Talk:Rocket engine/Archive 1

Archive 1

Exit mach numbers

"Exhaust speeds as high as 10 times the speed of sound at sea level are not uncommon." Who ever said this is wrong. Exit mach numbers in chemical rockets tend to be 2.8 - 4.0, not 10.

That was me, Mr. Anonymous. The speed of sound at sea level is ~340m/s. The exhaust velocity of the SSMEs are about 4500m/s. You do the maths. Hydrocarbon engines can be 330 seconds or more.WolfKeeper 10:52, 29 May 2006 (UTC)

You may have confused the speed multiplier given by a De Laval nozzle relative to the sonic velocity at the throat. They give speed increases of about that range. However the sonic speed in the hot gasses that are typically put through such nozzles is much higher than the sonic speed of air at STP, so the final speed is far higher.WolfKeeper 10:52, 29 May 2006 (UTC)

For example, if you put a hot gas through a De Laval nozzle where the speed of sound in the hot gas is 1000km/s, and the nozzle coefficient is 3, then the final velocity is 3 km/s.WolfKeeper 10:52, 29 May 2006 (UTC)

Reverted.WolfKeeper 10:52, 29 May 2006 (UTC)

Mass Drivers

Woflkeeper, you have a strange defintion of a rocket. Mass drivers belong in the same category as electric propulsion, including ion propulsion and plasma. You should remove those also. —The preceding unsigned comment was added by Cfrjlr (talk • contribs) 18:55, 11 February 2007 (UTC).

Mass drivers are reaction drives, they do not emit an exhaust jet. They lack a nozzle or a throat; as do Ion drives and most plasma drives. They are not normally considered to be rockets. The only exception currently in the article is VASIMR which has a magnetic throat and nozzle. I'm planning to largely remove but reference out to ion drives. There has to be some scope to the article otherwise it would end losing focus.WolfKeeper 19:16, 11 February 2007 (UTC)

Reorganize

OK. I will help reorganize this page, the whole space tech part of wikip is a mess and needs a lot of work, I wish I had more time. —The preceding unsigned comment was added by Cfrjlr (talk • contribs) 19:20, 11 February 2007 (UTC).

Reorg complete

OK I have completed the re-org. I hope you agree it makes the page easier to read, and improves the logical flow.Charles 20:02, 11 February 2007 (UTC)

Rocket engines → Spacecraft propulsion

Please see this request for discussion about the rocket engines redirect. Sdsds 21:18, 1 March 2007 (UTC)

Result was to redirect to Rocket engine

First sentence

I propose changing the first sentence of the article to read

A rocket engine is a reaction engine that takes all its reaction mass from within tankage and forms it into a high speed jet.

Are these the two attributes we think of as distinguishing a "rocket" engine from all other reaction engines? (The second sentence would then cover the common uses, and then after that would be coverage of Newton and combustion.) I hesitate to be WP:BOLD and make the change without prior discussion, because the current version has been so stable.... Also, this wording finesses the question of whether or not a convergent-divergent nozzle is part of the definition, or just a frequently-found characteristic of some rocket engines. Is saying "high speed jet" without saying how that jet was formed a good approach? (sdsds - talk) 18:36, 29 August 2007 (UTC)

Sounds fine and better than we have at the moment.WolfKeeper 19:01, 29 August 2007 (UTC)

Nuclear heating - state of the art?

Like most WP spaceflight articles, more impetus seems to have gone on speculation and SF concepts rather than actual practice. Consequently, certain areas are risible. Take this for example, in the over-long Nuclear Heating section:

"Antimatter containment issues, thermal issues, beyond current state of the art."

That's putting it mildly. Apart from accelerator experiments (high energy physics), there is no technology whatever that can handle industrially-significant quantities of antimatter. The engine concept is purely speculative.

Matt Whyndham 15:00, 29 August 2007 (UTC)

I think you will find lots of whole-hearted agreement with this sentiment. The questions are: "How to deal with stuff that's over the line?" and "Where to draw the line?" Given recent news, VASIMR would be my bright line between what belongs, and what doesn't. (sdsds - talk) 18:42, 29 August 2007 (UTC)

When the line becomes clear, the conceptual stuff should ideally be placed in Another Article. Matt Whyndham 08:24, 7 September 2007 (UTC)

Performance

WolfKeeper: This is to do with your section on performance.

"Rocket engine nozzles are surprisingly efficient heat engines for generating a high speed jet, as a for generating a high speed jet, as a consequence of the high combustion temperature and high compression ratio in accordance with the carnot cycle. For a vehicle employing a rocket engine the energetic efficiency is very good if the vehicle speed approaches or somewhat exceeds the exhaust velocity (relative to launch); but at low speeds the jet propulsion efficiency asymptotically approaches 0% at zero speed"

I am not at all sure about this, It doesn’t seem to make sense. Are we talking about efficiency in terms of thrust generated or speed? Suppose you have a heavy vehicle of a given mass needing a certain amount of thrust to propel it, does it mean that the thrust generated to move that vehicle will be less effective than if the same amount of thrust was applied to a lighter vehicle to move it faster? Surely if this was true none of the Apollo missions with their heavy payloads, would have been possible. What seems to be implied is that the thrust would be so ineffective at slow speeds(nearing zero efficiency) that the rocket would not get into the air at all! The statement seems to contradict Newton’s Third Law of motion, the thrust generated should be the same in both cases. In the former, the thrust generated is moving a relatively heavier mass and in the latter a lighter one, where does the efficiency come into it? Surely it would be simpler and more accurate to state that the efficiency of a rocket depends upon the propellant it uses, and the amount of thrust it can generate? DDjames 11:46, 11 September 2007 (UTC)

You need to consider carefully that speed is not at all the same thing as kinetic energy.

