ISRU On A Spaceship For Propellant Is For Beginner Space Civilizations

[Spacescifi]

I say this because while worldbuilding a non-overpowered space drive I realized this.

Let's say you have a rocket drive that can expend it's fuel of 90 tons in 70 minutes at 3g max acceleration or longer at lower accelerations.

To get that kind of rocket efficiency you need crazy high exhaust velocities as well as high energy combustion/detonation chamber temperatures higher than rockets we currently use... basically scifi territory unless you count NSWR drives which are radioactive death plume rockets.

If you want to last longer than 70 minutes at 3g you just carry 180 tons of fuel instead and either increase the amount of engine nozzles for higher thrust or otherwise lighten the ship's mass so that it will be similar to what you started with so you do not have to raise thrust to accomodate extra weight.

 

You do not need to carry ISRU equipment... you just have a mission plan and stick to it so you do not run out of propellant.

Want more wiggle room for scifi day of the week adventures?

Use my other non-overpowered space drive:

Impulse Warp Drive: Warps space ahead of your vessels that it moves in the opposite direction your vessel is currently accelerating with main engines... at the same rate of acceleration.

So what you do is accelerate at max acceleration for a moment and engage impulse warp, then shut off main rocket engines. Impulse warp will continually warp space past you at 3g until you shut it down... allowing you to traverse vast distances without needing a torch drive.

Why is it not overpowered? Well.. the ship's ACTUAL momentum/velocity never changed. If it hit something  while at warp it will be just the same as if it was not at warp.. since it's actual velocity and momentum vecotor would all reveal itself on impact.

Because of this using warp can be used to drop into low orbits to increase speed for free, then warp back and do it again and again.

This is the easiest way to make a relativistic kill vehicle but it is time consuming and any ships or probes nearby would notice what you are doing.

 

Conclusion: ISRU for propellant is best reserved for space stations or space or planet colony bases... as they can be better optimized to do a better job than spaceship can and thus better serve spaceships with propellant.

 

EDIT: That was a revelation I just had. Even with super efficient high thrust rocket engines spaceship mass to propellant ratios matter, as well as how many rocket nozzles you have.

With a single nozzle you need all the thrust and heat to come out a single nozzle, which may be too much for a single engine to pull off, so making multiple engines with nozzles is optimal.

 

And adding more or extra propellant to a vessel than what it was designed to carry WILL lower it's maximum thrust unless you increase the engine's maximum efficiency which won't happen unless you reduce the ship's mass since increasing performance is not possible if it is already at the maximum OR increase the number of engine nozzles so you can burn more fuel in less time making it less efficient but more thrusty.

Edited October 29, 2022 by Spacescifi

[wumpus]

Does it matter?  Learn the rocket equation first, then you can understand how rocket fuel works.  Then find a workable ISRU.

And as long as you are committed to "just magic", there's not point caring about which chemicals are used.

[Shpaget]

19 hours ago, Spacescifi said:

If you want to last longer than 70 minutes at 3g you just carry 180 tons of fuel instead and either increase the amount of engine nozzles for higher thrust or otherwise lighten the ship's mass

And just like that, you've solved all the problems of rocketry.

[Spacescifi]

5 hours ago, wumpus said:

Does it matter?  Learn the rocket equation first, then you can understand how rocket fuel works.  Then find a workable ISRU.

And as long as you are committed to "just magic", there's not point caring about which chemicals are used.

 

ISRU won't be required unless you have no other option.

 

ISRU also takes time. And when you add in travel and processing time it quickly becomes practically worthless since much of your delta v would be wrapped arouund constant refueling routes with little to no wiggle room unless you want to take months of detour coasting and orbital assists to lower fuel costs.

We are not interested in apace travel in scifi to merely refuel.

We are interested in visiting places of interest.

And the rocket equation is useful for calculating orbital maneuvers in order to determine how much propellant is needed to change to a particular new orbit, or to find the new orbit as the result of a particular propellant burn.

But when you have scifi tricks like impulse warp you can bypass this a bit.

