Nozzle Size Versus High Pressure And Scifi

[Spacescifi]

In my quest to design scifi SSTO's I realized that not only is waste heat a factor but so is the pressure coming from the exhaust plume on the way out.

 

Obviously heavy SSTO's would require performance and thrust higher than known chemical rockets, which also means their nozzle pressure would be higher than normal.

I tend to think that using a single main engine nozzle is good for such advanced rocket engines (be they fusion or whatever) since bigger is better when it comes to handling high pressure exhaust without cracking or breaking.

So in this discussion I am curious as to how big and thick such a nozzle would actually need to be for specific SSTO data that is shown below.

 

1. 100 ton SSTO capable of 3g pulse acceleration for 20,000 seconds of specific impulse.

2. 300 ton SSTO capable of 2g continous thrust for 10,000 seconds of specific impulse.

3. 1000 ton SSTO capable of 2g pulse acceleration for 1000 seconds of specific impulse.

 

Are we talking truly gigantic nozzles? Since obviously these are not airbreathing rockets, they are relying on pure thrust to reach space via fusion or something like that that is high performance.

Also I think regenerative cooling ringlets on the nozzle would be necessary so that propellant flowing through the nozzle could cool it.

 

Just wanted to know what the nozzle of dreamy torchdrive SSTO's would look like if we COULD make them.

 

I do not believe I am wrong in assuming that if torchdrive heavy SSTOs were a thing that singular large main engine nozzles would be preferred over multiple smaller ones, since extreme pressure is the price you pay for such high performance... the kind I believe would wreck the typical smaller rocket nozzle cluster you see with chemical rocketry.

Now you may say, why not make multiple smaller torchdrive or pulse rocket engines so that multiple smaller rocket nozzles could handle the pressure so you could just fire them all at once in tandem?

To that I say this.

High thrust fusion rocket and torchdrives are complex processes, and I do think that making multiple smaller torchdrive engines would be more complex and difficult than simply building one big one. Nevermind the fact that clusters of rockets in IRL are often used in case some fail others will still work, and a torchdrive by definition should be more reliable than that.

Compared to fusion or a torchdrive, chemical rocketry is downright simple, which is why they can utilize clusters of smaller rocket engines so often.

Also bigger nozzles are often avoided at launch because of atmospheric pressure changes as the vessel ascends... yet with the high thrust of a torchdrive.... the plume will be going out fast that atmospheric pressure won't really effect it much in any way that effects performance.

 

Edited July 17, 2022 by Spacescifi

[wumpus]

Look up the nozzle of the shuttle and any other "sustainer" 1.5 stage rockets.  They all had to deal with this issue.

And why do you keep banging on about designing things that make no sense?  If you want to design rockets, learn the rocket equation and all the rest of the physics and engineering.  If you want to write a book, either learn the rocket equation (there's a nifty little game that teaches it painlessly linked somewhere on this website) so your readers don't leave nasty reviews saying just how bad the so-called science in your science fiction is or simply don't include the details.  Between KSP and the internet, I'd expect a good chunk of anyone reading "space science fiction" to be reasonably aware of the rocket equation and why SSTO is stupid.  But if you simply include a belly lander that does SSTO, you can handwave it into "zip fuels" (the name of the search for fuels with higher than hydrolox Isp.  They don't appear to exist, but you never know what they might find) if you like.  Just don't include a pusher plate, the less details the better.  See how intersteller travel is covered with hyperspace, without anyone worrying about causality violations.

[Spacescifi]

24 minutes ago, wumpus said:

Look up the nozzle of the shuttle and any other "sustainer" 1.5 stage rockets.  They all had to deal with this issue.

And why do you keep banging on about designing things that make no sense?  If you want to design rockets, learn the rocket equation and all the rest of the physics and engineering.  If you want to write a book, either learn the rocket equation (there's a nifty little game that teaches it painlessly linked somewhere on this website) so your readers don't leave nasty reviews saying just how bad the so-called science in your science fiction is or simply don't include the details.  Between KSP and the internet, I'd expect a good chunk of anyone reading "space science fiction" to be reasonably aware of the rocket equation and why SSTO is stupid.  But if you simply include a belly lander that does SSTO, you can handwave it into "zip fuels" (the name of the search for fuels with higher than hydrolox Isp.  They don't appear to exist, but you never know what they might find) if you like.  Just don't include a pusher plate, the less details the better.  See how intersteller travel is covered with hyperspace, without anyone worrying about causality violations.

Heavy torchdrive SSTOs would change the nature and economy of space travel a lot.

 

1. Suddenly interplanetary tourism/trade becomes a thing, even if more expensive than spacestation trade.

2. Larger orbital stations could be built in less time due to heavy SSTO's hauling up the raw materials to assemble them.