Kinetic energy varies as the square of the speed- so at low velocity kinetic energy is negligible (tends to zero). This actually means that for a given constant acceleration, from stationary the initial rate of gain in kinetic energy (power) is *zero*; even though the speed is increasing at a constant rate. This is true in *all* cases, but is particularly annoying in rockets because the exhaust is taking away power.

Another example of this zero rate of gain of kinetic energy is if something is dropped under gravity, the power that gravity applies is initially zero, and increases with speed (the equation is actually deceptively simple: force * distance = energy; power is energy over time; force * distance / time = force * velocity; when stationary you have no velocity...)

If you still don't believe me do the calculus. If you don't know how to do the calculus, then I probably can't help you because kinetic energy is the integral of force, and it would take me too long to explain.WolfKeeper 06:39, 13 September 2007 (UTC)

Rockets are, barring minor variations, constant combustion engines, the thrust is not varied appreciably. The problem seems to be this:- The momentum of the jet equals the momentum of the rocket (i.e., m¹ x v¹ = m² x v²). While the momentum is conserved, the kinetic energy 1/2mv² is not.

Kinetic energy is not necessarily conserved per se, but total energy is *always* conserved. In the case of the rocket potential chemical energy is coming from the propellant, and also from the kinetic energy of the propellant when it is burnt (and yes that does make a difference- if you fail to account for that, rockets can appear 200% efficient at some parts of the flight!).WolfKeeper 06:40, 14 September 2007 (UTC)

So which equation is right and in what context? If a rocket is stationary (i.e., clamped onto a test pad ) does it generate thrust less effectively than a rocket that is moving.

No. Momentum is momentum.WolfKeeper 06:40, 14 September 2007 (UTC)

I can appreciate that if it is possible to propel the rocket at a speed slightly less than or equal to the jet exhaust, as is possible in space, the engine is giving its optimum output performance. What about a rocket traveling in the earth’s atmosphere at sea-level, is it completely ineffective?

You need to define what you mean by effective; it's gaining speed, but it may not be technically gaining kinetic energy at the moment it is stationary.WolfKeeper 06:40, 14 September 2007 (UTC)

Take another example. Fire a cannon under similar circumstances, what happens, the projectile travels for a certain distance, fire the same cannon in space and the projectile would travel for an almost infinite distance, does this mean that the canon is inefficient when it is fired at sea level and that it is only efficient when fired in space ?

The cannon probably would be said to be as efficient; but the projectiles loses energy on friction.WolfKeeper 06:40, 14 September 2007 (UTC)

Is this about performance or about efficiency, I am a bit confused.DDjames 06:02, 14 September 2007 (UTC)

It's about energy efficiency.WolfKeeper 06:40, 14 September 2007 (UTC)

Thankyou Wolfkeeper, most of what you have to say is acceptable. But I was wondering if you could add a section on momentum or thrust, since usually momentum is calclulated when the eqaution involves force in a given time while the kinetic energy equation is used when distance is involved. Since impulse plays such a large part in rocket dynamics, don't you think it would give the article a more balanced viewpoint if both equations were quoted. Because they are both equally important in any reference to rockets.DDjames 16:18, 14 September 2007 (UTC)

Impulse ends up in the exhaust as well though, so impulse per se is not really useful except in an instantaneous sense.

On the contrary impulse is the single most important factor in the functioning of a rocket. Remember that a rocket engine is basically working on the forces of recoil(reaction), where a large force is acting over a small time. Thus it would be completely wrong to say that impulse does not play an important part in the process, this holds true even if the fuel is burning ‘continuously’. A detailed discourse on the subject is not needed, just a reference to it with a link.DDjames 03:33, 16 September 2007 (UTC)

Impulse is very different from specific impulse. Due to the way that rockets work, specific impulse and thrust/weight are the very probably the two most important variables in spacecraft propulsion. A total impulse is sometimes specified for a particular manufacturers rocket engine and propellant system though.WolfKeeper 15:08, 16 September 2007 (UTC)

The useful nearest thing to that is probably the rocket equation and we reference that already. It would be inappropriate to copy much of that into the article.WolfKeeper 18:15, 14 September 2007 (UTC)

Fine with me, I am not trying to be contentious, I am merely trying to point out that while the conservation of momentum is maintained, kinetic energy is not. Yes, the total energy is maintained but there are losses all over the place, to turbulence, to heat, to sound and so on, each of which is variable according to circumstances. The difference between momentum and kinetic energy is rather like the difference between absolute units and gravitational units. Another point in favour of using the Momentum equation is that while momentum is a vector, kinetic energy is not, it is just a unit of energy, it certainly does not describe the movement of the rocket in any way. I think that the momentum equation gives a more accurate picture of how a rocket engine works, and this after all is an article about rocket engines. Maybe I’m wrong about this. DDjames 03:09, 16 September 2007 (UTC)

