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Re: Landing rockets upright is unnecessary?

Posted: Tue Feb 09, 2016 5:06 pm UTC
by Wolfkeeper
KarenRei wrote:
Sockmonkey wrote:I think what some people are suggesting is essentially a scramjet/ramjet where the flame-holder is a rocket engine that provides the initial startup thrust to get it up to ramjet operational speeds then throttles back as the ramjet portion starts providing more thrust.


Not exactly. The concept presented doesn't inherently have to deal with any combustion with the air brought in - the air is a working fluid. It could just as well be martian air or venerian air or whatnot.

A rocket's efficiency is at a maximum when all of the heat of combustion and all of the pressure have been fully expanded to ambient temperature and pressure.

No. That's a finite length nozzle, except in a perfect vacuum. And even there, there's a optimum nozzle length where increasing the length adds weight and slows you down overall.
This would require an infinitely large nozzle, so one does as much as they can with the nozzle that they have. The more gas involved (even gas that doesn't take part in the reaction), the more fully they can approach ambient for a given-sized nozzle.

No, and actually the space shuttle main engines went quite a long way below ambient at take-off. The flow was about three psi. It's just that going below ambient reduces the thrust. That was fine for the space shuttle, they wanted to make sure that the engine was working properly before take off.
The point of air-augmentation in rockets - whether or not it can also attain some degree of ramjet functionality - is to make use of the air as additional working fluid to increase your rocket's efficiency.

The problem is that the extra weight of the duct and associated equipment usually cancels out the extra performance advantages.

SABRe is about the only one that looks like it's a major advantage.

Re: Landing rockets upright is unnecessary?

Posted: Tue Feb 09, 2016 9:01 pm UTC
by KarenRei
Wolfkeeper wrote:
KarenRei wrote:A rocket's efficiency is at a maximum when all of the heat of combustion and all of the pressure have been fully expanded to ambient temperature and pressure.

No. That's a finite length nozzle, except in a perfect vacuum. And even there, there's a optimum nozzle length where increasing the length adds weight and slows you down overall.


When one talking about hypotheticals like infinite length nozzles, do you really think that we're considering things like skin drag? We're just talking about just the basics here. The optimal length and expansion ratio of a vacuum nozzle are both infinite. The optimal expansion ratio of nozzle for atmospheric operation is finite while the optimal length is infinite. This is captured by the length ratio parameter of the nozzle, which is measured proportional to the length of a 15° conical nozzle that achieves the same expansion ratio. For example, a 60% length ratio and a 10:1 expansion ratio begins at ~32° and ends at ~17°, while one with a 100% length ratio begins at ~22% and ends at ~6°. Because the stream is oriented more straight out the back with a longer length ratio and a variety of other effects, the rocket is more efficient - but then you're hit with all of the practical problems, the foremost of them being weight. It's a case of rapidly diminishing returns, so usually you end up with a length ratio of around 80% to balance out all factors.

The more gas involved (even gas that doesn't take part in the reaction), the more fully they can approach ambient for a given-sized nozzle.

No, and actually the space shuttle main engines went quite a long way below ambient at take-off. The flow was about three psi.


"Below ambient"? What are you talking about? What is the "ambient fuel ratio" of the atmosphere? What fuel ratio is "three psi". We're talking about fuel-oxidizer ratios here and why one runs fuel rich. Which the SSMEs absolutely do do.

The problem is that the extra weight of the duct and associated equipment usually cancels out the extra performance advantages.


Whether it's worth the weight is an entirely different question.

Re: Landing rockets upright is unnecessary?

Posted: Tue Feb 09, 2016 9:56 pm UTC
by sevenperforce
KarenRei wrote:
Wolfkeeper wrote:The problem is that the extra weight of the duct and associated equipment usually cancels out the extra performance advantages.


Whether it's worth the weight is an entirely different question.

Slapping ducts onto an existing rocket design is not very smart.

Building a rocket engine designed from the ground up for air augmentation, on the other hand...

Re: Landing rockets upright is unnecessary?

Posted: Wed Feb 10, 2016 1:11 am UTC
by Wolfkeeper
KarenRei wrote:
Wolfkeeper wrote:
KarenRei wrote:A rocket's efficiency is at a maximum when all of the heat of combustion and all of the pressure have been fully expanded to ambient temperature and pressure.

No. That's a finite length nozzle, except in a perfect vacuum. And even there, there's a optimum nozzle length where increasing the length adds weight and slows you down overall.