 

You still have to watch your fuel tank levels, just not as much as you can do gravity assists all day long once you get in range for marginally propellant burns.

 

Edited October 30, 2022 by Spacescifi

[kerbiloid]

It's easier to deliver a backup ship with tanks than to build a plant.

[Spacescifi]

9 minutes ago, kerbiloid said:

It's easier to deliver a backup ship with tanks than to build a plant.

 

Very true.

 

I guess with me anyway... I am far more interested with scifi spaceship design that is fitting for the kind of propulsion it uses.

In real life rocket shapes are optimal given their relatively short but hight thrust burn times to clear the atmosphere.

Only when you have higher energy to dump into your propulsion system or propellant without melting your engine when compared to modern space rockets do you get options for shapes other than rockets if we are lifting off an Earth-like planet.

Scifi often shows non-rocket looking spaceships flying like SSTOs to and from planets.... but to do that you would need vessels with performance similar to what I typed in the original post.

[Beamer]

When it comes to fuel it's all about the amount of acceleration you get from a certain mass of fuel for a certain mass of hardware. If you somehow managed to have a 'storage medium' for your fuel that is more energy dense than the actual fuel you can refine from it, then the refinement process is just a part of your engine and considered into the engine ISP and dry mass. There is no principle mathematical difference between converting Hydrogen and Oxygen to water with a certain amount of kinetic energy and converting some sort of more optimal "magical fuel storage crystal" to Hydrogen and Oxygen which is then converted to water with kinetic energy. In the end you have X mass of fuel, which can produce Y amount of acceleration for Z mass of hardware. Calling part of your engine an "ISRU" just complicates the maths, it doesn't change the answer.

If you want to beat the rocket equation for sci-fi purposes you either have to go for a more optimal way to convert matter to energy (like fission or fusion, you won't find huge leaps by chemical means anymore), leave your fuel at home (light sails, laser power) or pick it up along the way (ramjet), or forego any scientific explanation and just go with "magical fuel storage crystal" (looking at you Star Trek).

 

[Spacescifi]

19 hours ago, kerbiloid said:

It's easier to deliver a backup ship with tanks than to build a plant.

 

Actually on second thought doing that does little good... since the backup ship needs to spend propellant just to keep up with the  ship it is going to feed.

Unless you literally launched a bunch of tankers off a huge coil gun to ALREADY be where you are going when you get there, they will lose so much propellant along the way that it is an inefficient system that can be made more efficient by taking forever using ion engines.

 

3 hours ago, Beamer said:

When it comes to fuel it's all about the amount of acceleration you get from a certain mass of fuel for a certain mass of hardware. If you somehow managed to have a 'storage medium' for your fuel that is more energy dense than the actual fuel you can refine from it, then the refinement process is just a part of your engine and considered into the engine ISP and dry mass. There is no principle mathematical difference between converting Hydrogen and Oxygen to water with a certain amount of kinetic energy and converting some sort of more optimal "magical fuel storage crystal" to Hydrogen and Oxygen which is then converted to water with kinetic energy. In the end you have X mass of fuel, which can produce Y amount of acceleration for Z mass of hardware. Calling part of your engine an "ISRU" just complicates the maths, it doesn't change the answer.

If you want to beat the rocket equation for sci-fi purposes you either have to go for a more optimal way to convert matter to energy (like fission or fusion, you won't find huge leaps by chemical means anymore), leave your fuel at home (light sails, laser power) or pick it up along the way (ramjet), or forego any scientific explanation and just go with "magical fuel storage crystal" (looking at you Star Trek).

 

 

Fission and fusion both are kind of in the realm of science fiction if we want torchsips.

 

According to wikpedia the very reaction chamber that nuclear saltwater rockets  would use is at risk of melting from the heat.

Zubrin thinks increasing the flow rate of the propellant will solve that problem as it will carry heat away fast enough not to wreck the reaction chamber. But we won't know for sure until we actually build a NSWR.

Worse yet there are concerns of stray neutrons messing things up and even causing a subcritical nuclear reaction to go critical and.... BOOM!