Removed comments

I removed two very recent comments this one and this one when I reverted the page due to the former's bad ref tag (it wasn't closed properly, so it broke wiki formatting). I'm adding these two comments to their appropriate places. --slakr^\ talk / 05:51, 19 September 2007 (UTC)

Principle of Operation

Hi! Wolfkeeper, I thought that perhaps something on the lines of the following, could be added to make the article more comprehensible to the layman:

The thrust of a rocket is calculated by the equation F = ma, where M = the mass of the propellant ejected per second, and a its acceleration. The momentum delivered to a rocket varies directly with the mass and velocity of the gas ejected from the rocket and is based on Newton’s Third Law : For every action, there is an equal and opposite reaction. In the case of a rocket, the momentum of the escaping gases at the rear of the rocket is equal to the momentum of the rocket in the opposite direction and is based on the equation m¹ v¹ = m² v² where m¹ v¹ represents the momentum of the gases escaping at high velocity and m² v² represents the momentum of the rocket. The amount of thrust generated by a rocket depends upon the [[ http://www.en.wikipedia.org/wiki/Rocket_propellant%7Cthe propellant]] that is used and the design of the nozzle. DDjames 02:29, 18 September 2007 (UTC)

The momentum equation you quote above isn't really relevant, since it's only correct instantaneously, because the rocket is losing mass, and because different bits of the exhaust ends up doing different speeds depending on when in the burn it is ejected. F=ma is probably better expressed as F=d/dt(momentum).WolfKeeper 03:29, 18 September 2007 (UTC)

See http://hyperphysics.phy-astr.gsu.edu/hbase/rocket.html for how it's done.WolfKeeper 16:50, 18 September 2007 (UTC)

You are worng about this wolfkeeper, the momentum equation is the only equation that counts in rocketry, it is the kinetic equation that is irrelevant. You can check this from any source you like, including NASA. In any case I have perfomed an edit, check it out and see if it goes with the overall outlay.DDjames 11:37, 18 September 2007 (UTC)

None of the equations of physics are irrelevant in rocketry ;-)WolfKeeper 16:33, 18 September 2007 (UTC)

However, I removed your change because it was unreferenced. If you want to re-add you will need to add a good reference.WolfKeeper 16:33, 18 September 2007 (UTC)

Ok. Here is a reference from the NASA web-site: see the last paragraph at the bottom of the page. —Preceding unsigned comment added by DDjames (talk • contribs) 19:20, 18 September 2007 (UTC)

Firing a ball from a cannon is not a rocket engine; it's a reaction engine. A rocket engine needs a nozzle and a fluid exhaust. —Preceding unsigned comment added by Wolfkeeper (talk • contribs) 05:41, 19 September 2007

Wolfkeeper: You had deleted the sentences on thrust and mometum that I had added to this article on ‘Rocket Engines’ on the grounds that there were no proper references. The following quotes are taken from the Thiokol Corporations web-site on “Rocket Basics”. The Thiokol Corporation manufactures and supplies propellants and components for the space shuttle and other NASA projects: http://www.fas.org/man/dod-101/sys/missile/docs/RocketBasics.htm The first quotation (shown below) supports my use of the momentum equation:

“Newton's third law, the law of action and reaction, states that: for every force exerted by one mass on another, there is an equal and opposite reaction exerted by the second mass on the first. In the rocket, the expulsion of combustion products (gases) through the nozzle produces a reactive force on the rocket in the opposite direction which causes it to be propelled. “

The second quotation(shown below) substantiates my statement that thrust is equal to F = ma

“The first and most common term used in rocketry is thrust, which is a measure of the total force delivered by a rocket motor for each second of operation. Essentially, thrust is the product of mass times acceleration. In actual calculations, of course, gravity, pressure of the surrounding medium, and other considerations must be taken into account. “

An encyclopedia article on rocket engines that does not refer to Newton’s third Law in the form of m¹v¹ = m²v² or at best makes only a fleeting reference to it is deficient. Further the fact that thrust is not identified as essentially being due to F = ma, is also not doing its job.

The reference you mentioned does not show these particular equations as applying in the form you specify above to rocketry. Therefore their inclusion in the article would be unreferenced.WolfKeeper 21:04, 19 September 2007 (UTC)

In fact the article as it stands is misleading in that it seems to state that the only criteria for the performance of a rocket is the kinetic energy of the exhaust. Which is the main reason for the addition I had made. I hope that you will now allow the changes I am making to remain. P.S. I did not mean to infer that a canon was a rocket, it is not. But if you did fire a canon in space the projectile would travel almost infinitely at its final velocity, thereby fulfilling the 'energy efficiency" you refer to. DDjames 20:26, 19 September 2007 (UTC)

Vernier engine?

What's a "Vernier engine"? I found it mentioned in at least one NASA document but there's no Wikipedia entry with that title.

