Would a Fuel+Oxidizer piston engine be more efficient then simple rockets?

[coyotesfrontier]

Say you modify a piston engine to be able to function in non-oxygenated atmospheres, by using an internal oxidizer supply instead of air from the environment. Would this be more efficient then using rockets?

[Xd the great]

30 minutes ago, coyotesfrontier said:

Say you modify a piston engine to be able to function in non-oxygenated atmospheres, by using an internal oxidizer supply instead of air from the environment. Would this be more efficient then using rockets?

No, as piston engines only work on ground, or to drive propellers. They csnnot fly in space.

Unless you are talking about vehicles.

Then, the weight of such oxidizer would decrease your payload capscuty.

So, no.

[steve_v]

45 minutes ago, coyotesfrontier said:

Would this be more efficient then using rockets?

That's a question that is impossible to answer without extensive detail on the vehicle and engine design, it's operating velocity, and the density of the atmosphere.
In general (and assuming an aircraft, not a ground vehicle), anything that uses atmosphere as it's reaction mass is going to be more efficient (in terms of fuel expended vs ΔV) than a system that must carry reaction mass with it... Provided the conversion of fuel-energy to work done on that reaction mass by the engine system is of equal efficiency.
Read here for a start.

Edited August 1, 2019 by steve_v

[mikegarrison]

Piston engines are used to turn heat energy into rotational energy. But how are you going to turn that rotational energy into anything useful in space?

Compare a rocket engine sled car to a drag racing car. Yes, both rockets and the piston engine can accelerate the car down the track. But the difference is that the piston engine car needs to drive the wheels against the ground in order to accelerate, while the rocket car just directly accelerates.

If you are talking about driving around on some other body, then yeah, it would work. But most rovers on other bodies use electric power rather than combustion engines because it's more convenient to either just run off batteries or to get energy for free from the sun.

Edited August 1, 2019 by mikegarrison

[Wjolcz]

Fuel cells+electric motors would probably be much better than a heavy piston engine with ~50% efficency running on rocket fuel.

[tomf]

Some sort of engine a bit like a turbofan jet with injected oxidiser would probably be more efficient in an atmosphere than just using your fuel and oxidiser in a rocket engine. Heavier though.

If you are using Hydrogen or Methane and Oxygen you are going to be be better off using a fuel cell to generate electricity. Internal combustion engines tend to be pretty inefficient by comparison.

 

[kerbiloid]

If electric motors were good for flight, helicopters and cessnas would be using them for a while.
But they are too weak per mass. So, the crewed aviation uses internal combustion engines.

But an IC engine fuel  ~= (CH₂)_n

CH₂ + 1.5 O₂ → CO₂ + H₂O

So, per (12 + 2 * 1) = 14 kg of fuel you need (3 * 16 = 48) kg of oxygen.

So for 200 kg of your Cessna-Laythe fuel you should carry 700 kg of oxygen + tanks.

Edited August 1, 2019 by kerbiloid

[mikegarrison]

Electric motors are just fine for airplanes. The reason why internal combustion is generally used is because of the energy density and capacity of hydrocarbons versus batteries.

[Shpaget]

11 minutes ago, kerbiloid said:

If electric motors were good for flight, helicopters and cessnas would be using them for a while.
But they are too weak per mass.

Not quite. The reason why electric motors are not used for aircraft is the energy density of batteries, not motors.

For example:

A high performance petrol engine such as BMW V10 3L P84, which was used in Formula 1 in 2005) develops 690 kW at 7,5 kW/kg.

A comparable electric motor would be a Remy HVH250-090-POC3, which develops 300 kw with energy density of 8,9 kW/kg.

 

[steve_v]

25 minutes ago, kerbiloid said:

f electric motors were good for flight, helicopters and cessnas would be using them for a while.
But they are too weak per mass.

It's not the motor per-se that has a poor power-to-weight ratio, it's the chemical electricity storage system that powers it. We're better at storing energy in hydrocarbons than batteries is all.
Fuel-cells kinda suck in this regard compared to a combustion engine too, and their gaseous fuels tend to be a pain to store.

25 minutes ago, kerbiloid said:

Cessna-Laythe

Unless you are actually on Laythe... in which case you haver atmospheric oxygen to play with.

Ed. And I've been ninjad again

 

Edited August 1, 2019 by steve_v

[kerbiloid]

This inspires. So, an electroplane can be powered with a microwave or laser.

[steve_v]

Just now, kerbiloid said:

So, an electroplane can be powered with a microwave or laser.

Just as soon as someone invents an efficient beam-receiver and a way to keep the laser/microwave beam pointing at the right spot without creating KFC rain.

[kerbiloid]

19 minutes ago, steve_v said:

Just as soon as someone invents an efficient beam-receiver and a way to keep the laser/microwave beam pointing at the right spot without creating KFC rain.

This works in KSP, Fallout, and Red Alert. Why shouldn't irl?

***

Another question is: is there an aviation and ground technics propellant for combustion engines which can be ISRUed say on Mars from local resources.

Say, you need a drill away from base, but you don't want to carry a nuclear reactor everywhere.

The cheapest and low-level'est thing I could find out is the German WWII C-Stoff/T-Stoff (methanol + hydrazine hydrate) + HTP.
It can power several tonnes heavy planes, rovers, drills, etc.

Then you don't need pistons.

Edited August 1, 2019 by kerbiloid

[magnemoe]

1 hour ago, mikegarrison said:

Electric motors are just fine for airplanes. The reason why internal combustion is generally used is because of the energy density and capacity of hydrocarbons versus batteries.