Long story short... we do not even know if modern materials could sustain a reaction chamber that could survive a Nuclear Saltwater rocket burn.

 

So even NSWR may be fiction unless we can prove otherwise.

[Shpaget]

So, in concludion, you don't need ISRU, if you plan your mission such that you don't need the extra propellant.

Of course, it helps if you have the magic warp drive.

[Beamer]

3 hours ago, Spacescifi said:

So even NSWR may be fiction unless we can prove otherwise.

Nuclear propulsion is not science fiction, it is science. The fact that we do not have a working system is an engineering problem (or political, or ethical, or financial, take your pick), not a scientific one. We know how to create thermal energy through fission and we know many, many ways to convert thermal energy into kinetic energy. The Impulse Warp Drive idea you are proposing is creating free energy out of nothing. That is not an engineering problem, it is a scientific impossibility. There is a difference between these two and it's kinda important to understand if you want to come up with scientifically plausible solutions for hard sci-fi problems.

Sure you can go the magic crystal route, nothing wrong with that, but it's a bit pointless to try to explain that using actual science when actual science says it is impossible. Star Trek got away with it for a few shows by just throwing in some techno-babble and, most importantly, not trying to explain it. Star Wars barely even bothers with the techno-babble at all. I always enjoyed watching it (although the most recent one I have seen with their infinite improbability spore drive and yet another fascist alternate universe story-line is pushing the limits a bit IMHO), but whenever someone breaks out the tachyon beams or warps right into orbit of a planet we all know it's just a pretty fable, not science.

 

 

[Spacescifi]

On 10/31/2022 at 9:45 PM, Beamer said:

Nuclear propulsion is not science fiction, it is science. The fact that we do not have a working system is an engineering problem (or political, or ethical, or financial, take your pick), not a scientific one. We know how to create thermal energy through fission and we know many, many ways to convert thermal energy into kinetic energy. The Impulse Warp Drive idea you are proposing is creating free energy out of nothing. That is not an engineering problem, it is a scientific impossibility. There is a difference between these two and it's kinda important to understand if you want to come up with scientifically plausible solutions for hard sci-fi problems.

Sure you can go the magic crystal route, nothing wrong with that, but it's a bit pointless to try to explain that using actual science when actual science says it is impossible. Star Trek got away with it for a few shows by just throwing in some techno-babble and, most importantly, not trying to explain it. Star Wars barely even bothers with the techno-babble at all. I always enjoyed watching it (although the most recent one I have seen with their infinite improbability spore drive and yet another fascist alternate universe story-line is pushing the limits a bit IMHO), but whenever someone breaks out the tachyon beams or warps right into orbit of a planet we all know it's just a pretty fable, not science.

 

 

 

To be sure the impulse warp drive DOES rely on thrust from main engines to kickstart it... but after that... yeah... it is virtually free space warping.

So long as you have propellant in your tank to exhaust out your main engine you can also use it to set your impulse warp acceleration.

 

So nearly... free.

[magnemoe]

On 10/31/2022 at 10:31 PM, Beamer said:

When it comes to fuel it's all about the amount of acceleration you get from a certain mass of fuel for a certain mass of hardware. If you somehow managed to have a 'storage medium' for your fuel that is more energy dense than the actual fuel you can refine from it, then the refinement process is just a part of your engine and considered into the engine ISP and dry mass. There is no principle mathematical difference between converting Hydrogen and Oxygen to water with a certain amount of kinetic energy and converting some sort of more optimal "magical fuel storage crystal" to Hydrogen and Oxygen which is then converted to water with kinetic energy. In the end you have X mass of fuel, which can produce Y amount of acceleration for Z mass of hardware. Calling part of your engine an "ISRU" just complicates the maths, it doesn't change the answer.

If you want to beat the rocket equation for sci-fi purposes you either have to go for a more optimal way to convert matter to energy (like fission or fusion, you won't find huge leaps by chemical means anymore), leave your fuel at home (light sails, laser power) or pick it up along the way (ramjet), or forego any scientific explanation and just go with "magical fuel storage crystal" (looking at you Star Trek).

