Questions About Chemical Rocket Engine Reliability In A Scifi Setting....

[Spacescifi]

When I get in my car, I never do a T-minus countdown to when I turn my car keys to start the engine.

Why? Because I expect the engine to run properly, and with regular maintenence over several months between I know it will.

If only chemical rocket engines were like that!

The Scenario:

I am using my uber mono-repellant exhaust nozzle idea in conjunction with plain old chemical rocketry.

 

I want my chemical rocket engines to be more reliable than todays. No countdown... just ignition and go. No more explosions on the launch pad or engine failures midflight.

 

Would making the engine of heavier more durable materials be the way to go? I could certainly afford it given the uber thrust I have available.

 

Why do rocket engines fail? Could we not build one on the ground that would not fail? Would it have to be heavier? Could you avoid countdowns and just go like an automobile?

I get that IRL we have to make rocket engines as light as possible, but that is not an issue in this scenario since the nozzles themselves are generating uber thrust to the exhaust.

[kerbiloid]

Even if a rocket engine is able to survive 100 full-time burns, 2 minutes long each, its total lifespan is still just three hours.

Most of real engines can survive for about ten minutes.

It's not that short. 
A gun shot lasts for 0.001 .. 0.01 s, so its total lifespan is just several minutes in the best case.

Do you want such car engines?

[AckSed]

Fascinating idea.[/davros]

You would want to overbuild to perdition and back. The RL10 is a fair example of a 'fuel and go' rocket engine already: its expander cycle and pumps were originally designed for the classified Suntan hydrogen spyplane, and are under-run for their capacity. One figure I've seen quoted is 4000 seconds of continuous operation with minimal maintenance. What makes it fiddly is the use of cryogenic fuel and oxidiser, requiring pre-chilling.

If you have a room-temperature monopropellant with the heat capacity of a cryogenic liquid, the specific impulse of a dual-fuel rocket and high-tech heat-exchangers to run the expander cycle, then yes, you could get away with start-stop operation.

I recommend Everyday Astronaut's article on rocket engine cycles if you want to know how expander cycle works, and its drawbacks.

[Shpaget]

9 hours ago, Spacescifi said:

I want my chemical rocket engines to be more reliable than todays.

What's stopping you from having it? It has been said so many times, but here it goes again: Just have it any way you want, and don't overexplain.

[Spacescifi]

4 hours ago, AckSed said:

Fascinating idea.[/davros]

You would want to overbuild to perdition and back. The RL10 is a fair example of a 'fuel and go' rocket engine already: its expander cycle and pumps were originally designed for the classified Suntan hydrogen spyplane, and are under-run for their capacity. One figure I've seen quoted is 4000 seconds of continuous operation with minimal maintenance. What makes it fiddly is the use of cryogenic fuel and oxidiser, requiring pre-chilling.

If you have a room-temperature monopropellant with the heat capacity of a cryogenic liquid, the specific impulse of a dual-fuel rocket and high-tech heat-exchangers to run the expander cycle, then yes, you could get away with start-stop operation.

I recommend Everyday Astronaut's article on rocket engine cycles if you want to know how expander cycle works, and its drawbacks.

 

So that is why automobiles are so reliable? Room temperature monopropellant AKA gasoline?

Hmmm... I did not realize that was why rocket launches were complex.

 

So I guess the standard hydrolox rocket engine would take time to either chill the propellant or something before lift off.

55 minutes ago, Shpaget said:

What's stopping you from having it? It has been said so many times, but here it goes again: Just have it any way you want, and don't overexplain.

 

I already know.

 

It is still interesting and humbling to know that reality is rarely ever simple.

 

What happens is that when I learn reality does not allow for something... I just skip it and use fiction.

 

I really thought I could at LEAST use chemical rocketry even if the repellant nozzles are total fiction.

 

But turns out I cannot even do that it seems... since no room temperature propellant I know of has the heat capacity of cryogenic propellants.

 

So back to just using total fictional engines with anti-grav ray exhaust.