When one talking about hypotheticals infinite length nozzles, do you really think that we're considering things like skin drag? We're just talking about just the basics here. The optimal length and expansion ratio of a vacuum nozzle are both infinite.

Nope. Beyond a certain expansion ratio the vapour freezes solid. You can't expand solid material any further.

Nor is an infinite length nozzle optimal, because the nozzle weighs something.

The optimal expansion ratio of nozzle for atmospheric operation is finite while the optimal length is infinite.

Not for any realistic definition of 'optimal', because the nozzle weighs something, and twice as much when it's twice as long.

This is captured by the length ratio parameter of the nozzle, which is measured proportional to the length of a 15° conical nozzle that achieves the same expansion ratio. For example, a 60% length ratio and a 10:1 expansion ratio begins at ~32° and ends at ~17°, while one with a 100% length ratio begins at ~22% and ends at ~6°. Because the stream is oriented more straight out the back with a longer length ratio and a variety of other effects, the rocket is more efficient - but then you're hit with all of the practical problems, the foremost of them being weight. It's a case of rapidly diminishing returns, so usually you end up with a length ratio of around 80% to balance out all factors.

Yes, and the difference makes maybe 1% of the Isp. It's mostly irrelevant. The reason 80% nozzles are used is simply weight, they're lighter.

The more gas involved (even gas that doesn't take part in the reaction), the more fully they can approach ambient for a given-sized nozzle.

No, and actually the space shuttle main engines went quite a long way below ambient at take-off. The flow was about three psi.


"Below ambient"? What are you talking about? What is the "ambient fuel ratio" of the atmosphere?

I don't have a fucking clue what you're talking about. Most people refer to the exit pressure of a rocket nozzle relative to ambient pressure. The absolute exit pressure of the SSME was around 3psi, compared to sea level pressure of 15 psi; the jet was literally emitted as a partial vacuum. NASA did something sneaky at the rim to prevent the flow separating.

What fuel ratio is "three psi". We're talking about fuel-oxidizer ratios here and why one runs fuel rich. Which the SSMEs absolutely do do.


High fuel ratios are used at relatively low pressures to improve the Isp; at high pressure they run nearer to stochiometric.

The problem is that the extra weight of the duct and associated equipment usually cancels out the extra performance advantages.


Whether it's worth the weight is an entirely different question.

Not usually, except probably SABRE.

sevenperforce wrote:
KarenRei wrote:
Wolfkeeper wrote:The problem is that the extra weight of the duct and associated equipment usually cancels out the extra performance advantages.


Whether it's worth the weight is an entirely different question.

Slapping ducts onto an existing rocket design is not very smart.

Building a rocket engine designed from the ground up for air augmentation, on the other hand...


Not really, it's been studied loads of time, and virtually never works, not even scramjets seem to be SSTO class; they hit issues around Mach 15 with reaction rates. Only SABRE seems to offer significant real benefit that I've seen, that precooler seems to be a huge win.

Re: Landing rockets upright is unnecessary?

Posted: Wed Feb 10, 2016 2:30 pm UTC
by KarenRei
Nope. Beyond a certain expansion ratio the vapour freezes solid. You can't expand solid material any further.


Now you're back to the earlier, primary point: the advantages of having a working fluid (that's one of many).

Nor is an infinite length nozzle optimal, because the nozzle weighs something.


It's almost like you're not even reading.

No, and actually the space shuttle main engines went quite a long way below ambient at take-off. The flow was about three psi.


"Below ambient"? What are you talking about? What is the "ambient fuel ratio" of the atmosphere?

I don't have a fucking clue what you're talking about.


You and me both. You brought up the concept of the SSME being "below ambient" in response to my stating that it doesn't burn a stoichiometric ratio of H2 and LOX, which is an absurd non sequitur. Care to try again? I'll lead once again: the SSME (and most rockets) run fuel rich to increase the amount of low molecular weight/low boiling point substances, ideally hydrogen, in the exhaust, in order to act as a working fluid.

What fuel ratio is "three psi". We're talking about fuel-oxidizer ratios here and why one runs fuel rich. Which the SSMEs absolutely do do.


High fuel ratios are used at relatively low pressures to improve the Isp; at high pressure they run nearer to stochiometric.