Agree,  the Expanse engines are not realistic but its nothing stopping you for accelerating an fairly lightweight ship at say .2 g for a month or two using fusion. 
Antimatter can take you much farther, its starship fuel. It has some safety issues however. 

KSP is pretty stupid here in that you can use an fuel cell to run an mining and isru opperation, yes it work on earth as you can run an pump on an oil well on part of the gas from it, but that only work as you burn the gas with air. So you sacrifice some gas to get out more gas and oil. 
That does not work outside an biosphere. 

[Beamer]

 

In KSP the ore weighs the same as the fuel you create from it, so it's never a good idea to refine ore on your ship. Just add the same weight in fuel tanks and you can leave the heavy ISRU behind. Of course if you're shipping a miner that has ore storage and refinery anyway you might as well fill up the ore tanks, but that's a special case.

The most unrealistic part is the 100% mass conversion, I think the only way you could manage that in reality is if the 'ore' is pure H2O and your rockets run on hydrolox, and you somehow manage to do your conversion and storage of hydrogen without the 10% plus leakage losses common in such processes. In real life the situation is always going to be that your ore weighs more than the refined fuel you get from it, so although KSP turns the dial to the 'easy' side, they at least didn't turn it all the way to 11 where ore+refinery is actually lighter than the equivalent amount of fuel. Which is what would be required for refining your fuel en route to be efficient, i.e. the magical fuel crystals.

 

[Nuke]

i like the idea of thermal engines which can use anything as remass, granted with a wide range of specific impulses. i think this would be best used for asteroid mining or inter-moon operations (say in the jovian system). you could use anything as propellant provided you can liquefy it/compress it and purify it, and so long as it doesnt react with parts of your engine. the heat source is irrelevant, it can be nuclear, beamed, etc. you just need enough to get you to your next destination, and harvest whatever local resource your engine will like most. 

Edited November 3, 2022 by Nuke

[sevenperforce]

On 10/29/2022 at 7:03 PM, Spacescifi said:

Let's say you have a rocket drive that can expend it's fuel of 90 tons in 70 minutes at 3g max acceleration or longer at lower accelerations.

Oh dear, this again.

Acceleration is NOT a feature of a "rocket drive" at all. A rocket engine produces thrust. Acceleration is the result of dividing thrust by mass. Let me say it again. Rocket engines DO NOT produce acceleration; they produce thrust. Period.

And because the mass of a vehicle changes over time, the acceleration of a vehicle changes over time, assuming constant thrust. Specifically, it goes up.

Since you said "3g max acceleration" I'm going to assume (probably wrongly) that you're thinking of something sane. For a vehicle to have a 70-minute burn time with 90 tonnes of propellant and achieve 3 gees at burnout, then (clearly) it must be burning 21.4 kg per second and it must be yeeting those 21.4 kg/s out the back end at a sufficient exhaust velocity that the impulse to the dry mass of the vehicle is 29.43 m/s².

This gives a range of values depending on the dry mass of the vehicle and thus the corresponding mass ratio. Here are some comparisons:

Ratio of Fuel to Dry Mass	Thrust (kN)	Isp (sec)	Initial Acceleration (gees)	Total Δv (km/s)
24:1 (typical for some chemical upper stages)	110	465	0.12	14.7
6:1 (typical for a laden first stage rocket)	441	2,100	0.43	40.1
3:1 (enough thrust to get off the ground)	1,177	5,606	1.00	76.2
1:1 (typical for aircraft like a 747 or a B-2)	2,649	12,618	1.50	85.8

Having a specific impulse in the range of 2,000-5,000 seconds isn't completely out of the question; that's the theoretical specific impulse range of your typical open-cycle gas-core nuclear rocket. But of course a gas-core nuclear rocket sprays out rather unpleasantly radioactive exhaust, and it's doubtful that you'll be able to get the amount of thrust you need. The NASA studies of gas-core designs found they wouldn't produce enough thrust to even lift half the weight of their pressure vessel alone, and that's before factoring in the weight of a moderator/reflector and radiators.

