails to debunk Boethius' formulas, and which in my browser has disabled the copy-paste functionality of a normal web site.

http://www.braeunig.us/space/propuls.htm

But if you scroll down to Monopropellant Engines on this page, you will see a lot of interesting gibberish about small boosters and course correction rockets. The fuel used is supposed to be hydrazine. Here is what it says (copied from the HTML source and changed to phpBB forum code):

By far the most widely used type of propulsion for spacecraft attitude and velocity control is monopropellant hydrazine. Its excellent handling characteristics, relative stability under normal storage conditions, and clean decomposition products have made it the standard. The general sequence of operations in a hydrazine thruster is:

• When the attitude control system signals for thruster operation, an electric solenoid valve opens allowing hydrazine to flow. The action may be pulsed (as short as 5 ms) or long duration (steady state).
• The pressure in the propellant tank forces liquid hydrazine into the injector. It enters as a spray into the thrust chamber and contacts the catalyst beds.
• The catalyst bed consists of alumina pellets impregnated with iridium. Incoming hydrazine heats to its vaporizing point by contact with the catalyst bed and with the hot gases leaving the catalyst particles. The temperature of the hydrazine rises to a point where the rate of its decomposition becomes so high that the chemical reactions are self-sustaining.
• By controlling the flow variables and the geometry of the catalyst chamber, a designer can tailor the proportion of chemical products, the exhaust temperature, the molecular weight, and thus the enthalpy for a given application. For a thruster application where specific impulse is paramount, the designer attempts to provide 30-40% ammonia dissociation, which is about the lowest percentage that can be maintained reliably. For gas-generator application, where lower temperature gases are usually desired, the designer provides for higher levels of ammonia dissociation.
• Finally, in a space thruster, the hydrazine decomposition products leave the catalyst bed and exit from the chamber through a high expansion ratio exhaust nozzle to produce thrust.

Monopropellant hydrazine thrusters typically produce a specific impulse of about 230 to 240 seconds.

Now why would they specifically make a high expansion nozzle for direction if vacuum is already giving an enormous helping hand in the matter?

[Boethius]

Simon, what you are talking about is achieving orbit rather than operation in a vacuum. I locked the thread since the discussion became more about getting things up to the vacuum rather than the science within the vacuum.

Why is this pertinent? Because, presuming we are already In the vacuum in a successful orbit, time no longer matters in the equation, since the ship is essentially moving and accelerating "for free". Hence, if the 50,000 kg mass of gas were ejected a little at a time, the tiny opposite-and-equal reaction on the ship will build up and it doesn't have to come out as a chunk, right? Presuming stable orbital pattern between gravity and momentum, right?

My only question regarding this would be to subtract Boethius' 2km/s speed of depressurization from the net velocity of the forced gas — given our assumed limit is indeed 2km/s. With a fuel speed of 4km/s, minus speed of 'free expansion', and you should still have a 2km/s value of exploding this fuel. Boethius, please explain the 2km/s speed better if you can, in your opinion. You have said 'instantly' before but what does that mean? Must the 2km/s speed be multiplied by however many molecules-wide the hole is offering depressurization? Are we talking about a much faster rate of "free expansion" for every stream of gas molecules squeezing out?

Hello Hoi,

my feeling is the rocket gas exhausted at 4 km/s will enter the vacuum at 6 km/s (4 km/s + 2 km/s).

The real question is will the increased velocity have any effect on the ability of gas to do work and produce thrust in a vacuum?

I say no, it does not.

This is because gas molecules can't accelerate inside the vacuum and hence they cannot do work.
F=MxA (force = mass x acceleration). If Acceleration = 0 then Force =0.
Which agrees with the formula for work done by a gas
Work = Pressure x Volume
When pressure = 0, work = 0.
When force = 0, work = 0;

Without a force there is no work. Note that for the same reason a massless object cannot impart a force on the ship.

What happens to the gas when it exits the nozzle
Each molecule leaves the nozzle and fans out into space traveling into infinity without interacting with any other molecules. The speed at which it leaves the nozzle is the speed it continues with throughout its journey. An object whose speed does not change experiences no acceleration. This is different from the atmospheric case where the first exhaust molecules slow down due to resistance from the air and the second wave of exhaust collides with the first, etc... Gravity also acts as a force accelerating gas molecules in the earth's atmosphere.