Fuel ratios are not measured in PSI, so I am still baffled by your prior statements. And no, the SSME does not vary its fuel ratio with ambient pressure. Saturn V varied its fuel ratio on some of its stages (S-II and S-IVB), but that was to change the thrust to ISP ratio (aka, to get more thrust when gravity losses are higher, more ISP when they were lower). That is to say, the S-II underwent EMR (Engine Mixture Ratio shift) 5 minutes 20 seconds after ignition, dropping from a mass ratio of 5,5:1 to 4,5:1 (aka, more fuel rich). This dropped the thrust from 1,15m lbf to 0,91m lbf but increased the ISP slightly from 423sec to 427sec. The same sort of thing happened in the S-IVB. Note that in both stages ambient pressure was pretty much irrelevant during the ratio shift. S-II ignited 2:44 into the burn.

Not really, it's been studied loads of time, and virtually never works, not even scramjets seem to be SSTO class


And you keep jumping back to scramjets, as if they're some sort of "ultimate incarnation" version of air augmentation, when the whole point of air augmentation is designed to be an ideal middle ground between scramjets and conventional nozzles, as they don't face the frozen combustion problems of scramjets. And no, they have not been "studied loads of times, and virtually never works". They have received some study, with generally positive results, but no large scale programs - like 99% of advanced rocketry concepts. Advanced rocketry concepts are cheap to come up with and do the theoretical work on, but the money to turn them into real deliverable systems is scarce.

Re: Landing rockets upright is unnecessary?

Posted: Wed Feb 10, 2016 3:00 pm UTC
by sevenperforce
For some reason, people really seem to think that if we can't get scramjets to work, then we definitely won't be able to get air-augmented/ducted rockets to work.

This is not correct. Scramjets are the most challenging sort of air-augmented engine imaginable.

Back to the OP topic: I get that landing a rocket on its tail is an inherently unstable arrangement. It's just that's virtually the only way to use the ascent engines to slow down.

Re: Landing rockets upright is unnecessary?

Posted: Wed Feb 10, 2016 8:58 pm UTC
by gmalivuk
KarenRei wrote:You brought up the concept of the SSME being "below ambient" in response to my stating that it doesn't burn a stoichiometric ratio of H2 and LOX, which is an absurd non sequitur.
That is false. You yourself started talking about ambient pressure first:
Wolfkeeper wrote:
KarenRei wrote:This would require an infinitely large nozzle, so one does as much as they can with the nozzle that they have. The more gas involved (even gas that doesn't take part in the reaction), the more fully they can approach ambient for a given-sized nozzle.
No, and actually the space shuttle main engines went quite a long way below ambient at take-off.

Re: Landing rockets upright is unnecessary?

Posted: Wed Feb 10, 2016 10:31 pm UTC
by KarenRei
gmalivuk wrote:
KarenRei wrote:You brought up the concept of the SSME being "below ambient" in response to my stating that it doesn't burn a stoichiometric ratio of H2 and LOX, which is an absurd non sequitur.
That is false. You yourself started talking about ambient pressure first:
Wolfkeeper wrote:
KarenRei wrote:This would require an infinitely large nozzle, so one does as much as they can with the nozzle that they have. The more gas involved (even gas that doesn't take part in the reaction), the more fully they can approach ambient for a given-sized nozzle.
No, and actually the space shuttle main engines went quite a long way below ambient at take-off.


Again, you're mixing up the expansion ratio - which yields a finite optimal area when ambient pressure is not zero - and nozzle length - which in an idealized situation (no skin drag, weight irrelevant, etc) - is optimally infinite. I've already pointed this out to you once, how many more times do I have to do so? The parameter in question here is the nozzle length ratio.

Re, "ambient": I'm not talking about pressure, I'm talking about ambient temperature. The maximum thermodynamic efficiency of a system is the Carnot limit, based on the difference between the hot reservoir and the cold reservoir. Mixing in augmented air reduces the temperature difference and thus the carnot efficiency, but offers a corresponding net increase in the working mass, canceling the difference out. But by achieving a lower exhaust temperature, you achieve more condensation reactions in the exhaust; for compounds with particularly high enthalpy of condensation, there can be significant energy released in this manner. However, you can't do any better than the ambient temperature, since that's the temperature of the air you're injecting into the exhaust

There is of course a lot of implementation complexity, well beyond things like skin drag and weight. For example, higher temperatures yield higher sonic velocities which in turn yield faster flow through the nozzle, so if your augmented air is going through the nozzle (not a fundamental requirement, many if not most designs simply have it intersect the exhaust stream), then you're choking the nozzle at a lower velocity. On the other hand, lower molecular weights increase sonic velocities as well, and atmospheric air is a lower molecular weight gas than some combustion products (CO2, Al2O3, etc) - and low molecular weight gases tend to have lower viscous drag as well. But then again, air is not in the ballpark of hydrogen in this regard. A longer nozzle may help with reaction freezing issues in terms of giving more length of time to burn, but at the same time, you're reducing the burn temperature (but unevenly), which can hinder or even prevent reactions. And on and on - it gets complex. And this is without getting into the potential of some fuel combustion with atmospheric air.

