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DDE

NTRs at sea level

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I've been toying with vertical SSTO rocketships over the last few days. In particular, I had quite high hopes for @Nertea's pebble bed lOx-augmented modded NTRs, however, they still turned out to be more mass than wallop at take-off.

So I have this non-KSP question: why do vacuum-oriented chemical and nuclear rockets lose this much thrust ASL, and why then are nuclear lightbulbs frequently touted as the SSTO engine of choice in hard sci-fi?

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The optimal condition is when the pressure at the nozzle exaust is equal to the ambient pressure. Liftoff engine have a short nozzle with a high exit pressure (less than 1 atm but higher the vaccum). Vaccum nozzles are longer with the lowest possible exit pressure. When you use a vaccum engine ASL you are lossing performance .

Take a look here for more details: http://www.braeunig.us/space/propuls.htm

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Let me see if I get this right:

All rocket engines are made less efficient by the presence of an atmosphere, the air pressure resists the expasion of the exhaust, reducing the attainable exhaust velocity and reducing efficiency.

To make a rocket more efficient, the larger the nozzle the better to extract energy from the exhaust expansion, the more expansion of the exhaust that takes place within the nozzle, the more energy can be extracted from it (in practice this means: the more the exhaust can be accelerated). In an atmosphere there is an upper limit on how much benefit you can get from a larger nozzle. At some point the exhaust will "detach" from the nozzle (due to atmspheric pressure preventing further expansion) and any extra nozzle beyond this point is dead weight.

In an atmosphere, the pressure of the exhaust gases at the nozzle exit ought to just about equal local atmospheric pressure - so any given nozzle has an altitude at which its performance is optimum.

In vacuum, you can make your nozzle as large as you like, other practical matters place limits on it instead, and after a certain size the returns will be diminishing.

 

So its not that sea-level-rated engines work better with a smaller nozzle, its that adding a larger one wouldn't do you any good. 

 

As for NTR's, as far as I understand it, there is nothing about NTRs which makes them operate differently than normal rockets with respect o the presence of an atmosphere, all of the above applies to them equally. Its their TWR which makes them less suitable for ascent (radiation concerns notwithstanding), very high thrust versions would need a very large reactor which would be extremely heavy. I've actually never seen a nuclear lightbulb that often in sci-fi, but they are just about the highest-performance concept in terms of NTRs so if any NTR is going to get you off the ground it will be one of those.

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32 minutes ago, p1t1o said:

Let me see if I get this right:

All rocket engines are made less efficient by the presence of an atmosphere, the air pressure resists the expasion of the exhaust, reducing the attainable exhaust velocity and reducing efficiency.

To make a rocket more efficient, the larger the nozzle the better to extract energy from the exhaust expansion, the more expansion of the exhaust that takes place within the nozzle, the more energy can be extracted from it (in practice this means: the more the exhaust can be accelerated). In an atmosphere there is an upper limit on how much benefit you can get from a larger nozzle. At some point the exhaust will "detach" from the nozzle (due to atmspheric pressure preventing further expansion) and any extra nozzle beyond this point is dead weight.

In an atmosphere, the pressure of the exhaust gases at the nozzle exit ought to just about equal local atmospheric pressure - so any given nozzle has an altitude at which its performance is optimum.

In vacuum, you can make your nozzle as large as you like, other practical matters place limits on it instead, and after a certain size the returns will be diminishing.

 

So its not that sea-level-rated engines work better with a smaller nozzle, its that adding a larger one wouldn't do you any good. 

 

As for NTR's, as far as I understand it, there is nothing about NTRs which makes them operate differently than normal rockets with respect o the presence of an atmosphere, all of the above applies to them equally. Its their TWR which makes them less suitable for ascent (radiation concerns notwithstanding), very high thrust versions would need a very large reactor which would be extremely heavy. I've actually never seen a nuclear lightbulb that often in sci-fi, but they are just about the highest-performance concept in terms of NTRs so if any NTR is going to get you off the ground it will be one of those.