It's just a small engine used to supplement the bigger engine that allows you to change the overall thrust or the direction of the thrust slightly. For example you can use a Vernier engine for roll control.WolfKeeper 23:09, 14 August 2006 (UTC)

Note that vernier thruster was created June 10, 2006, but is still marked as a "stub". (sdsds - talk) 04:17, 20 September 2007 (UTC)

Moving material to reaction engine article

Much of the material currently in the lead paragraph of the article would be better placed in the reaction engine article, which the lead sentence references in its definition. "Reaction engine" is the place to discuss the basic physics common to all such engines. (sdsds - talk) 23:28, 19 September 2007 (UTC)

Not to put to fine a point on it, those equations are wrong. F=ma assumes m is constant (which is not true in a rocket) and the m1 v1 = m2 v2 only applies to impulsive events when two masses m1,m2 end up travelling at speeds v1, v2. That's not what happens in reaction engines in the general case (OK, mass drivers, if you want to add it to that one article, go right ahead.)WolfKeeper 23:44, 19 September 2007 (UTC)

If you don't agree with me, fine, perhaps I am wrong, it has happened, in which case get me a good reference, in the meantime, I'm taking it out for being unreferenced.WolfKeeper 23:44, 19 September 2007 (UTC)

I think the concept of tying the statements in these articles back to specific passages in reference works is a great one! That way, everyone (at least everyone who has access to a copy of the reference work) can check to see if the editor has correctly construed the material. Luckily, there are a few freely available reference works, that every editor can access easily. I encourage everyone to add to the list of these; personally I will content myself with: Huzel, D. K. and Huang, D. H. (1971). NASA SP-125, Design of Liquid Propellant Rocket Engines (2nd ed.). NASA.{{cite book}}: CS1 maint: multiple names: authors list (link) The .pdf file is large and awkward, and if anyone knows of an ascii version of this I would be grateful to be told about it, but the material it covers just in chapter 1, section 1, called, "The Generation of Thrust by a Rocket Engine" should be sufficient for much of the material that is appropriate for an encyclopedia article. Does anyone have trouble (in theory, or practice) with using this as a reference? (sdsds - talk) 04:11, 20 September 2007 (UTC)

Absolutely I have a problem with this, is this an encyclopedia or a text book? Look, Rockets are all about Newtons Third Law, go to the most sophisticated site you can name and it is still true! Basically rockets are all about the momentum equation, the present data is not sufficient to illustrate this. I laid off this topic for a couple of weeks hoping that someone else would see the problem, but this does not seem to be the case. In laymans terms to talk about the efficiency of a rocket in terms of how it would perform in space is ridiculous. Please do not let wikipedia go to the dog s! We need an expert moderator!DDjames 09:57, 5 October 2007 (UTC)

Hello DDjames. Thank-you for attempting to explain your view. My first response was to add the standard {{talkheader}} template to the top of this page. Among other things -- all of which are worth reading -- that standard template mentions, "Be polite, Assume good faith, No personal attacks, Be welcoming." I hope we can all join together in acknowledging the importance of adhering to those guidelines!

To respond directly to your concern - you are correct that the principles of Newtonian physics are sufficient to explain the dynamics of vehicles propelled by rocket engines, when those principles are correctly applied. We all agree on that, I think. We apparently haven't yet reached concensus on what level of understanding of those principles we can expect from the readers of this article. Personally I am generally satisfied with the level of understanding required to read the "Principle of operation" section. (Although I note the section is currently entirely unreferenced.) I suppose the "Performance" section is where we might not yet agree on the level of sophistication appropriate for the article to assume of the reader. While calculus is not exactly required for this material, a comprehensive treatment that avoids calculus is going to be tough! In particular, the use of "m-dot" notation to represent the time-derivative of mass probably presupposes too much. Do you agree? (sdsds - talk) 06:42, 10 October 2007 (UTC)

Chemistry?

I am a bit troubled by the wording of this section, seeming as it does to claim that petrol is much more energetic per unit mass than, say, LOX/RP1. I think that the difference is actually small when the entire mass of reactants is taken into account, as I think they really must be for a sensible comparison to be made. To my mind, simply neglecting to count the mass of the atmospheric oxygen needed for an automobile engine is really just a failure to do the accounting properly. Strictly speaking, I suppose one should even count the total mass of the air used by terrestrial engines, including nitrogen, water vapor, argon, etc.

Does anyone object to a revision of this section with this in mind? (Since the issue encompasses much of the section's content, it may make more sense to drop it as a separate section altogether, and merge the remnants with some other part of the article.) Wwheaton (talk) 07:23, 13 January 2008 (UTC)

No, no. The energy of RP1 and petrol are about the same. But the mass of LOX is twice that of RP1 and doesn't directly add any energy. So LOX/RP1 has one third the energy of petrol.- (User) WolfKeeper (Talk) 22:28, 24 February 2008 (UTC)

Your point about the atmosphere seems good on the surface, but ultimately the energy density refers to the specific energy of the *propellant* which is what you carry around and hence costs you, so a definition including the atmosphere isn't helpful.- (User) WolfKeeper (Talk) 22:28, 24 February 2008 (UTC)

To make this concrete, consider two engines, one rocket, the other airbreathing that both propel a similar mass aircraft under otherwise identical conditions (with same actual exhaust velocity). Which would go further?- (User) WolfKeeper (Talk) 22:28, 24 February 2008 (UTC)