If you want to get up above 12,000 seconds of specific impulse, you're going to need a better energy source. Antimatter could do the trick. If you had perfect conversion of potential energy to kinetic energy, then you'd be accelerating your 21.4 kg/s of propellant to 124 km/s, requiring 164.5 GW of power. That's going to require you to burn about one milligram of antimatter per second, or a total of 4.2 kilograms over your entire journey. Of course, you won't, because the maximum efficiency of any heat engine (the Carnot cycle) is around 70%. That means 30% of your energy is lost to heat, so you're actually going to need 6 kilograms of antimatter. But that means we're going to have to have a way to reject 70.5 GJ of thermal energy per second.

There are two ways you can do this. The first is by using an open-cycle cooling loop where you're just dumping part of your propellant overboard. That propellant will of course achieve very high temperature (that's the point) so it will basically be an entirely separate rocket engine similar to a regular NTR, except the heat it's receiving is waste heat from the antimatter rocket rather than from a nuclear reactor. Assuming liquid hydrogen propellant and a coolant exhaust specific impulse on the order of 1000 seconds (but now benefiting from the 70% limit of the Carnot cycle), each kilogram of coolant is carrying away 69 MJ of heat and producing an impulse of 9.8 kN. But at this rate, you'd need to be dumping over 1000 kilograms per second just to deal with the waste heat of the engine, and our propellant budget is only 21.4 kg/s.

So we turn to the second approach: a closed-loop cooling cycle where the coolant runs through a series of radiators and is then injected into the reaction chamber as the main source of propellant. That will work, but it will approximately double the weight of your engine.

But let's suppose we ignore all of the waste heat issues entirely.

Some antimatter engines have T/W ratios on the order of 4:1, and so if we're just gonna handwave and say this is achievable here, we'll end up with an engine that weighs around 67 tonnes. This leaves 23 tonnes for payload, structure, and propellant tanks.

On 10/29/2022 at 7:03 PM, Spacescifi said:

If you want to last longer than 70 minutes at 3g you just carry 180 tons of fuel instead and either increase the amount of engine nozzles for higher thrust or otherwise lighten the ship's mass so that it will be similar to what you started with so you do not have to raise thrust to accomodate extra weight.

If you double the amount of propellant then you cut your acceleration in half. If you double your thrust to accommodate this, you use up your propellant twice as fast, and you're right back where you started.

You can't exactly "lighten the ships mass" when your engine and propellant alone are 87% of the ship's laden mass.

And if you're trying to accelerate at 3 gees the entire time...that makes no sense. Engines don't produce acceleration; they produce thrust. As your mass goes down, your acceleration will go up.

On 10/29/2022 at 7:03 PM, Spacescifi said:

Impulse Warp Drive: Warps space ahead of your vessels that it moves in the opposite direction your vessel is currently accelerating with main engines... at the same rate of acceleration.

So what you do is accelerate at max acceleration for a moment and engage impulse warp, then shut off main rocket engines. Impulse warp will continually warp space past you at 3g until you shut it down... allowing you to traverse vast distances without needing a torch drive.

Why is it not overpowered? Well.. the ship's ACTUAL momentum/velocity never changed. If it hit something  while at warp it will be just the same as if it was not at warp.. since it's actual velocity and momentum vecotor would all reveal itself on impact.

So space is accelerating past you, but you're not gaining kinetic energy? Ok, let's imagine that.

If you have this, you don't need 70 minutes of whatever wacky acceleration you're imagining.

I'm assuming you can't activate this impulse warp in atmosphere?

So you just need about 2 km/s to get out of the atmosphere and point in the desired direction, and then engage impulse warp.

At three gees of acceleration, that's going to be a burn time of just about 68 seconds, and you're going to only need a fuel to dry mass ratio of about 1:1. 

The problem is this: when you drop out of warp, you said you have the "actual velocity and momentum vecotor [sic]" from the beginning. But actual velocity relative to what? Relative to your home planet? Relative to your destination? Earth is moving at 30 km/s around the sun. If I point my vehicle toward Jupiter and warp toward it at 3 gees, then drop out of warp near Jupiter, I'm going to be outpacing Jupiter by a whopping 7 km/s. And that's if Jupiter is perfectly lined up with Earth so that our velocity vectors are pointing in the same direction. If our velocity vectors are pointing in different directions, it gets even worse.