 

Conclusion: Heavy torchdrive SSTO's would open space resources/colonization up wide for the taking by mankind as never before.

Edited July 16, 2022 by Spacescifi

[.50calBMG]

You keep saying that these designs are for scifi, so that's where the designs will stay. These drives don't exist anywhere but the movies, and the odds of you figuring them out on a game forum are less than miniscule. I don't mean to come across as condescending, but there are much smarter people than us working on these problems, and they admit that we won't have them any time soon.

[wumpus]

A few notes:

Torchdrive implies constant thrust between planets, and typically 1g acceleration (which provides natural gravity for Earth dwelling people).  There's little reason for them to land.  SSTOs require TWR>1 (or wings for lift.  But anyone with the tech to do SSTO [and especially torchships] won't be using wings).

Sure it will change humanity.  But would it change humanity more than teleportation, FTL travel, instant communication, replicators, reversible digital circuits (i.e. very low power), and magical batteries?  The replicators, magical batteries and what not are far more possible than SSTO and torchships.  Look at the equations already.  Or just think about how a school for magic would change humanity, because that's what SSTO (and moreso torchships) really are.

PS: .001g "torchships" are a thing.  Well, maybe with a few more zeros.  But ion drives should provide more than a few tricks for moving cargo (don't expect them to be fast enough to transport people directly).

[kerbiloid]

The Martian colonists could use the big nozzle as a dome for their habitat.

[Shpaget]

Didn't F1 nozzle size cause quite a few head itches to the engineers, because of the size?

[Spacescifi]

48 minutes ago, Shpaget said:

Didn't F1 nozzle size cause quite a few head itches to the engineers, because of the size?

The F1 was a challenge because it relied on chemical combustion for propulsion. Chemical combustion in larger chambers becomes harder somehow I read.

 

As per the OP the main engine would either use pulsed or continous fusion reactions for propulsion so there is no inefficiency based on chemical propulsion with the main engines.

 

[KSK]

I have no idea where to start with the first and third examples (pulse propulsion) but here are some rough figures for the second example.

I’m assuming that the 300 ton ship is 50% propellant by mass for a dry mass of 150 tons.

You don’t measure delta-V in seconds but a 2g acceleration applied for 10,000 seconds gives you a delta-V of approximately 200 km/s.

Plugging all that into the rocket equation, and assuming my arithmetic is correct, I get a required Isp of 29,411.

Maximum required thrust is approximately 6MN (300,000kg x 2g rounded up). That’s slightly less than the thrust of a single Rocketdyne F1 (1st stage engine for the Saturn V)

So from that, you might imagine that your fictional spacecraft requires an F1 sized nozzle? Not so fast.

For a perfectly expanded nozzle ( a more or less invalid assumption but let’s roll with it - this engine is bonkers anyway) where exhaust exit pressure is equal to external ambient pressure, Isp equals thrust divided by mass flow x g. 

For an Isp of 29,411, that works out at a mass flow of about 21 kg per second. Incidentally, the exhaust velocity is about 288 km/s. Or 0.1% of light speed if you prefer.

I’m afraid that calculating exhaust temperature and chamber pressures for this thing are beyond me at 11:00 at night. Long story short though, you don’t need a big nozzle at all - 21kg/s is peanuts for a rocket engine.

Quite how one builds a rocket engine with such a ridiculous specific impulse and exhaust velocity, I leave as an exercise for the reader. Just don’t go pointing it at any poor Kzinti, OK?

[KSK]

1 hour ago, Shpaget said:

Didn't F1 nozzle size cause quite a few head itches to the engineers, because of the size?

Only indirectly. The big problem was combustion instability, which made the injector design rather challenging.

You’re probably already aware but just to spell things out for the sake of this thread, the injector is the part of the engine which controls the flow of fuel and oxidiser into the combustion chamber.

A successful injector design gives you a nice even combustion across the engine so all the exhaust goes in the right direction.

An unsuccessful injector design doesn’t. Which means you get some regions of the engine where combustion is relatively fast and some regions where it’s relatively slow. That gives you hot-spots and/or random explosions in your combustion chamber, which tends to result in chamber wall burn-through followed, in short order, by an engine rich exhaust.

I always think of combustion instability as being a bit like spinning a plate on a pole. If everything is perfectly balanced, that plate will stay up there for a good long time. But if you get a small wobble, that very quickly becomes a big wobble, which inevitably ends up with a plate free pole and a lot of broken bits.

Edited July 16, 2022 by KSK

[Spacescifi]

53 minutes ago, KSK said:

I have no idea where to start with the first and third examples (pulse propulsion) but here are some rough figures for the second example.

I’m assuming that the 300 ton ship is 50% propellant by mass for a dry mass of 150 tons.