Gee, Wolfkeeper, I guess I really disagree if the section title is "Chemistry". In comparing the energetics of chemical reactions, I think one always includes all the reactants (or the propellants, in this context). Propellants to my mind include both fuel and oxidizers, wherever you get them. The energy is not in the fuel or the oxidizer, but in the combination. So from a chemical point of view, there is no difference, because the total energy and total mass of the chemical reaction, O2+2H2 ==>> 2H2O is unchanged, and thus the specific energy (= total energy per unit total mass) is also unchanged. Of course if you are flying in the atmosphere, it makes excellent sense to make use of the oxygen that is available for free! (If you were jetting around in Jupiter's atmosphere, you would have to carry your LOX, and draw H2 from the atmosphere there. But that would not change the chemistry.) Anyhow, I would say that for a rocket in space, or a scramjet on Earth (or on Jupiter), the energetics are unchanged. Otherwise we would have to say the "energy density of petrol" depends on where you live (ie, the availability and thus the price of oxygen), rather than being a property of the propellants themselves. Certainly the different contexts have vastly different and important practical consequences, no one could deny that. In your comparison example, the air-breather (at sea level) would surely go further, but I think that would have nothing to do with chemistry.

But I guess this is more a discussion of semantics and the proper meanings of words than about substance, so maybe we should solicit some other opinions? I am a more of a scientist myself, certainly not an aerospace engineer, so perhaps others will see it differently. I suspect chemists would take my side, but that is maybe not the last word. I would not want to confuse our "average reader", whoever that might be. Cheers, Bill Wwheaton (talk) 04:11, 25 February 2008 (UTC)

I am neither chemist nor aerospace engineer, and the phrase "specific reaction energy density" -- which appears in the first sentence of the "Chemistry" section -- has no special meaning for me. So I did a google search for that exact phrase, and the only "hit" was this wikipedia article. That leads me to ask the question: what is the source for the sentence in which that phrase appears? (sdsds - talk) 04:28, 25 February 2008 (UTC)

Yes, I agree, that phrase doesn't make sense really.- (User) WolfKeeper (Talk) 06:17, 25 February 2008 (UTC)

Good question. I hope I did not introduce it, though I did work on that section. I may have been trying to do minimal violence to the earlier version, I dunno. I assume "density" is redundant or meaningless here, meaning "per unit volume" or "per unit area" or some such, not important in the context of rocket engines, where to my mind, "specific energy" means energy per unit mass.

But that in itself does not really answer the question, since the issue seems to center around the mass of what? I would think "total mass of reactants", in the context of chemical reaction energy, especially since for a rocket engine we have to carry it all along. But the way the section was phrased originally (when I first touched it), it addressed comparisons between rockets and other kinds of motors. There are lots of google hits on {chemistry "specific energy"} (126,000 I see), about half apparently meaning particular energy, as "an atomic transition happens at a specific [ie, particular] energy", etc. Discarding those, almost everything I see is energy per unit mass, but I've not yet seen a definitive answer to the "mass of what?" question. The Wiki article on "energy density" makes it clear that comparisons within and across fields are common. I'll sleep on it.... Bill Wwheaton (talk) 06:40, 25 February 2008 (UTC)

After thinking about it a bit more, I guess I feel we are maybe tilting at windmills, since this article is about rocket engines. Probably there is no need to use such awkward terms, or settle on their correct usage across a wide range of subfields. I think it is correct to say that in the context of chemical rocket engines, which do carry along all their propellants, chemical energy per unit mass is what matters, except for the detail about the ratio of specific heats (and of course numerous practical side issues, e.g. about stability [can't use ozone this month...]). So I think I would favor changing the murky "specific reaction energy density (energy per unit mass)" to just "specific energy (energy per unit mass)". We could perhaps say "chemical energy per unit mass" without causing confusion. Then maybe we should do something about the next-to-last paragraph of the section. I think it is factually right, but maybe adds little that is essential. It is a relic, heavily modified, of the earlier version of before Jan 13. If no one objects, I think I will go ahead and fix that first line. Best, Bill Wwheaton (talk) 19:40, 25 February 2008 (UTC)

Size letters

I came here looking for an explanation of the rocket size letter system, but I couldn't find any info here, nor could I find a link to it. Does anyone have an idea how this could be included in the article? --Doradus (talk) 13:09, 21 January 2009 (UTC)

Hmm, we could perhaps link to it at the bottom under see-also. Those are model rocket motors, and if you go there you find it discusses it some, but then points you to Model rocket motor classification where it lives.- (User) Wolfkeeper (Talk) 14:00, 21 January 2009 (UTC)

hybrid rocket article

I just wanted to drop a quick note here that the hybrid rocket article needs a lot of work and I have started doing some modifications. There is essentially no discussion associated with that article and I want to try and recruit some others to contribute. Thanks Jonny.dyer (talk) 04:26, 23 April 2009 (UTC)

Optimal expansion

I take issue with the description of optimal expansion of a nozzle:

For optimal performance the pressure of the gas at the end of the nozzle should just equal the ambient pressure: if the exhaust's pressure is lower than the ambient pressure, then the vehicle will be slowed by the difference in pressure between the top of the engine and the exit; on the other hand, if the exhaust's pressure is higher, then exhaust pressure that could have been converted into thrust is not converted, and energy is wasted.