True, batteries has an lower energy density even if you have to also carry the oxygen. 
The Elektron rocket makes sense as you don't need the expensive turbopump so you looses some efficiency but saves cost and probably increase reliability. 

Benefit of electricity is simplicity and reliability and that you can recharge 
 

[Wjolcz]

2 hours ago, kerbiloid said:

This works in KSP, Fallout, and Red Alert. Why shouldn't irl?

There are at least three things wrong with this question.

[kerbiloid]

7 minutes ago, Wjolcz said:

There are at least three things wrong with this question.

In 1960s they didn't have even this.

[satnet]

Chemical rocket engines have very high thermal efficiency, bordering on ideal. An efficient chemical rocket has about 70% thermal efficiency. Combined gas and steam turbines used in some maritime applications are about 65% efficient. Jet engines have about 40% thermal efficiency. Gasoline engines are about 30-35% thermally efficient, diesel can reach about 40%. The low efficiency of piston engines is the reason electric cars make sense. Batteries have about 1/10th the energy density of hydrocarbons, but are much more efficient at converting that energy into usable work (90% vs 35%), so you can still get a useful range out of them in spite of the huge initial disparity (though it is large enough that piston engines still have longer ranges unless you devote more mass to batteries).

Where rocket engines do poorly is overall propulsive efficiency, which includes converting that usable energy into propulsion. This is because rockets need to carry all of their reaction mass with them. The most efficient way to propel something is to move a large reaction mass with a small, but opposite velocity. This is because momentum is m*v meaning an increase in either mass or velocity has an equal change to momentum, but kinetic energy is ¹/₂*m*v² meaning that increasing velocity requires significantly more energy, so the most energy efficient route is to use larger reaction mass and smaller velocity. Rockets can't use a large reaction mass for higher propulsive efficiency because this increases the amount of mass that needs to be moved which negates the advantage of using a large reaction mass, so it is ultimately more efficient for a rocket to devote that usable energy into a high exhaust velocity and reduce the amount of exhaust mass. Aircraft, boats, and cars all have the advantage of a constant supply of reaction mass they don't need to carry so they can devote their usable energy to moving a large reaction mass.

NOTE: All of these percentages are approximate and most are on the high end of the range.

[Nuke]

im assuming we are talking either atmospheric engines for aircraft flying in non-oxygen atmosphere or an engine for say a moon buggy. internal combustion engines are technically bi-propellant engines, with the ambient atmosphere being the oxidizer tank. fuels used for internal combustion engines are also significantly less energetic and harder to ignite than rocket fuels. using lox+lh2 and would likely eliminate the need for a compression stroke as the fuel is already in a compressed state when injected. seems like you would have the benefits of a 2 stroke without the usual losses that come with it. use of hypergolic fuels or monopropellants would simplify the engines a lot further, eliminating the need for an ignition system.

[Snark]

On 8/1/2019 at 1:12 AM, coyotesfrontier said:

Say you modify a piston engine to be able to function in non-oxygenated atmospheres, by using an internal oxidizer supply instead of air from the environment. Would this be more efficient then using rockets?

As folks have said:  totally depends on the situation, and what you're trying to do.  You need to be more specific.  What kind of atmosphere?  What gaseous mix?  What temperature and pressure?  What sort of vehicle do you have in mind-- an airplane, a rover?  How fast do you want it to go?  How long does it need to be able to operate?  etc. etc.

Electricity's a great option for many applications, not least of which is that in many circumstances you can make it without needing to expend consumables (e.g. solar panels, RTGs).  But whether or not solar panels or RTG are viable for a given application depends on circumstances.  Solar only works if you're close enough to the sun to generate the power levels that you need, works only in the daytime, depends on the orientation of the panels relative to the sun, and is vulnerable to clouds / dust / dirt / etc.  RTGs run for years and years, but they're heavy relative to the amount of power they generate-- great for rovers, probably not so great for airplanes.

Piston engines are heat engines, so their maximum possible efficiency is limited by Carnot's theorem, which is determined by the temperature of the combustion chamber relative to the environment.  So how efficient that's going to be depends a lot on what you're burning and what environment you're in.

Note that one thing that both fuel cells and internal-combustion engines have going for them, on Earth, is that they get their oxidizer for free from the ambient air, meaning that they only need to carry the fuel.  In certain atmospheres, you could turn that on its head:  instead of storing only fuel and getting oxidizer from the air, you store only oxidizer and get fuel from the air.  For example, if you're on Titan, the atmosphere is over 5% methane.  So, in principle, you could have a vehicle that simply stores oxygen (or some other oxidizer) as its "fuel", and then uses atmospheric methane to react with.  Whether that's at all useful relative to an RTG or something is another matter, but it's an interesting idea to think about. 

 

 

[kerbiloid]

And while the nitrogen-oxygen atmosphere consists of oxidizer and inert gas, you can use it directly,
a fuel-rich atmosphere (like on Titan) contains a lot of other flammable gases which kill any internal combustion engine as a bad petrol, though much faster.

Then, usually you need several times more oxidizer than fuel (because CO2 = 12 : 2 * 16 = 3 : 8, H2O =  1 * 2 : 16 = 1 : 8).
So, if you have to carry oxidizer, not sure if extracting fuel from atmosphere by some onboard plant makes sense than carrying some amount of prepared fuel from base.

As a result, the question is: is the atmosphere enough aerodynamically good that you should prefer an aviation engine + wings, rather than a rocket engine, for the same amount of propellant onboard,

[Snark]

Or indeed a buoyant balloon, which would work great in certain atmospheres, has the advantage of making local gravity strength irrelevant, and may or may not achieve your purpose, depending on what the mission is.