On 10/29/2022 at 7:03 PM, Spacescifi said:

With a single nozzle you need all the thrust and heat to come out a single nozzle, which may be too much for a single engine to pull off

Why?

Why would multiple nozzles be any different from a single nozzle?

In terms of thrust and heat, this makes no sense at all.

On 10/29/2022 at 7:03 PM, Spacescifi said:

increase the number of engine nozzles so you can burn more fuel in less time making it less efficient but more thrusty.

Increasing the number of engines doesn't make it less efficient. It increases your dry mass but the specific impulse isn't going to change.

Please just learn the rocket equation.

Edited November 3, 2022 by sevenperforce

[Spacescifi]

53 minutes ago, sevenperforce said:

Oh dear, this again.

Acceleration is NOT a feature of a "rocket drive" at all. A rocket engine produces thrust. Acceleration is the result of dividing thrust by mass. Let me say it again. Rocket engines DO NOT produce acceleration; they produce thrust. Period.

And because the mass of a vehicle changes over time, the acceleration of a vehicle changes over time, assuming constant thrust. Specifically, it goes up.

Since you said "3g max acceleration" I'm going to assume (probably wrongly) that you're thinking of something sane. For a vehicle to have a 70-minute burn time with 90 tonnes of propellant and achieve 3 gees at burnout, then (clearly) it must be burning 21.4 kg per second and it must be yeeting those 21.4 kg/s out the back end at a sufficient exhaust velocity that the impulse to the dry mass of the vehicle is 29.43 m/s².

This gives a range of values depending on the dry mass of the vehicle and thus the corresponding mass ratio. Here are some comparisons:

Ratio of Fuel to Dry Mass	Thrust (kN)	Isp (sec)	Initial Acceleration (gees)	Total Δv (km/s)
24:1 (typical for some chemical upper stages)	110	465	0.12	14.7
6:1 (typical for a laden first stage rocket)	441	2,100	0.43	40.1
3:1 (enough thrust to get off the ground)	1,177	5,606	1.00	76.2
1:1 (typical for aircraft like a 747 or a B-2)	2,649	12,618	1.50	85.8

Having a specific impulse in the range of 2,000-5,000 seconds isn't completely out of the question; that's the theoretical specific impulse range of your typical open-cycle gas-core nuclear rocket. But of course a gas-core nuclear rocket sprays out rather unpleasantly radioactive exhaust, and it's doubtful that you'll be able to get the amount of thrust you need. The NASA studies of gas-core designs found they wouldn't produce enough thrust to even lift half the weight of their pressure vessel alone, and that's before factoring in the weight of a moderator/reflector and radiators.

If you want to get up above 12,000 seconds of specific impulse, you're going to need a better energy source. Antimatter could do the trick. If you had perfect conversion of potential energy to kinetic energy, then you'd be accelerating your 21.4 kg/s of propellant to 124 km/s, requiring 164.5 GW of power. That's going to require you to burn about one milligram of antimatter per second, or a total of 4.2 kilograms over your entire journey. Of course, you won't, because the maximum efficiency of any heat engine (the Carnot cycle) is around 70%. That means 30% of your energy is lost to heat, so you're actually going to need 6 kilograms of antimatter. But that means we're going to have to have a way to reject 70.5 GJ of thermal energy per second.

There are two ways you can do this. The first is by using an open-cycle cooling loop where you're just dumping part of your propellant overboard. That propellant will of course achieve very high temperature (that's the point) so it will basically be an entirely separate rocket engine similar to a regular NTR, except the heat it's receiving is waste heat from the antimatter rocket rather than from a nuclear reactor. Assuming liquid hydrogen propellant and a coolant exhaust specific impulse on the order of 1000 seconds (but now benefiting from the 70% limit of the Carnot cycle), each kilogram of coolant is carrying away 69 MJ of heat and producing an impulse of 9.8 kN. But at this rate, you'd need to be dumping over 1000 kilograms per second just to deal with the waste heat of the engine, and our propellant budget is only 21.4 kg/s.