Thus the molecules exhausted into a vacuum have momentum (via their velocity) but momentum unspent is not a force. One way to visualize this is that the gas is is never ejected into space, it always simply drifts away, even if at a high velocity.

Nozzle design and the ability of exhaust to produce force in a vacuum
I have seen some web sites that claim special nozzles can baffle exhaust gasses inhibiting them from taking clear paths into space, causing them to bounce off of each other, creating a sort of fake atmosphere inside and around the nozzle. Gas molecules colliding with each other satisfies the conditions for the gas to do work. They could generate a force outside of the ship and impart some of that force to the rocket. From what I have seen only a fraction of the molecules would interact. These web sites analyses of gasses in the nozzle go against the standard rocket theory that the gasses do not "push" the ship.

[sceppy]

The way to look at it in a simpler way, is to think of the super hot thrusting exhaust gases 'consuming' the air directly underneath them.
At it's hottest...which would be the white hot part of the thrusting flame , it is creating a close 'vacuum' and that vacuum has to be filled, because the atmosphere wants to rectify the difference in pressure to equalise it's normal atmospheric pressure...and will do so, all the way up...or as long as the rocket fuel keeps attacking it.
If you look at any rocket exhaust, you will notice the different colours of the hot gases coming from it.
Each colour is an indication of the amount of oxygen that is pushing against that thrust... meaning, there's virtually none pushing against the white hot flame, because all of that oxygen is getting consumed and is sprinting like hell, at it...like the charge of the light brigade. Yet the wider cloud of exhaust, and the thick cotton wool density of its span, shows you the volume fighting back.
The atmosphere has back up in stronger numbers than the heat can consume.
The higher the thrust, the further down into the dense atmosphere that thrust can go but the faster the atmosphere will fight back basically catching the gases and eventually stopping them in their tracks (cooling them down).

Have you ever been carried out of an arena with the crowd?

No matter what happens on earth, you can only move something with an action and equal and opposite reaction, no matter how it's dressed up.

What you 'cannot' do, (N.A.S.A's vacuum rocket version)is run faster than a manual friction treadmill but you can expend all of your energy trying.

[Heiwa]

The way to look at it in a simpler way, is to think of the super hot thrusting exhaust gases 'consuming' the air directly underneath them.

Simpler? The rocket fuel combustion and expansion takes place inside the engine and the hot gases, hardly visible, just escape through the nozzle into the external vacuum and pollutes the vacuum making it a polluted vacuum full of gases. There is evidently no air or anything outside the nozzle in space/vacuum. But when gas is passing the nozzle into the vacuum, evidently the vacuum is affected (polluted, diluted with gas).

BTW - it is the escaping hot gases giving the rocket a kick in the ass that drives the rocket. Outside vacuum, air, atmosphere, water doesn't matter the least. Does anybody believe that the exhaust of a car drives the car (and not the internal combustion in the engine). Pls - lock this thread ASAP.

[sceppy]

The way to look at it in a simpler way, is to think of the super hot thrusting exhaust gases 'consuming' the air directly underneath them.

Simpler? The rocket fuel combustion and expansion takes place inside the engine and the hot gases, hardly visible, just escape through the nozzle into the external vacuum and pollutes the vacuum making it a polluted vacuum full of gases. There is evidently no air or anything outside the nozzle in space/vacuum. But when gas is passing the nozzle into the vacuum, evidently the vacuum is affected (polluted, diluted with gas).

BTW - it is the escaping hot gases giving the rocket a kick in the ass that drives the rocket. Outside vacuum, air, atmosphere, water doesn't matter the least. Does anybody believe that the exhaust of a car drives the car (and not the internal combustion in the engine). Pls - lock this thread ASAP.

Cars have wheels.
Try and drive your car when it's sat on bricks.

[simonshack]

Does anybody believe that the exhaust of a car drives the car (and not the internal combustion in the engine). Pls - lock this thread ASAP.

No - I, for one, do not believe the exhaust of a car drives the car. I believe no one does.

Instead, it would seem that your contention is that if the exhaust of a car would be expelled fast enough, it would nicely propel the car forward in outer space, due to the fuel mass being expelled from the car's engine (providing it ran on oxygen/hydrogen bipropellant). Am I correct in thinking that this is your contention?

[Boethius]

Fascinatingly mainstream web site which fails to debunk Boethius' formulas, and which in my browser has disabled the copy-paste functionality of a normal web site.

http://www.braeunig.us/space/propuls.htm

I would like to have a conversation with the braeunig folks. From their site:

Consider a rocket drifting in gravity free space [...]Assume there are no external forces, such as gravity or air resistance.