This is not an accurate description of why optimal expansion is optimal. Specifically, if the nozzle exit pressure is higher than atmospheric thrust is still generated but it is due to the pressure*area of the nozzle exit rather than the additional momentum that would have been imparted to the exhaust from further expansion. Saying that the thrust is "lost" is simply incorrect - it is just a less efficient conversion. This is clearly illustrated in eq. 2-14 of Sutton which gives $F={\dot {m}}v_{2}+(p_{2}-p_{3})A_{2}$. Eq. 3-29 gives the dependency of ${\dot {m}}v$ on $p_{2}-p_{3}$ and it is clear that the where Cf obtains an optimum is not obvious a-priori from this equation. A more accurate description would be that the momentum thrust is a more efficient use of the enthalpy in the flow than pressure thrust and thus one would like to maximize the contribution of the momentum thrust. Jonny.dyer (talk) 04:24, 23 April 2009 (UTC)

That's no good as an explanation; why would you not just maximise the momentum thrust and send the exhaust pressure down to vacuum pressures?- (User) Wolfkeeper (Talk) 13:26, 23 April 2009 (UTC)

Because you do generate a negative component of thrust exactly from the equation I sighted above. I am not saying that the end result isn't the same - the optimum is at P2 = P3. But the explanation for why that is is misleading at best. Specifically, the claim that "then the exhaust pressure that could have been converted into thrust is not converted" will confuse those that have seen the thrust equation above and wonder why there is a POSITIVE term obtained from exit pressure > ambient pressure. (continued below) Jonny.dyer (talk) 14:58, 23 April 2009 (UTC)

In fact, you cannot get the optimal expansion point from that equation; the V in the equation and Pe are not independent to changes in the expansion ratio. It is true that it turns out that Pe = Pa at the optimum expansion, but you need other equations to show that. It is useful to know that Pe = Pa when you have the equation though.- (User) Wolfkeeper (Talk) 17:24, 23 April 2009 (UTC)

It's probably easiest to see this simply in terms of pressures on the nozzle. Keeping mass flow constant, in terms of rings of metal in the nozzle; any metal that is present beyond the point where the pressure equals the ambient is slowing your vehicle down; the net thrust from that ring is negative as the atmospheric pressure on the outside is higher than pressure of the gas on the inside. Similarly if you could add metal to the nozzle and gain thrust because there is pressure above ambient there, then it's positive to add more nozzle. The exhaust velocity is irrelevant to see this; it doesn't matter what the exhaust velocity is, it's the net thrust of the engine that matters, which is simply the integral of pressures times area. However, clearly the peak thrust for any given mass flow is going to give you the maximum effective exhaust velocity, and it's easy to calculate what it is from the thrust and the mass flow.- (User) Wolfkeeper (Talk) 13:26, 23 April 2009 (UTC)

I understand this perfectly. Again, I am not taking issue with the general result, I am taking issue with the explanation. I think an explanation looking at it from a force balance point of view (as you do above) is much more relevant. The problem is whether you are looking at the nozzle as a "module" in isolation (as the thrust equation effectively does) or the whole engine from a pressure balance point of view. For someone interested in calculating the thrust a rocket produces, the thrust equation is the most useful construct and the explanation for optimum expansion on the page is will seem to contradict it. I understand that this was supposed to be an intuitive explanation for the lay man, but it is not intuitive - it is confusing. I think a diagram with a force balance would be much clearer as to why thrust is optimized when P2 = P3. How about this - I will work on one and submit it on the discussion page for you and others to comment on...Jonny.dyer (talk) 14:58, 23 April 2009 (UTC)

principle of operation

Good and useful article, but I think some of the principles shown at the beginning, especially the diagrams, could be amended. They inidcate that the primary mechanism of producing thrust are the difference in area between the front and rear surfaces of the combustion chamber, which is a tiny effect (the school teacher's explanation). In fact, as correctly shown later in the article, the primary mechanism is the momentum change as the mass flow is accelerated through the nozzle. --Raywilkinson (talk) 06:32, 24 April 2009 (UTC)

Actually, it's not a small effect, a typical nozzle coefficient is only about 2 or so, so half the thrust is due to the difference in areas.- (User) Wolfkeeper (Talk) 13:08, 24 April 2009 (UTC)

SSME highest Isp engine ever to fly?

According to its article (and the Delta IV and Delta III articles), the RL10B-2 is capable of an Isp of up to 462 seconds, whereas the SSME is quoted on its article as being capable of a vacuum Isp of up to 452.5 seconds. Presumably the figures came from Astronautix, which gives the same values. --Jatkins ^{(talk - contribs)} 17:52, 26 July 2009 (UTC)

Closed cycle and/or regenerative cycle engines

I believe these are different principles of supplying fuel or keeping the nozzle cool on a rocket engine. ISTR one was preferred by the Soviet Union and American engines like the F-1 and J-1 use the other. I sort of expected some rocket scientist or other would have set some explanation of the difference out here, or am dreaming? I couldn't see one. Soarhead77 (talk) 21:45, 28 July 2009 (UTC)

Is this a fifth type of cycle?