So we turn to the second approach: a closed-loop cooling cycle where the coolant runs through a series of radiators and is then injected into the reaction chamber as the main source of propellant. That will work, but it will approximately double the weight of your engine.

But let's suppose we ignore all of the waste heat issues entirely.

Some antimatter engines have T/W ratios on the order of 4:1, and so if we're just gonna handwave and say this is achievable here, we'll end up with an engine that weighs around 67 tonnes. This leaves 23 tonnes for payload, structure, and propellant tanks.

If you double the amount of propellant then you cut your acceleration in half. If you double your thrust to accommodate this, you use up your propellant twice as fast, and you're right back where you started.

You can't exactly "lighten the ships mass" when your engine and propellant alone are 87% of the ship's laden mass.

And if you're trying to accelerate at 3 gees the entire time...that makes no sense. Engines don't produce acceleration; they produce thrust. As your mass goes down, your acceleration will go up.

So space is accelerating past you, but you're not gaining kinetic energy? Ok, let's imagine that.

If you have this, you don't need 70 minutes of whatever wacky acceleration you're imagining.

I'm assuming you can't activate this impulse warp in atmosphere?

So you just need about 2 km/s to get out of the atmosphere and point in the desired direction, and then engage impulse warp.

At three gees of acceleration, that's going to be a burn time of just about 68 seconds, and you're going to only need a fuel to dry mass ratio of about 1:1. 

The problem is this: when you drop out of warp, you said you have the "actual velocity and momentum vecotor [sic]" from the beginning. But actual velocity relative to what? Relative to your home planet? Relative to your destination? Earth is moving at 30 km/s around the sun. If I point my vehicle toward Jupiter and warp toward it at 3 gees, then drop out of warp near Jupiter, I'm going to be outpacing Jupiter by a whopping 7 km/s. And that's if Jupiter is perfectly lined up with Earth so that our velocity vectors are pointing in the same direction. If our velocity vectors are pointing in different directions, it gets even worse.

Why?

Why would multiple nozzles be any different from a single nozzle?

In terms of thrust and heat, this makes no sense at all.

Increasing the number of engines doesn't make it less efficient. It increases your dry mass but the specific impulse isn't going to change.

Please just learn the rocket equation.

 

Thank you for more or less killing off the idea of The Expanse TV series... at least it would not look the same at all.

 

Still there is some hope for gas core that could improve performance by combining it with MHD tech.

http://www.projectrho.com/public_html/rocket/power.php#nukemhd

 

 

About Impulse Warp: Your speed and velocity and inertia will be the same it was when you engaged the impulse warp drive.

So yes  I am aware this can be weaponized easily due to the wild differences in relative speeds in space... still one would have plenty of notice since 3g may be fast but still takes more than enough time for anyone to detect and prepare for any such RKV attack.

The main benefits for impulse warp are dropping out of warp to gain orbital or lose orbital speed as well as make maneuvers to evade that normally would be impossible.

Missiles without impulse drives are essentially obsolete unless they are near enough to overtake a fleeing vessel they would run out of fuel chasing it.

 

As for thrust, I presumed more engines of similar thrust would grant overall more thrust tham a single one on it's own... at the cost of burning more propellant.

 

IRL more engines than less are used since it is easier to cluster a bunch of less powerful engines together for higher thrust than make a single uber engine that can do the work of a whole cluster of engines by itself.

Edited November 3, 2022 by Spacescifi

[sevenperforce]

1 minute ago, Spacescifi said:

Thank you for more or less killing off the idea of The Expanse TV series... at least it would not look the same at all.

The Epstein Drive in that series is a torch drive that employs Brachistochrone trajectories.

4 minutes ago, Spacescifi said:

Still there is some hope for gas core that could improve performance by combining it with MHD tech.