You don’t measure delta-V in seconds but a 2g acceleration applied for 10,000 seconds gives you a delta-V of approximately 200 km/s.

Plugging all that into the rocket equation, and assuming my arithmetic is correct, I get a required Isp of 29,411.

Maximum required thrust is approximately 6MN (300,000kg x 2g rounded up). That’s slightly less than the thrust of a single Rocketdyne F1 (1st stage engine for the Saturn V)

So from that, you might imagine that your fictional spacecraft requires an F1 sized nozzle? Not so fast.

For a perfectly expanded nozzle ( a more or less invalid assumption but let’s roll with it - this engine is bonkers anyway) where exhaust exit pressure is equal to external ambient pressure, Isp equals thrust divided by mass flow x g. 

For an Isp of 29,411, that works out at a mass flow of about 21 kg per second. Incidentally, the exhaust velocity is about 288 km/s. Or 0.1% of light speed if you prefer.

I’m afraid that calculating exhaust temperature and chamber pressures for this thing are beyond me at 11:00 at night. Long story short though, you don’t need a big nozzle at all - 21kg/s is peanuts for a rocket engine.

Quite how one builds a rocket engine with such a ridiculous specific impulse and exhaust velocity, I leave as an exercise for the reader. Just don’t go pointing it at any poor Kzinti, OK?

 

I see..... so to even warrant a large and thick nozzle I would need an SSTO both massive and heavy.... in other words like project Orion only a fusion rocket is propelling it.

Thanks... so apparently I can go large... but yeah... the plume is likely going to blow up anything it hits nearby.

Leaving craters at the launch site unless you drive it off a ramp over the ocean and boost from there.

 

Finally SSTOs can upsurp the supremacy of TSTO.... at the cost of leaving craters in the ground everytime the lift off unless they do it over the ocean.

Edited July 16, 2022 by Spacescifi

[Spacescifi]

In other words once we get into the 1000 ton or 5000 ton SSTO range.... F1 size nozzles start to become kind of necessary.... right?

[sevenperforce]

I think one of the reasons these perennially sprouting threads give us all headaches (and yet induce us all to answer) is that they are SO close to being reasonable questions, but they aren't. Like, we inevitably imagine that by explaining it just one more time, then something will "click" and the questions will start to make sense.

And they WOULD be fascinating questions IF the correct assumptions were applied so that they made sense. But no matter how many ways or how many times it is explained, nothing ever clicks. The questions never actually start making sense.

Essentially, we are being continually nerd sniped.

Like, this question:

7 hours ago, Spacescifi said:

I am curious as to how big and thick such a nozzle would actually need to be for specific SSTO data that is shown below.

1. 100 ton SSTO capable of 3g pulse acceleration for 20,000 seconds of delta v.

2. 300 ton SSTO capable of 2g continous thrust for 10,000 seconds of delta v.

3. 1000 ton SSTO capable of 2g pulse acceleration for 1000 seconds of delta v.

This question LOOKS reasonable. It involves apparent maths. It cries out for a thoughtful response. And yet it is unanswerable because it is based on a fundamental lack of understanding of the basic principles of rocket science.

Specifically, it's based on the OP's apparent notion that the "seconds of delta v" carried by a spaceship are somehow directly related to the engine or nozzle design. As @KSK pointed out, delta-v is not measured in seconds, but in meters per second, so it is already unclear whether the OP is trying to talk about burn duration or about m/s of dV. But even more fundamentally, almost all of the "specific SSTO data" are irrelevant because neither the staging design nor the pulsed vs continuous nature nor the total stage dV have any bearing on the pressures experienced by the engine; that only comes down to one thing:

1. 100 ton SSTO capable of 3g pulse acceleration for 20,000 seconds of delta v
2. 300 ton SSTO capable of 2g continous thrust for 10,000 seconds of delta v
3. 1000 ton SSTO capable of 2g pulse acceleration for 1000 seconds of delta v

The only information relevant to estimate the pressures sustained is what remains: total mass and acceleration rate, which together provide thrust. To push a 100 tonne vehicle at 3 gees you'll need a thrust of 2.2 MN. To push a 300 tonne vehicle at 2 gees you'll need a thrust of 4.4 MN. To push a 1000 tonne vehicle at 2 gees you'll need a thrust of 14.6 MN.

These are big numbers, but not that big. A single Raptor 2 produces 2.3 Newtons at sea level, more than enough to push a 100 tonne vehicle at 3 gees. A single F-1 engine produced 6.7 MN, significantly more than what you'd need to push your 300 tonne vehicle at 2 gees; if you don't mind using dual nozzles the RD-180 will get you to 4.1 MN which is nearly where you want to be. The original Shuttle SRBs produced 14.7 MN of thrust each, plenty to push 1000 tonnes at over 2 gees.