In the main article there are small sections covering four cycles of rocket engine:

• Pressure fed cycle
• Expander cycle
• Gas Generator cycle
• Staged Combustion cycle

Is there a cycle called a regenerative cycle? Is this one of the four above, or is it different? Soarhead77 (talk) 21:56, 28 July 2009 (UTC)

There's regenerative cooling, which is cooling the nozzle with a cryogenic fuel, such as liquid hydrogen. The metal jackets around the Space Shuttle main engine nozzle and the F-1 are examples of this. --Jatkins ^{(talk - contribs)} 16:42, 30 July 2009 (UTC)

Move and rename an image

Any views on moving File:Hydrogen Oxygen External Combustion Engine.png down the page to rest next to the "staged combustion cycle" paragraph? The caption would need to be changed to reflect the relevance. --Old Moonraker (talk) 22:21, 16 December 2009 (UTC)

Ecologic rocket engines

The page Zero-emission_rocket_propulsion could be linked here and the article can be expanded. 91.182.161.120 (talk) 14:21, 26 January 2010 (UTC)

External combustion engine

I think a "rocket engine" can also be considered a external combustion engine. I thus recommend merging. 81.241.109.50 (talk) 07:44, 14 January 2010 (UTC)

No, some rockets are external combustion engines, but most are actually internal combustion engine.

The terms 'external' and 'internal' combustion are correctly considered to be somewhat misnomers; internal combustion engines work by using the combustion products directly to create power; whereas external combustion engines use a heat exchanger to transfer the heat from the combustion process into a separate working fluid (usually steam) that does the work. Because rockets use the combustion products acting directly on the nozzle to generate kinetic energy, they are very typically internal combustion engines. There are a few rockets that use nuclear power; these are more or less external 'combustion' engines though. Hope this helps.- Wolfkeeper 15:07, 26 January 2010 (UTC)

The aeolipile is an external combustion rocket engine on a bearing.- Wolfkeeper 15:07, 26 January 2010 (UTC)

Combustion chamber: stoichiometric temperature??

"Unlike in air-breathing jet engines, no atmospheric nitrogen is present to dilute and cool the combustion, and the temperature can reach true stoichiometric." What's a stoichiometric temperature? I'm guessing from the comment about nitrogen that this is describing how the ratio of the reactants can be perfectly balanced, but surely this situation is perfectly possible in atmospheric combustion? Is this just a poorly worded (and incorrect) point, or is there something else to it? — Preceding unsigned comment added by 86.185.20.243 (talk) 15:45, 5 August 2011 (UTC)

Yeah, this section needs to be fixed, but I'm not sure how. One suggestion is to replace "stoichiometric" with "stoichiometric value" (referring, I think, to the "maximum" adiabatic flame temperature). I'm not sure if that was the intention or if it would be correct in meaning.

The point is that "temperature" is just another word for "mean kinetic energy of the gas molecules", which is (slightly simplistically) the kinetic energy of the exhaust gas coming out of the nozzle. If inert nitrogen is mixed in, that reduces the energy, and thus the exhaust velocity. "Stoichiometric" just means the ratio from the chemical balance equations, with no extraneous material (ie, nitrogen) mixed in to dilute the energy of combustion.

Here are two complications: first, the molecules rotate and tumble, as well as streaming out. Some kinetic energy is generally effectively lost from the stoichiometric values due to that. Second, the thermodynamic equilibrium composition of the exhaust is not generally the same as the products of the simple chemical reaction equation suggest. For example, H2 & O2 burning to make steam seems simple, but at high temperature, steam breaks down into H2 & O2, so the exhaust contains a mixture of incompletely burned gases. High pressure pushes this towards pure steam. (Which is part of the reason the SSMEs ran at such high pressure. Note that this goes the wrong way as the gas expands out the nozzle.) Also a little excess hydrogen beyond the stoichiometric value also improves the exhaust velocity by lowering the mean weight of the product molecules in the exhaust, from 18 for water towards H2. Wwheaton (talk) 14:53, 14 September 2012 (UTC)

rocket dynamics

can u explain what is rocket dynamics/ — Preceding unsigned comment added by 117.211.88.171 (talk) 08:25, 14 September 2012 (UTC)

I don't find that term in the article, only in the old discussion (subject, "Performance") above. Generally in physics, "statics" refers to the forces among things that are not moving; "kinematics" to simple descriptions of motion, and "dynamics" to changes in motion due to forces, as involving gravity, thrust, momentum and energy of exhaust and vehicle, etc. Tutorial issues like this are not very appropriate for article talk pages; if you register as a Wikipedia user, you get your own talk page, where other users may answer your particular questions at more length. Wwheaton (talk) 15:11, 14 September 2012 (UTC)

Gas Core Reactor Section

I cannot find any examples of Gas Core Reactors ever being built (if it happened it was likely in the 60s, there are sources I can't access from then and the 70s), thus making it 'entirely theoretical' per the language of other entries). However I also can't find a source saying they haven't been built. This is the sort of thing citation needed is for, but the error exists in the negative space. As a new editor I'd appreciate some guidance on how to handle this issue. RequiaAngelite (talk) 17:10, 14 July 2015 (UTC)

Please explain your revert

@GliderMaven: Per WP:Revert please explain why you reverted my edit. Note that the term 'oberth effect' does not feature even once in my rocket engine engineering textbook (searchable ebook version), and removed information should not be added without a source.