Edited February 11, 2023 by Spacescifi

[AckSed]

31 minutes ago, Spacescifi said:

So that is why automobiles are so reliable? Room temperature monopropellant AKA gasoline?

Well, yes, but also no. Comparing aerospace to the car industry is not a matter of the propellant, it's like... Imagine if every single car manufacturer had begun business trying to build a dragster as their main passenger car. There would be a whole load of blown engines along the way, a lot of impractical fuels like nitromethane or methanol and only the richest customers would commute in one. Instead the vast majority of cars are all about getting from A to B along a road at about 30-60 mph.

It's about meeting much less extreme requirements than riding the recoil of a continuous flamethrower to reach supersonic speeds and leaving the atmosphere.

Besides, if you want to get technical, a car engine is a bi-propellant, as it uses atmospheric oxygen.

31 minutes ago, Spacescifi said:

But turns out I cannot even do that it seems... since no room temperature propellant I know of has the heat capacity of cryogenic propellants.

Now hold on. Again, this is "it depends". High-concentration 85% hydrogen peroxide has been used in Britain's Black Arrow, a bi-propellant rocket, and was used to cool the nozzle.

Since science fiction's on the table, Diatomic Metastable Helium is a godlike solid monopropellant activated with a bit of heat, but it's also hard to make. If you'll allow a bit of handwaving to actually produce the stuff, this is your substance.

[Spacescifi]

18 minutes ago, AckSed said:

Well, yes, but also no. Comparing aerospace to the car industry is not a matter of the propellant, it's like... Imagine if every single car manufacturer had begun business trying to build a dragster as their main passenger car. There would be a whole load of blown engines along the way, a lot of impractical fuels like nitromethane or methanol and only the richest customers would commute in one. Instead the vast majority of cars are all about getting from A to B along a road at about 30-60 mph.

It's about meeting much less extreme requirements than riding the recoil of a continuous flamethrower to reach supersonic speeds and leaving the atmosphere.

Besides, if you want to get technical, a car engine is a bi-propellant, as it uses atmospheric oxygen.

Now hold on. Again, this is "it depends". High-concentration 85% hydrogen peroxide has been used in Britain's Black Arrow, a bi-propellant rocket, and was used to cool the nozzle.

Since science fiction's on the table, Diatomic Metastable Helium is a godlike solid monopropellant activated with a bit of heat, but it's also hard to make. If you'll allow a bit of handwaving to actually produce the stuff, this is your substance.

I see... lower performance equals easier ignition.. zero countdowns, and a lot of mileage... which is the converse of what high performance entails.

Edited February 11, 2023 by Spacescifi

[AckSed]

36 minutes ago, Spacescifi said:

I see... lower performance equals easier ignition.. zero countdowns, and a lot of mileage... which is the converse of what high performance entails.

There is a lot to be said for not pushing the envelope and optimising for reliability.

A real-life rocket manufacturer, Rocket Lab, has set out to make a 'boring' rocket engine for the Neutron rocket, that was based on liquid methane and liquid oxygen. The aim was and is to be able to loft a payload and return to the launch site, ready to be fuelled again. They switched from the proven gas-generator cycle to the incredibly complex full-flow staged combustion cycle, but are deliberately under-driving it, because it is actually less stressful on the pre-burners that power the turbines that power the turbopumps.

However, if you have energy densities that high in your accelerating nozzles, that store that much electrical energy, you already have so much energy to spare it would make a nuclear reactor seem like a petrol generator. All you have to do is create a lower grade and optimise whatever this material is for safety, install it in the engine bay as your battery/powerplant, then charge it on the ground.

Resistojets are some of the simplest rockets going, as all they do is electrically heat propellant. Tungsten has one of the highest melting points so a tungsten resistojet can get pretty hot, and flowing H2 through one gives respectable impulse and some thrust. Hell, since you have power to spare, it can be augmented with an electric arc to be even hotter. Add in your accelerating nozzle to augment the thrust and you have your SSTO.