Re: Landing rockets upright is unnecessary?

Posted: Wed Feb 10, 2016 11:18 pm UTC
by gmalivuk
I'm not mixing up anything "again", as that was my first post on the topic.

As for the rest, clearing up the simple miscommunication of you talking about temperature and Wolfkeeper talking about pressure could have been done a lot faster and less condescendingly.

Re: Landing rockets upright is unnecessary?

Posted: Fri Feb 12, 2016 1:04 am UTC
by ijuin
sevenperforce wrote:
Back to the OP topic: I get that landing a rocket on its tail is an inherently unstable arrangement. It's just that's virtually the only way to use the ascent engines to slow down.


I think that the simplest thing to do to increase stability is to lower the center of mass and widen the landing legs. Yes, this will incur some mass penalty, but presumably less than adding delta-v for hover time would require.

Re: Landing rockets upright is unnecessary?

Posted: Fri Feb 12, 2016 2:14 pm UTC
by sevenperforce
ijuin wrote:
sevenperforce wrote:
Back to the OP topic: I get that landing a rocket on its tail is an inherently unstable arrangement. It's just that's virtually the only way to use the ascent engines to slow down.


I think that the simplest thing to do to increase stability is to lower the center of mass and widen the landing legs. Yes, this will incur some mass penalty, but presumably less than adding delta-v for hover time would require.

Yeah, as unstable as the Falcon 9 Stage 1 Booster landings look, they aren't too bad; the mass is all down there at the bottom because the tanks are virtually empty.

You can't really do that with a manned vehicle, though, because you need to have your crew module near the top of the rocket. Hence the problem with tail-first propulsive landings of manned spacecraft.

Re: Landing rockets upright is unnecessary?

Posted: Fri Feb 12, 2016 7:01 pm UTC
by sevenperforce
VTOL spaceplane:

Image

It's just over 90 meters long. With an internal tank volume exceeding that of the Space Shuttle's external tank, it can carry a huge amount of fuel. The rounded-body form factor allows the tanks to form structural elements, reducing total weight.

The 30 forward main engines, eight on each centerbody sidewall and seven on each inner wing section, cause a vortex to form which travels down the length of the shuttle on each side:

Image

This vortex is "caught" by the two rear-facing engines and eight additional angled thrusters to direct the full augmented flow backward:

Image

During takeoff, all downward-pointed engines (the rear wing-mounted engines and the centerbody engines) fire simultaneously with sufficient force to lift the shuttle straight up off the ground, air being pulled down between the wings and the centerbody to augment thrust. Then, the rear engines fire to propel the shuttle forward, while the vortex is initiated by throttling up the upward-pointed wing-mounted engines. This mode, while requiring additional thrust and a very specific design, allows for the safest possible takeoff and landing operation with maximal reusability.

The crew module contains its own fuel tank and six of the engines used for vertical takeoff, as well as four of the engines used for forward thrust. In any abort scenario, it can automatically detach from the rest of the craft and land propulsively. It is capable of independent re-entry.

Re: Landing rockets upright is unnecessary?

Posted: Sat Feb 13, 2016 9:03 am UTC
by ijuin
sevenperforce wrote:
ijuin wrote:
sevenperforce wrote:
Back to the OP topic: I get that landing a rocket on its tail is an inherently unstable arrangement. It's just that's virtually the only way to use the ascent engines to slow down.


I think that the simplest thing to do to increase stability is to lower the center of mass and widen the landing legs. Yes, this will incur some mass penalty, but presumably less than adding delta-v for hover time would require.

Yeah, as unstable as the Falcon 9 Stage 1 Booster landings look, they aren't too bad; the mass is all down there at the bottom because the tanks are virtually empty.

You can't really do that with a manned vehicle, though, because you need to have your crew module near the top of the rocket. Hence the problem with tail-first propulsive landings of manned spacecraft.


If the center of mass can not be brought any lower, then the landing legs will have to spread wider in order to widen the support base--ideally we want the rocket to be able to tilt considerably (e.g. 30 degrees) before it becomes more favorable for it to fall onto its side rather than falling into the upright position.

Re: Landing rockets upright is unnecessary?