So if you want to get an idea of how large or thick the nozzle needs to be for any desired application,  just look at the size of the actual nozzles of actual engines that actually produce that much thrust.

7 hours ago, Spacescifi said:

heavy SSTO's would require performance and thrust higher than known chemical rockets, which also means their nozzle pressure would be higher than normal.

First of all, it's SSTOs, not SSTO's. There's no possessive. Ugh.

But more to the point, this assumption is wrong. In order to get increased performance in the world of orbital rocketry, you need to increase exhaust velocity. That doesn't necessarily translate to higher nozzle pressures, though. Behold the magic of a de Laval nozzle:

The green "p" line above is the gas pressure. The beauty of a de Laval nozzle is that when you pass the shock at the nozzle throat, your exhaust flow becomes supersonic and so pressure and temperature both drop precipitously in favor of increased gas velocity. All other things being equal, an engine with higher performance will have higher chamber pressure and higher chamber temperature than an engine with lower performance, but once the gas leaves the chamber and enters the nozzle its pressure and temperature fall exponentially, so you never actually need a "large and thick" nozzle. Unless, of course, you just want a thicc nozzle for aesthetics...in which case, get it.

16 minutes ago, Spacescifi said:

44 minutes ago, KSK said:

Long story short though, you don’t need a big nozzle at all - 21kg/s is peanuts for a rocket engine.

Quite how one builds a rocket engine with such a ridiculous specific impulse and exhaust velocity, I leave as an exercise for the reader. Just don’t go pointing it at any poor Kzinti, OK?

I see..... so to even warrant a large and thick nozzle I would need an SSTO both massive and heavy.... in other words like project Orion only a fusion rocket is propelling it.

Thanks... so apparently I can go large... but yeah... the plume is likely going to blow up anything it hits nearby.

No part of what @KSK said would suggest, imply, or encourage the conclusion you subsequently drew.

7 hours ago, Spacescifi said:

I do not believe I am wrong in assuming that if torchdrive heavy SSTOs were a thing that singular large main engine nozzles would be preferred over multiple smaller ones, since extreme pressure is the price you pay for such high performance... the kind I believe would wreck the typical smaller rocket nozzle cluster you see with chemical rocketry.

I do believe you are wrong in so assuming.

As I explained, the pressure in the nozzle drops off exponentially, so there is not a linear (or even near-linear) relationship between specific impulse and nozzle pressure.

But the question you should be asking is whether a single large combustion chamber is better at handling high temperatures and pressures than multiple smaller combustion chambers. The answer to that question is a resounding NO.

The square-cube law is often your friend in the world of engineering. Bigger is usually better, because the volume in a given space increases faster than the surface area of the enclosing solid. For example, a 10-meter spherical tank will carry more propellant for its weight (at a given tank pressure) than a 5-meter spherical tank. That's geometry for you. But that doesn't always translate. In particular, it doesn't translate to rocket engine combustion chambers. Combustion chambers are receiving a constant flow of propellant and so volume is constant; making it bigger doesn't change the tensile strength requirements of the chamber wall. Chamber wall pressure depends on internal area, which is quadratic; chamber wall weight depends on thickness times circumference, which is also quadratic. Double the pressure and you double the weight. Bigger is not better and bigger is not worse.

NOTE: I'm using the term "combustion chamber" here but it does not mean we're talking about conventional chemical combustion. A combustion chamber is merely shorthand for "that place where exhaust gets really energetic" -- it doesn't matter whether the potential energy comes from chemical sources or nuclear sources or whatever other sources you invent.

Bigger chambers do have a tendency to induce combustion instability which is why the Russians love their single-turbopump multi-chambered engines. Further proof that a bigger chamber doesn't actually help and may often hurt.

8 hours ago, Spacescifi said:

Just wanted to know what the nozzle of dreamy torchdrive SSTO's would look like if we COULD make them.

You may or may not know what a torchdrive is.

If you're talking about an engine which can execute a true Brachistochrone trajectory between planets then it's not going to be dependent on anything remotely similar to chemical rockets. It's like asking, "What would a car look like if it could fly like a plane but without wings?" It's just not a meaningful question.

 If you just mean a spaceship that can produce high thrust and high specific impulse, enough to fly between planets on a single stage, then you're just going to need a high-density, high-power, and high-specific-energy propellant combination. There are any number of ways to achieve this in a science fiction universe. Pick one. 

But for any spaceship which is capable of providing single-stage-to-destination performance with a de Laval nozzle, the engine design can be as big or as small as you want it. But a cluster of small engines is usually better, because it allows you better throttling, better differential pitch and yaw control, and lower overall loading on your roll gimbals.