The follow up edit is by far even more incorrect, again, please source the statement. — Preceding unsigned comment added by RequiaAngelite (talk • contribs) 05:32, 6 August 2015 (UTC)

The Oberth effect article has a poorly written introduction which might be contributing to this confusion. It is written passively, using the lead sentence to introduce an (unverified) "Oberth effect", which seems to be a synonym for "slingshot maneuver" (which we redirect to Gravity assist. The lead sentence should summarize what the Oberth effect is, and talk about the maneuver in later sentences. Oberth maneuver redirects to Oberth effect, which is arguably wrong; it probably should redirect to Gravity assist. There also seems to be no verification of usage of the term "Oberth maneuver".

Bottom line: I think you may be right (but I can't prove it one way or the other.) JustinTime55 (talk) 13:24, 6 August 2015 (UTC)

No, the Oberth Maneuver is a powered flyby of a planet, and doesn't use the slingshot effect. The Oberth effect is simply that the use of a rocket at high speed generates more useful power; that is what happens during the Oberth Maneuver because the vehicle is moving at higher speed at periapsis. If you think about it the Oberth effect also happens at all points of a rocket's flight; the useful power output is continuously increasing as it accelerates.GliderMaven (talk) 14:08, 6 August 2015 (UTC)

What is the exit pressure to which rockets can be throttled down?

"Rockets can usually be throttled down to an exit pressure of about one-third of ambient pressure"

This statement has been uncited. An editor Mduvigneaud (talk · contribs) (who admits he isn't educated in rocketry) doubted the pressure could be less than ambient, not understanding Bernoulli's principle. GliderMaven (talk · contribs) then reverted my reversion of his edit. @GliderMaven:, did you read his version, or did you just make a gut-reaction reversion because you didn't like my edit to the nozzle pictre caption (which is a separate issue in itself)? There are some questions and problems which have to be straightened out before we can correct this statement:

• By "exit pressure", are you talking about the total pressure in the chamber, or the exit static pressure at the throat? The latter usually is below ambient pressure (in the atmosphere; it obviously can't go below 0 in vacuum).
• How does the exit pressure even relate to throttling the engine, which we define as reducing the propellant flow rate? The exit static pressure is determined by the throat static pressure and the ratio of exit to throat area, and by the total chamber pressure. How does the chamber pressure (which for a liquid-fuel rocket is determined either by turbopumps or by a pressurant such as helium) vary with the propellant burn? JustinTime55 (talk) 17:36, 14 September 2015 (UTC)

The exit pressure is the sideways pressure of the jet as it exits the nozzle. In rockets both the chamber pressure and exit pressure is proportional to the propellant flow rate (because once the nozzle chokes, the speed is constant, as is the temperature but the density/pressure varies PV=nRT). If the exit pressure is below ambient (overexpanded), then the jet is narrowed by air pressure; if the jet pressure is above then it's underexpanded, and the jet will expand once it's left the rocket. In a vacuum as you say, you can obviously never be overexpanded.GliderMaven (talk) 18:46, 14 September 2015 (UTC)

I would prefer to revert my revision and allow those who know more about the subject than me improve it. My original edit (which I intended to add here to the Talk section) was an attempt to clarify wording. Does anyone object to me reverting my revision? User:Mduvigneaud (talk) 01:36, 19 September 2015 (UTC)

Yes, those of us who are more knowledgeable would object very much. The statement was uncited, but GliderMaven now has inserted a citation, so the statement should be vverifiable. And it is quite possible, as I said, for the exit pressure to go below ambient (as long as ambient is not a vacuum), and it quite frequently does at low altitudes, due to the high nozzle expansion ratios rockets use; see Rocket engine#Nozzles. JustinTime55 (talk) 13:22, 21 September 2015 (UTC)

Actually, I would tend to agree with Mduvigneaud (talk · contribs), or at least, keeping the explanation with the diagram is clearer than the way it currently is.GliderMaven (talk) 15:07, 21 September 2015 (UTC)

There is no reason to move the diagram back to the throttling section; it does not illustrate throttling, but the different expansion ratio regimes of the nozzle, which is a completely different issue. There is no diagram which illustrates throttling, or why the engine can operate below ambient pressure (which has nothing to do with whether it is throttled or not.) JustinTime55 (talk) 15:16, 21 September 2015 (UTC)

Um. Yeah it does. If you throttle a rocket down by 50%, the exhaust jet pressure also goes down by 50%. Think about it.GliderMaven (talk) 01:46, 23 September 2015 (UTC)

The reason all of this "below ambient pressure" stuff happens isn't Bernoulli, it's because the jet is supersonic; changes in pressure can't travel up a supersonic jet because they travel at the speed of sound (pretty much by definition that's what sound is, changes in pressure.)GliderMaven (talk) 15:07, 21 September 2015 (UTC)

Fine, if you want to add an explanation for the lower pressure in the throttling section, go right ahead. (I would say a complete explanation of why the pressure can be lower would include both effects, though.) But the question remains: is the statement "throttled down to one-third of ambient pressure" actually verified by the Sutton book, or is it not? JustinTime55 (talk) 15:23, 21 September 2015 (UTC)

Actually, it's unnecessary to add any more explanation, because it's already there in Back pressure and optimal expansion:

Nozzle efficiency is affected by operation in the atmosphere because atmospheric pressure changes with altitude; but due to the supersonic speeds of the gas exiting from a rocket engine, the pressure of the jet may be either below or above ambient, and equilibrium between the two is not reached at all altitudes (see diagram).

JustinTime55 (talk) 20:37, 21 September 2015 (UTC)