Edited February 11, 2023 by AckSed

[magnemoe]

32 minutes ago, AckSed said:

Well, yes, but also no. Comparing aerospace to the car industry is not a matter of the propellant, it's like... Imagine if every single car manufacturer had begun business trying to build a dragster as their main passenger car. There would be a whole load of blown engines along the way, a lot of impractical fuels like nitromethane or methanol and only the richest customers would commute in one. Instead the vast majority of cars are all about getting from A to B along a road at about 30-60 mph.

It's about meeting much less extreme requirements than riding the recoil of a continuous flamethrower to reach supersonic speeds and leaving the atmosphere.

Besides, if you want to get technical, a car engine is a bi-propellant, as it uses atmospheric oxygen.

This, dragsters are the most extreme excample but I heard formula 1 cars do an full breakdown on engines between races. Not for training but an engine fail her will not cost you an race. 
Car engines are idiotic reliable because its an main selling point. Yes lots of power, low fuel use and emissions are great but not if you have to rebuild the engine every 100.000 km  
Also they mostly run at low throttle, this is why electrical cars works so well but electrical planes are questionable. 

It would be much better to compare future space travel to air travel.  Pilots still do check lists. Now cars has the benefit that if they stop they might block traffic or get rear ended. Its worse in an plane. 
Spaceplanes would still be hangar queens compared to passenger planes. You might get it down to the level of high performance military jets then you have flying the Skylon as long as the B-52  simply as its more of an extreme opperation. 
Deep space is another thing but here you use other engines. 
On the other hand SpaceX has become pretty good at reuse and spamming engines give you some redundancy or how many B-36 flights had engine out

 

[Spacescifi]

2 hours ago, AckSed said:

There is a lot to be said for not pushing the envelope and optimising for reliability.

A real-life rocket manufacturer, Rocket Lab, has set out to make a 'boring' rocket engine for the Neutron rocket, that was based on liquid methane and liquid oxygen. The aim was and is to be able to loft a payload and return to the launch site, ready to be fuelled again. They switched from the proven gas-generator cycle to the incredibly complex full-flow staged combustion cycle, but are deliberately under-driving it, because it is actually less stressful on the pre-burners that power the turbines that power the turbopumps.

However, if you have energy densities that high in your accelerating nozzles, that store that much electrical energy, you already have so much energy to spare it would make a nuclear reactor seem like a petrol generator. All you have to do is create a lower grade and optimise whatever this material is for safety, install it in the engine bay as your battery/powerplant, then charge it on the ground.

Resistojets are some of the simplest rockets going, as all they do is electrically heat propellant. Tungsten has one of the highest melting points so a tungsten resistojet can get pretty hot, and flowing H2 through one gives respectable impulse and some thrust. Hell, since you have power to spare, it can be augmented with an electric arc to be even hotter. Add in your accelerating nozzle to augment the thrust and you have your SSTO.

Yes tungsten can get really hot... no where near enough to SSTO off LH though I think.

 

Still... provided the hydrogen won't ablate it (do not see why it would, oxidizer probably would though), it could provide thrust and as you said the uber repellant nozzles could do the rest.

 

Don't electric arcs tend to ablate resistojets though?

 

That is a problem with long term reliability because it would have to be replaced regularly.

[AckSed]

The resistojet is essentially a giant toaster element. The liquid H2 keeps it cool enough that the kiloamps of current passing through the resistor heating elements do not melt it down, but I don't know how prone to arcing it is. You could control it well enough by limiting the electricity flowing and the amount of gas coming through for a great range of throttleability. Designing one might be comparable to creating a solid-core nuclear engine, but without the radioactivity. Though if you are charging up the nozzles, I might go ahead and assume you already have the power system to handle all that electricity.

Arcjets actually use electric arcs to heat the propellant, and their electrodes do wear down over time. But designing for reliability in that is not impossible... it might be like changing your spark plugs.

Edited February 11, 2023 by AckSed