26 minutes ago, Spacescifi said:

In other words once we get into the 1000 ton or 5000 ton SSTO range.... F1 size nozzles start to become kind of necessary.... right?

Nope, not even slightly.

The Superheavy booster carries 3600 tonnes of propellant and it could easily reach orbit in a single stage if launched without Starship. It does not need an F-1 engine nozzle.

45 minutes ago, Spacescifi said:

Leaving craters at the launch site unless drive it off a ramp over the ocean and boost from there.

When and where in an addled universe did you come up with this ramp idea?

Just launch from a floating platform and use pumps to create a water deluge.

Or just launch from underwater.

Ramps are one of the worst ideas I have seen from you, to date, so far.

[Spacescifi]

1 hour ago, sevenperforce said:

I think one of the reasons these perennially sprouting threads give us all headaches (and yet induce us all to answer) is that they are SO close to being reasonable questions, but they aren't. Like, we inevitably imagine that by explaining it just one more time, then something will "click" and the questions will start to make sense.

And they WOULD be fascinating questions IF the correct assumptions were applied so that they made sense. But no matter how many ways or how many times it is explained, nothing ever clicks. The questions never actually start making sense.

Essentially, we are being continually nerd sniped.

Like, this question:

This question LOOKS reasonable. It involves apparent maths. It cries out for a thoughtful response. And yet it is unanswerable because it is based on a fundamental lack of understanding of the basic principles of rocket science.

Specifically, it's based on the OP's apparent notion that the "seconds of delta v" carried by a spaceship are somehow directly related to the engine or nozzle design. As @KSK pointed out, delta-v is not measured in seconds, but in meters per second, so it is already unclear whether the OP is trying to talk about burn duration or about m/s of dV. But even more fundamentally, almost all of the "specific SSTO data" are irrelevant because neither the staging design nor the pulsed vs continuous nature nor the total stage dV have any bearing on the pressures experienced by the engine; that only comes down to one thing:

1. 100 ton SSTO capable of 3g pulse acceleration for 20,000 seconds of delta v
2. 300 ton SSTO capable of 2g continous thrust for 10,000 seconds of delta v
3. 1000 ton SSTO capable of 2g pulse acceleration for 1000 seconds of delta v

The only information relevant to estimate the pressures sustained is what remains: total mass and acceleration rate, which together provide thrust. To push a 100 tonne vehicle at 3 gees you'll need a thrust of 2.2 MN. To push a 300 tonne vehicle at 2 gees you'll need a thrust of 4.4 MN. To push a 1000 tonne vehicle at 2 gees you'll need a thrust of 14.6 MN.

These are big numbers, but not that big. A single Raptor 2 produces 2.3 Newtons at sea level, more than enough to push a 100 tonne vehicle at 3 gees. A single F-1 engine produced 6.7 MN, significantly more than what you'd need to push your 300 tonne vehicle at 2 gees; if you don't mind using dual nozzles the RD-180 will get you to 4.1 MN which is nearly where you want to be. The original Shuttle SRBs produced 14.7 MN of thrust each, plenty to push 1000 tonnes at over 2 gees.

So if you want to get an idea of how large or thick the nozzle needs to be for any desired application,  just look at the size of the actual nozzles of actual engines that actually produce that much thrust.

First of all, it's SSTOs, not SSTO's. There's no possessive. Ugh.

But more to the point, this assumption is wrong. In order to get increased performance in the world of orbital rocketry, you need to increase exhaust velocity. That doesn't necessarily translate to higher nozzle pressures, though. Behold the magic of a de Laval nozzle:

The green "p" line above is the gas pressure. The beauty of a de Laval nozzle is that when you pass the shock at the nozzle throat, your exhaust flow becomes supersonic and so pressure and temperature both drop precipitously in favor of increased gas velocity. All other things being equal, an engine with higher performance will have higher chamber pressure and higher chamber temperature than an engine with lower performance, but once the gas leaves the chamber and enters the nozzle its pressure and temperature fall exponentially, so you never actually need a "large and thick" nozzle. Unless, of course, you just want a thicc nozzle for aesthetics...in which case, get it.

No part of what @KSK said would suggest, imply, or encourage the conclusion you subsequently drew.

I do believe you are wrong in so assuming.

As I explained, the pressure in the nozzle drops off exponentially, so there is not a linear (or even near-linear) relationship between specific impulse and nozzle pressure.

But the question you should be asking is whether a single large combustion chamber is better at handling high temperatures and pressures than multiple smaller combustion chambers. The answer to that question is a resounding NO.