An NTR has a exhaust vector velocity at the back of the engine that steadily  builds pressure unitl it passes the radioactive material from that point momentum and plasma/gas dynamics act to  creat thrust.. The light proced by a chemical rocket is indicative of remaining reactions that are occurring in the nozzle and later as the gas hits the air and ignites. In NTR this is not the cases, the hydrogen is heated, it accelerates, it is realeased and cools down. It it gets got enough plasma forms then it recombines. All of this occurs before the nozzle, nothing after.

 

I ponder the OP wants to go.. The problem with NTR is TWR, low, And heat contsinment problem is high. Its not something easily scaled for launch. This is a post orbital dV engine, primarily designed for a long burn from parking orbit to planetary targets. As applied it has been generally rejected by everyone. 

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On ‎11‎.‎07‎.‎2016 at 7:26 PM, PB666 said:

I ponder the OP wants to go.. The problem with NTR is TWR, low, And heat contsinment problem is high. Its not something easily scaled for launch. This is a post orbital dV engine, primarily designed for a long burn from parking orbit to planetary targets. As applied it has been generally rejected by everyone. 

What I'm pondering is why there aren't that many solutions that push for mass flows sufficient for launch, by using lOx afterburners and hotter core designs (up to an including gas cores).

Cooling isn't that of a problem; reaction mass is the coolant.

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2 hours ago, DDE said:

What I'm pondering is why there aren't that many solutions that push for mass flows sufficient for launch, by using lOx afterburners and hotter core designs (up to an including gas cores).

Cooling isn't that of a problem; reaction mass is the coolant.

Trading reaction mass for cooling will destroy your ISP (the whole point of the NTR).

There is also the issue for stop/starting your reaction and cooling after your burn.  You can't just remove fuel and oxidizer to stop a nuclear reaction.  I'm guessing that you would need to separate your reactor mass (uranium) as well as add control rods, something that nuclear plants either don't need/can't do (I suspect anything that would provide a "fast SCRAM" would already be implemented, thus implying that it is rather hard engineering problem).

I wouldn't put too much faith in afterburners.  It first assumes you aren't heating your fuel up enough (otherwise heating it up any more would melt rocket internals/nozzles).  It should (barely) work in the nozzles, but only after expansion had allowed the gasses to cool off enough.  You would then be heating up low amounts of fuel back up to exhaust velocity but have to deal with somehow having air entering you nozzle, a difficult situation (maybe some sort of two-stage aerospike?).  In any event expect to lost nearly (if not more) as you gain.

"Because science fiction stories" isn't a good means to drive the methods of engineering.  "Because I want a flying car*/tricorder/warp drive" might generate good results.  Trying to build semiconductors by using positrons to etch the chips "because Issac Asimov" won't generate anything good.

* forget the "flying car" and go for short/zero takeoff and landing.  But don't expect to be good at both flying and taxiing.

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On 7/11/2016 at 5:32 PM, p1t1o said:

All rocket engines are made less efficient by the presence of an atmosphere, the air pressure resists the expasion of the exhaust, reducing the attainable exhaust velocity and reducing efficiency.

Yes. But. You're missing the elephant in the room.

In order to get away from earth, you need a sufficiently high TWR. If you want to do that with hydrogen, you need insane amounts of it. Slightly a propos video:

 

That atmospheric ISP is lower than vacuum doesn't help either, but that effect is comparatively small (~25% performance loss at sea level) and besides, you have that problem with *any* fuel. The bigger challenge is that a low-density fuel requires much bigger pipes and pumps to move the same mass/time, and also much wider engines to expel the low-density exhaust.

A high TWR is much easier to achieve if you toss out denser propellants. Using kerolox, you get like 6-8 times as much thrust out of a similar-sized engine (compared to hydrolox); and with solids it gets better still. Heavier molecules mean lower exhaust speed (=lower ISP), but on first stages the tradeoff is very much worth it.

This doesn't speak against NTRs, per se. But they should probably use water or mercury as a propellant. I guess the former has been explored in some detail, and theoretical performance data can be found if you bother to look for it.