Posted: Sat Feb 13, 2016 1:43 pm UTC
by sevenperforce
ijuin wrote:
sevenperforce wrote:Yeah, as unstable as the Falcon 9 Stage 1 Booster landings look, they aren't too bad; the mass is all down there at the bottom because the tanks are virtually empty.

You can't really do that with a manned vehicle, though, because you need to have your crew module near the top of the rocket. Hence the problem with tail-first propulsive landings of manned spacecraft.


If the center of mass can not be brought any lower, then the landing legs will have to spread wider in order to widen the support base--ideally we want the rocket to be able to tilt considerably (e.g. 30 degrees) before it becomes more favorable for it to fall onto its side rather than falling into the upright position.

Unfortunately, weight cost for wider landing legs becomes non-negligible quite rapidly.

Re: Landing rockets upright is unnecessary?

Posted: Sat Feb 13, 2016 6:31 pm UTC
by aph
So, is there a way to simulate landing in that kerbal game or some kind of a simulator? Could you just use the bullet library, some thrusters down and on the side and a control algorithm?

Re: Landing rockets upright is unnecessary?

Posted: Sun Feb 14, 2016 12:06 am UTC
by ijuin
sevenperforce wrote:Unfortunately, weight cost for wider landing legs becomes non-negligible quite rapidly.


Obviously, but it may be worth it for the landing legs to be somewhat wider than they are at present, even though not nearly as wide as I had earlier suggested. There is a "sweet spot" somewhere in here.

Re: Landing rockets upright is unnecessary?

Posted: Mon Feb 15, 2016 5:16 am UTC
by Copper Bezel
Wolfkeeper wrote:The absolute exit pressure of the SSME was around 3psi, compared to sea level pressure of 15 psi; the jet was literally emitted as a partial vacuum. NASA did something sneaky at the rim to prevent the flow separating.

Okay, this is not related to the discussion that this datum was a part of, but it indicates to me that I don't know how rockets or in fact gases work. So I want to make sure I'm getting all the technical terms right here in asking this, but why then does the burny stuff not then suck back up the fire hole and forget about the whole "expanding" and "pushing a rocket" business?

Re: Landing rockets upright is unnecessary?

Posted: Mon Feb 15, 2016 6:11 am UTC
by sevenperforce
Good question.

The flow doesn't get sucked back up into the bell because it's already moving at around 8,400 miles per hour. Although a pressure drop can result in a net force ((P1 - P2)*A), rockets operate more efficiently by simple momentum exchange. Ejecting a superheated mixture of water and diatomic hydrogen at over 2 miles per second results in far more net force than the backpressure force.

An ideal rocket bell is designed to match the ambient pressure to eliminate this loss, because you lose efficiency rapidly if your exhaust stream is over-expanded to too low a pressure and ends up getting choked by the ambient atmosphere. But for the SSMEs, the gains of a longer nozzle once outside of the atmosphere more than made up for the backpressure loss of sea-level overexpansion.

Re: Landing rockets upright is unnecessary?

Posted: Mon Feb 15, 2016 11:38 am UTC
by Copper Bezel
Huh. That's really interesting, and it tracks with what I know of the implosion that follows an explosion, etc. I guess that makes sense, that what I'd expect from ideal gas laws and things doesn't apply when a supersonic flow is involved.

Re: Landing rockets upright is unnecessary?

Posted: Mon Feb 15, 2016 2:48 pm UTC
by sevenperforce
The backpressure does cause the flow to slow down, but it's still going to be moving rather speedily. The more serious problem is that air getting sucked up into the rocket bell can cause unstable flow separation, potentually making the thrust vector fluctuate wildly. As a result, most engine bells intended for use from sea level all the way up to space err on the side of underexpansion. You can see this really obviously if you watch the Apollo launches; the early stages end up with really wide, puffy exhaust plumes because their pressure was much higher than ambient. With the SSMEs, NASA apparently used a jet of hot gas around the lip of the exhaust bell to prevent flow separation and thus achieve better vacuum Isp in exchange for a slightly lower launch ISP.

Re: Landing rockets upright is unnecessary?

Posted: Mon Feb 15, 2016 3:47 pm UTC
by Neil_Boekend
Unstable flow separation inside a rocket engine sounds like a spectacularly bad situation

Re: Landing rockets upright is unnecessary?

Posted: Mon Feb 15, 2016 3:57 pm UTC
by sevenperforce
Neil_Boekend wrote:Unstable flow separation inside a rocket engine sounds like a spectacularly bad situation

Considering that induced/controlled flow separation has been used as a primary means of gimballing...yeah, exceedingly bad. You-will-not-go-to-space-today-or-maybe-ever bad.