The square-cube law is often your friend in the world of engineering. Bigger is usually better, because the volume in a given space increases faster than the surface area of the enclosing solid. For example, a 10-meter spherical tank will carry more propellant for its weight (at a given tank pressure) than a 5-meter spherical tank. That's geometry for you. But that doesn't always translate. In particular, it doesn't translate to rocket engine combustion chambers. Combustion chambers are receiving a constant flow of propellant and so volume is constant; making it bigger doesn't change the tensile strength requirements of the chamber wall. Chamber wall pressure depends on internal area, which is quadratic; chamber wall weight depends on thickness times circumference, which is also quadratic. Double the pressure and you double the weight. Bigger is not better and bigger is not worse.

NOTE: I'm using the term "combustion chamber" here but it does not mean we're talking about conventional chemical combustion. A combustion chamber is merely shorthand for "that place where exhaust gets really energetic" -- it doesn't matter whether the potential energy comes from chemical sources or nuclear sources or whatever other sources you invent.

Bigger chambers do have a tendency to induce combustion instability which is why the Russians love their single-turbopump multi-chambered engines. Further proof that a bigger chamber doesn't actually help and may often hurt.

You may or may not know what a torchdrive is.

If you're talking about an engine which can execute a true Brachistochrone trajectory between planets then it's not going to be dependent on anything remotely similar to chemical rockets. It's like asking, "What would a car look like if it could fly like a plane but without wings?" It's just not a meaningful question.

 If you just mean a spaceship that can produce high thrust and high specific impulse, enough to fly between planets on a single stage, then you're just going to need a high-density, high-power, and high-specific-energy propellant combination. There are any number of ways to achieve this in a science fiction universe. Pick one. 

But for any spaceship which is capable of providing single-stage-to-destination performance with a de Laval nozzle, the engine design can be as big or as small as you want it. But a cluster of small engines is usually better, because it allows you better throttling, better differential pitch and yaw control, and lower overall loading on your roll gimbals.

Nope, not even slightly.

The Superheavy booster carries 3600 tonnes of propellant and it could easily reach orbit in a single stage if launched without Starship. It does not need an F-1 engine nozzle.

When and where in an addled universe did you come up with this ramp idea?

Just launch from a floating platform and use pumps to create a water deluge.

Or just launch from underwater.

Ramps are one of the worst ideas I have seen from you, to date, so far.

 

I like ramps though so.... nah I'm kidding. I do like ramps though lol.

 

Engineers don't care about likes and dislikes so much as a design works for efficiency though.

 

Thick nozzles I also like. I was thinking throttling was easy as using less or more fusion fuel and magnetic compression.

Since my idea of a fusion torch would likely be pulsed and ignited via magnetic compression of fuel pellets.

 

Nonetheless if the ship pulls a Star Wars and lands and takes off again from some random planet I think it's going to leave a mark on the spot it launches from.

Edited July 17, 2022 by Spacescifi

[Spacescifi]

And I goofed up.

 

I meant to say specific impulse NOT delta v for the seconds.

Edited July 17, 2022 by Spacescifi

[kerbiloid]

11 hours ago, Spacescifi said:

Obviously heavy SSTO's would require performance and thrust higher than known chemical rockets, which also means their nozzle pressure would be higher than normal.

Their nozzle pressure will be absolutely same, because it's defined by the material strength limit of either nozzle or tank (if pressure-fed).

Everything other is a bad engineering, because carries excessive mass.

[KSK]

8 hours ago, Spacescifi said:

I see..... so to even warrant a large and thick nozzle I would need an SSTO both massive and heavy.... in other words like project Orion only a fusion rocket is propelling it.

What is this obsession with large and thick nozzles?

8 hours ago, Spacescifi said:

In other words once we get into the 1000 ton or 5000 ton SSTO range.... F1 size nozzles start to become kind of necessary.... right?

No.  Setting aside your goof for the moment and working with the numbers I already gave you, that 5000 ton SSTO will require 16.6 times the thrust of your 300 ton SSTO. Which means that you need a mass flow rate of 349 kg/s (rounded up).  That's still trivial. For comparison, the mass flow rate for the actual F1 engine was 2,578 kg/s.  That's one engine mind, not the five attached to the actual Saturn V. Yes, the Saturn V was a beast.

6 hours ago, Spacescifi said:

I like ramps though so.... nah I'm kidding. I do like ramps though lol.

Engineers don't care about likes and dislikes so much as a design works for efficiency though.

Thick nozzles I also like. I was thinking throttling was easy as using less or more fusion fuel and magnetic compression.

Well there's a big part of the problem with these threads. Rather than starting with a set of requirements and figuring out something about how those requirements would be met, you're picking an arbitrary 'cool' thing and trying to shoehorn everything else into a design that requires that 'cool' thing.  And really, why nozzle thickness?

In reality , the correct (but unhelpful for the purposes of this thread) answer to "how thick does the nozzle need to be?" is "as thick as necessary and no more". And in fiction - of all the details of a spaceship to focus one, why in all of Heinlein's multiverses do we care about nozzle thickness?