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9 hours ago, DDE said:

What I'm pondering is why there aren't that many solutions that push for mass flows sufficient for launch, by using lOx afterburners and hotter core designs (up to an including gas cores).

Cooling isn't that of a problem; reaction mass is the coolant.

Cooling is a big problem, you are not talking about pressure based acceleration, where aprt of the energy is stored in the form of potential, its pretty much all driven by thermal motion on the outside opthe core, its all heat, the higher the ISp the hotter it is, the add to that trapping aspect of increasing volume for a bigger engine, and you are in meltdown territory, you coukd increase the size of the engine at the expense of ISP until you are down in SSME territory. 

The core including the engine an uranium has a relatively high mass fraction, at low ISP most of the energy in the core is simply wasted as heat without any ejecta, once the rection starts heat production is difficult to stop. The most extensive use is for as high as temperature as possible for as long as possoble burning as much fuel as possible to as high as ISP as possible. Burning thorough a launch just does make any sense. 

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11 hours ago, wumpus said:

Trading reaction mass for cooling will destroy your ISP (the whole point of the NTR).

There is also the issue for stop/starting your reaction and cooling after your burn.  You can't just remove fuel and oxidizer to stop a nuclear reaction.  I'm guessing that you would need to separate your reactor mass (uranium) as well as add control rods, something that nuclear plants either don't need/can't do (I suspect anything that would provide a "fast SCRAM" would already be implemented, thus implying that it is rather hard engineering problem).

Then go hotter. That's why I've mentioned gas-core lightbulbs, they should pack enough energy to keep up Isp despite higher mass flow.

NERVA had control drums, other systems would presumably carefully evacuate the fuel - such as UF6 gas - to containment.

11 hours ago, wumpus said:

I wouldn't put too much faith in afterburners.  It first assumes you aren't heating your fuel up enough (otherwise heating it up any more would melt rocket internals/nozzles).  It should (barely) work in the nozzles, but only after expansion had allowed the gasses to cool off enough.  You would then be heating up low amounts of fuel back up to exhaust velocity but have to deal with somehow having air entering you nozzle, a difficult situation (maybe some sort of two-stage aerospike?).  In any event expect to lost nearly (if not more) as you gain.

By afterburner I meant injecting liquid oxygen into preheated hydrogen for a conventional hydrolox reaction in the de Laval nozzle. NASA's contractors seem to think that such a LANTR would work.

11 hours ago, Laie said:

Yes. But. You're missing the elephant in the room.

In order to get away from earth, you need a sufficiently high TWR. If you want to do that with hydrogen, you need insane amounts of it.

That atmospheric ISP is lower than vacuum doesn't help either, but that effect is comparatively small (~25% performance loss at sea level) and besides, you have that problem with *any* fuel. The bigger challenge is that a low-density fuel requires much bigger pipes and pumps to move the same mass/time, and also much wider engines to expel the low-density exhaust.

A high TWR is much easier to achieve if you toss out denser propellants. Using kerolox, you get like 6-8 times as much thrust out of a similar-sized engine (compared to hydrolox); and with solids it gets better still. Heavier molecules mean lower exhaust speed (=lower ISP), but on first stages the tradeoff is very much worth it.

This doesn't speak against NTRs, per se. But they should probably use water or mercury as a propellant. I guess the former has been explored in some detail, and theoretical performance data can be found if you bother to look for it.

Clearly Delta IVs are missing the elephant in the room as well.

As to water NTRs, to quote the guy over at https://childrenofadeadearth.wordpress.com ,

Quote

The trouble with combustion rockets, particularly the LOX/LH2 rocket, is the propellants. As mentioned in the previous post (Slosh Baffles), each propellant tank has an ultimate mass ratio ceiling. Roughly speaking, higher density propellants have higher allowed mass ratios. Given standard tank materials, water has an excellent mass ratio ceiling (in the hundreds), while hydrogen has an awful mass ratio limit (< 10 generally).

Which is why shipbuilders in his quote-game-unquote ended up gravitating to methane and water NTRs as well as methane-fluorine rockets; because the game is about combat, the size of requisite tankage is a major limitation.

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