These are the same preconditions I have been using for my analyses although some people have argued that these assumptions are flawed...

This is called the principle of conservation of linear momentum.[C of M]

They rely heavily on this principle to explain rocket propulsion in the vacuum. C of M, however, is intended to explain systems where forces are exchanged such as a cue ball striking the 8-ball, or a gun firing a bullet.

The problem with using C of M for rockets in space is that you end up dividing by zero and so your results are invalid.
C of M requires that the momentum conserved is the result of an equal/opposite force applied to two different objects.

To construct a proper C of M analysis you first set up two forces equal and opposite then you re-write them in terms of change in momentum of the objects involved.

In a space rocket one of the objects is exhaust gas entering the vacuum. If it experiences no force then your C of M analysis ends there.

Force = 0, C of M = invalid.

Confirmation of my C of M analysis
Another critical problem I have with the NASA types is that they don't check to see if their results agree with Newton's Law F=MA. Gasses in space do not experience acceleration. There is none due to gravity and none due to air resistance. An object moving in space keeps moving with constant velocity, acceleration = 0, until it's either struck by an asteroid or falls into some other massive object's gravitational field.

A ship in space that is not moving can't get started using exhaust. A ship that is moving can't speed up or slow down using exhaust. In both cases, stopped or moving, acceleration will be 0 in for the space rocket.

So with acceleration = 0, force = 0 and without force there is no C of M.

A mass deltaM has been ejected from the rocket

This is a critical difference I have with the NASA types. A mass is not ejected from the rocket.
Ejection requires force and you cannot force gas into a vacuum.

NB
If Newton's 2nd Law disagrees with Conservation of Momentum then there is an error in the analysis.

[Heiwa]

Does anybody believe that the exhaust of a car drives the car (and not the internal combustion in the engine). Pls - lock this thread ASAP.

No - I, for one, do not believe the exhaust of a car drives the car. I believe no one does.

Instead, it would seem that your contention is that if the exhaust of a car would be expelled fast enough, it would nicely propel the car forward in outer space, due to the fuel mass being expelled from the car's engine (providing it ran on oxygen/hydrogen bipropellant). Am I correct in thinking that this is your contention?

The combustion inside a car Otto or Diesel engine cylinder drives the pistons/shaft etc of the engine connected in the end to the wheels, etc. The combustion (expansion of gases) inside a rocket engine simply creates the thrust on the rocket to which the engine is attached. In both cases the combustion gases escape from the engines as exhaust.
Evidently you can also attach a rocket engine to a car and drive it on a dry salt lake in the USA. It has been done by some crazy people. It is like a jet plane not taking off.

[I, Gestalta]

Your comparison requires space to be paved.

[sceppy]

The combustion inside a car Otto or Diesel engine cylinder drives the pistons/shaft etc of the engine connected in the end to the wheels, etc. The combustion (expansion of gases) inside a rocket engine simply creates the thrust on the rocket to which the engine is attached. In both cases the combustion gases escape from the engines as exhaust.
Evidently you can also attach a rocket engine to a car and drive it on a dry salt lake in the USA. It has been done by some crazy people. It is like a jet plane not taking off.

The engine drives the wheels of a car on a surface of friction.
A jet car, or plane, works by sucking in air...compressing it with fuel and expelling it out of the back, which creates a high pressure through the front... and a low pressure out of the back, through burning thrust, which forces the air to push back against that thrust... propelling the jet, or jet car forward.

If rockets work how we are told they work, then we all should be able to drive our vehicles by pressing our foot down on the accelerator and racing along the road with the gear stick in NEUTRAL.

[Heiwa]

Yes, a jet engine is different from a rocket engine. A jet engine doesn't work in vacuum. Nor does a car engine. Only rocket engines work in vacuum.

[brianv]

A Rocket Engine might "work" in a vacuum, but will it supply thrust to propel the rocket in the emptiness of space?

A fixed-wing aircraft generates forward thrust when air is pushed in the direction opposite to flight. This can be done in several ways including by the spinning blades of a propeller, or a rotating fan pushing air out from the back of a jet engine, or by ejecting hot gases from a rocket engine. The forward thrust is proportional to the mass of the airstream multiplied by the difference in velocity of the airstream.