6 hours ago, Spacescifi said:

And I goofed up.
I meant to say specific impulse NOT delta v for the seconds.

Well in that case, a 300 ton spacecraft, capable of 2g acceleration, with an engine Isp of 10,000 seconds will require the engine to have a mass flow rate of just over 61 kg/s. Still nowhere near F1 engine mass flows.

[Edit:  Sanity check. The sea level Isp for the F1 was 263 seconds.  Using the approximate formula that I've been using for mass flow rate, I get an F1 mass flow of about 3,000 kg per second.  That's not shockingly far off the actual value of 2,578 kg given that I doubt that F1 was anywhere close to ideally expanded.  So yeah, the numbers I'm giving you for mass flow don't appear to be complete nonsense.]

Incidentally, to reach orbit (lets go with a delta-V of 10 km/s here for the sake of argument), that same spacecraft will need to be approximately 90% propellant. So, out of your 300 ton spacecraft, 30 tons of that is spacecraft and 270 tons of it is propellant.

Edited July 17, 2022 by KSK

[kerbiloid]

3 hours ago, KSK said:

the mass flow rate for the actual F1 engine was 2,578 kg/s.

Brick density ~= 2 500 kg/m³.

One cubic meter of bricks per second.

3 hours ago, KSK said:

And really, why nozzle thickness?

A pusher plate. 
A concave, nozzle-shaped, crypto-pusherplate.

[Spacescifi]

11 hours ago, KSK said:

What is this obsession with large and thick nozzles?

No.  Setting aside your goof for the moment and working with the numbers I already gave you, that 5000 ton SSTO will require 16.6 times the thrust of your 300 ton SSTO. Which means that you need a mass flow rate of 349 kg/s (rounded up).  That's still trivial. For comparison, the mass flow rate for the actual F1 engine was 2,578 kg/s.  That's one engine mind, not the five attached to the actual Saturn V. Yes, the Saturn V was a beast.

Well there's a big part of the problem with these threads. Rather than starting with a set of requirements and figuring out something about how those requirements would be met, you're picking an arbitrary 'cool' thing and trying to shoehorn everything else into a design that requires that 'cool' thing.  And really, why nozzle thickness?

In reality , the correct (but unhelpful for the purposes of this thread) answer to "how thick does the nozzle need to be?" is "as thick as necessary and no more". And in fiction - of all the details of a spaceship to focus one, why in all of Heinlein's multiverses do we care about nozzle thickness?

Well in that case, a 300 ton spacecraft, capable of 2g acceleration, with an engine Isp of 10,000 seconds will require the engine to have a mass flow rate of just over 61 kg/s. Still nowhere near F1 engine mass flows.

[Edit:  Sanity check. The sea level Isp for the F1 was 263 seconds.  Using the approximate formula that I've been using for mass flow rate, I get an F1 mass flow of about 3,000 kg per second.  That's not shockingly far off the actual value of 2,578 kg given that I doubt that F1 was anywhere close to ideally expanded.  So yeah, the numbers I'm giving you for mass flow don't appear to be complete nonsense.]

Incidentally, to reach orbit (lets go with a delta-V of 10 km/s here for the sake of argument), that same spacecraft will need to be approximately 90% propellant. So, out of your 300 ton spacecraft, 30 tons of that is spacecraft and 270 tons of it is propellant.

 

Is there any practical reason to go with a thicker nozzle IRL? I was thinking of several:

1. Armor it so it is not riddled with holes from space lasers easily.

2. Or stuff it with regenerative coolant lining to keep it cool from waste heat.

 

Those are the only reasons right?

Besides me just liking the look of a giant rocket plume coming out of a single big and thick nozzle from a spaceship?

 

[kerbiloid]

46 minutes ago, Spacescifi said:

Is there any practical reason to go with a thicker nozzle IRL?

No. Because they can't be effectively cooled.

[KSK]

Armoring against space lasers? It's kind of setting specific and there are probably easier targets to hit but yeah, I guess.

For regenerative cooling, I would think the thinnest possible nozzle wall consistent with mechanical requirements would actually be helpful. A regeneratively cooled combustion chamber and/or nozzle is basically a double walled vessel with a whole lot of small tubes between the two walls to carry cold propellant.  The thinner the nozzle wall, the easier it is to conduct heat away to that cold propellant?

I think.

This is definitely more of a question for the actual aerospace folks on here.

[sevenperforce]

12 minutes ago, Spacescifi said:

Is there any practical reason to go with a thicker nozzle IRL? I was thinking of several:

1. Armor it so it is not riddled with holes from space lasers easily.

2. Or stuff it with regenerative coolant lining to keep it cool from waste heat.

Those are the only reasons right?