An airbreathing gas core MHD engine could get very high specific impulse and reasonably high thrust, yes, but thrust drops off once you're out of the atmosphere. Of course, that's perfectly fine. You don't need high constant acceleration for some specified period of time. I don't know why you keep insisting otherwise. Your total dV is your total dV, whether you dump it in 70 minutes or 20 minutes or fifteen hours.

10 minutes ago, Spacescifi said:

About Impulse Warp: Your speed and velocity and inertia will be the same it was when you engaged the impulse warp drive.

Saying "speed and velocity" is like saying "temperature and hotness" -- one includes the other.

If your velocity vector is the same, then it needs to be your velocity vector relative to something. Is it your velocity vector relative to the Sun?

 

[Spacescifi]

26 minutes ago, sevenperforce said:

The Epstein Drive in that series is a torch drive that employs Brachistochrone trajectories.

An airbreathing gas core MHD engine could get very high specific impulse and reasonably high thrust, yes, but thrust drops off once you're out of the atmosphere. Of course, that's perfectly fine. You don't need high constant acceleration for some specified period of time. I don't know why you keep insisting otherwise. Your total dV is your total dV, whether you dump it in 70 minutes or 20 minutes or fifteen hours.

Saying "speed and velocity" is like saying "temperature and hotness" -- one includes the other.

If your velocity vector is the same, then it needs to be your velocity vector relative to something. Is it your velocity vector relative to the Sun?

 

 

I am not sure what answer you are looking for but I will try to answer.

Obviously anything launched from any planet retains at least that planet's orbital vector unless it thrusts to change that.

Ever play asteroids or Space War? I know you have... we all have.

Impulse warp would allow you to seemingly accelerate across the screen with only a single burn, but when you shut off warp your would retain your original velocity vector before warp.

 

What that means is you can do some cool stuff like fly to ram an asteroid or moon and flip and burn right before hitting it and warp in the opposite direction at the same acceleration... basically cheating inertia... until you shut off warp anyway

 

By the same token in real life you could use this and rinse and repeat to gain a bunch of speed from gravity at the cost falling backward and warping forward again with a single burn again and again.

Edited November 3, 2022 by Spacescifi

[Nuke]

torch ships are cool, and the expanse is awesome for using them and laying their ships out in a way that makes sense for the technology. technically star trek's impulse drives are torch drives, but they still lay their ships out like a damn ocean liner. they have inertial dampeners and artificial gravity, so they can modify inertial frames and gravity. which begs the question why dont they use that tech for propulsion and leave the torch drive in a museum somewhere. 

despite this the expanse is not quite realistic in terms of thermal management. torch ships are not an irl impossibility per se, but its going to cost you some radiator mass. you could do something cool in military engagments, like target the radiators to what amounts to a missile with a flak warhead. this would cripple the engine and force a surrender. or you could  cook off your engine in a vain attempt at escape, and use it to dramatic purpose in story telling. the expanse is smart in that it pushes the realism in a way normal tv watchers can understand, but omits some details. for the same reason the radiators were removed from discovery in 2001. its an added detail that most people wont understand (unless they are an automotive mechanic, a nuclear power plant technician or an hvac specialist, even a pc builder will understand waste heat). 

i dont like it when a story dumbs itself down for the sake of the audience, it just makes people dumb not giving them stuff to think about. i tend to think that if your story is interesting enough you really dont need to skimp on the details and the details can reinforce the story (the expanse does this, even though they might not go far enough for some, they deserve credit for what they do). one of the reasons andor does so well i think is that they actually care about the characters and the details and dont try to explain everything or obfuscate it from the audience. not every story needs to be brain off popcorn in face. 

Edited November 4, 2022 by Nuke

[KSK]

10 hours ago, sevenperforce said:

*snip*

Please just learn the rocket equation.

Or have a look online for rocket equation calculators. Apologies for the advert spam but this one looks good.  

Tsiolkovsky Rocket Equation Calculator • Model Rockets • Online Unit Converters (translatorscafe.com)

Dry mass of rocket, wet mass of rocket, specific impulse or exhaust velocity, required velocity change. Input any three and it'll calculate the fourth for you.