Besides me just liking the look of a giant rocket plume coming out of a single big and thick nozzle from a spaceship?

If you just like the look of a giant rocket plume coming out of a single giant nozzle on a spaceship, then say so. Rule of cool is totally valid.

But that's not what you've been saying.

You've been saying, "I think engineering principles would require a large spaceship to have a single large nozzle, isn't that right?" And then we say no, it isn't right, and you come up with new, even more ill-founded reasons why it really is right.

If you've already decided what you want your fictional spaceship to look like and you are having trouble coming up with an in-universe explanation, that's fine. We can think of in-universe explanations easily enough. No one is really going to pay close attention to why the rocket looks the way it does and your explanations don't have to be perfect.

Start with your aesthetic and work from there.

For example, if you want a really large nozzle as part of your vehicle aesthetic, just lean into the square-cube law. Say that your propulsion system uses a variant of inertial confinement fusion with some sort of critical mass ignition element, so engines need to be gigantic in order to function at all. Boom, there you go.

 

6 hours ago, KSK said:

For regenerative cooling, I would think the thinnest possible nozzle wall consistent with mechanical requirements would actually be helpful. A regeneratively cooled combustion chamber and/or nozzle is basically a double walled vessel with a whole lot of small tubes between the two walls to carry cold propellant.  The thinner the nozzle wall, the easier it is to conduct heat away to that cold propellant?

I think.

This is definitely more of a question for the actual aerospace folks on here.

Yes, you are correct. The "regenerative coolant lining" proposed by @Spacescifi sounds more like ablative cooling than regenerative cooling.

If you want a very thick nozzle for whatever aesthetic reason, make it so and then handwave it.

For instance, you could propose an engine which uses inertial confinement fusion to induce helical magnetic fields which in turn produce eddy currents which are conducted to the nozzle, yeeting the propellant out of the nozzle much faster than an ordinary de Laval nozzle could sustain. Then you could just say that the super thick nozzle is necessary due to needing enough ferromagnetic material in the nozzle to properly conduct the eddy currents and shape the exhaust field.

Easy.

[kerbiloid]

In XXIV century they discovered that a 40 m wide nozzle is a game changer, because the flame turbulences merge into one huge spherical plasmoid.

The huge, 30 m in diameter, ball of plasma gets thrown back and explodes, additionally pushing the rocket from back, like in the known archaic Orion project.

To survive that, the nozzle has to be very thick (preferrably - forged) and attached to the ship via a double-cascade set of pistons.

[SunlitZelkova]

@Spacescifi I recommend following @sevenperforce’s advice regarding design.

If your story is not intended for public consumption, I suppose it wouldn’t matter, but in general it is better to say “I just want it this way for the story” or “I just want it this way because it looks cool” than coming up with some sort of detailed engineering reason.

I haven’t looked into the details, but from what I can tell Christopher Nolan claimed that Interstellar was going to be a hyper realistic movie. Despite being a great work of art in its own right, IIRC it is/was the bane of this forum because of that claim.

The same thing happens with me with alternate histories. There are people out there who write great alt-hist stories but then they claim it is “historically accurate [as to what would have happened]” and they destroy themselves and their work because there is always a hole that someone will find, and the work will be thrown into jeopardy by it.

[magnemoe]

On 7/16/2022 at 7:26 PM, wumpus said:

A few notes:

Torchdrive implies constant thrust between planets, and typically 1g acceleration (which provides natural gravity for Earth dwelling people).  There's little reason for them to land.  SSTOs require TWR>1 (or wings for lift.  But anyone with the tech to do SSTO [and especially torchships] won't be using wings).

Sure it will change humanity.  But would it change humanity more than teleportation, FTL travel, instant communication, replicators, reversible digital circuits (i.e. very low power), and magical batteries?  The replicators, magical batteries and what not are far more possible than SSTO and torchships.  Look at the equations already.  Or just think about how a school for magic would change humanity, because that's what SSTO (and moreso torchships) really are.

PS: .001g "torchships" are a thing.  Well, maybe with a few more zeros.  But ion drives should provide more than a few tricks for moving cargo (don't expect them to be fast enough to transport people directly).

This, the craft you use for getting into orbit will not be the one you use between planets. 
In space you don't care much about radiation coming out of the engine, more so most realistic fusion engines is an framework of magnets, stuff who would not work in an atmosphere. 

So you use an laser or microwave pumped SSTO spaceplane with reaction mass only then you leave the atmosphere. 
Then you tend to dock with an space station and transfer to your interplanetary ship. 
You will only stay in the SSTO for half a day and most of that is in freefall so it has an aircraft interior. 
On the interplanetary ship you want cabins and spin gravity,