Engines switching operation mode?

[Braun]

Hey.

I came accross an Alien technical handbook for marines, quickly skimmed through the pages and found blueprint and descriptions for the UD-4L(?) dropship.

It features Ram-Rockets, that switch operation mode from

normal air breathing > ramjet > rocket.

Is that possible in KSP already?

[shadowfax]

The main conceptual difference between an air breathing engine and a rocket engine is that the rocket engine carries its oxidizer with it along with the fuel, while the jet engine picks up its oxidizer in flight by sucking air into itself.

So an engine that could switch from a rocket engine to an air breathing engine would be pointless, because you\'re already carrying your oxidizer around with you for it to be a rocket, and you\'re carrying it in the proper amount for the fuel to be burned with it. So if you start using air as your oxidizer, now you have all this stored oxidizer that you\'re carrying around and which you can\'t use because you\'ve already burned the fuel you were supposed to use it with, which is a weight penalty for 0 gain.

The smarter design would be air breathing engines for atmospheric flight and a rocket engine for non-atmospheric flight. And you can already make a ship with that in KSP.

[Wyrm]

An engine that could switch from being a jet engine to a rocket would not be pointless. A jet engine gets its oxidizer from the surrounding atmosphere, which means that your vehicle doesn\'t need to carry that oxidizer with it. This makes a smaller rocket for the same payload. The increased mass of the exhaust vs. the input fuel means you can lower the speed of the exhaust without sacrificing force, so the effective Isp is larger for a jet engine than for an equivalent rocket engine using the same reactants. This, again, raises delta-V.

If the two engines are separate, in order to get both the advantages of a rocket (operates in space) and a jet engine (operates on free ambient oxidizer), you need two engines — one for atmospheric flight, and one for space flight. That\'s a weight penalty, resulting in a lower delta-V. If you have an engine that can switch between these two modes, air breathing in an atmosphere, a rocket in space, then you don\'t pay that weight penalty.

[Accelerando]

Except for the cooling system I presume is necessary to chill incoming air à la Skylon. But how much mass does that add?

[Braun]

the manual stated the fuel would be flowing around the engine for conditioning (heating, compression, spray injection).

operation mode would be controlled by the \'air injector cone\' (don\'t remember its real name) thats pressed into the collector the more pressure there is in the burn chamber.

open during low atmosphere air breathing operation, half closed during high atmosphere ramjet and fully closed when in trans atmosphere rocket mode.

One would need to write a complete new engine for that including a plugin, I pressume.

[Kreuzung]

One would need to write a complete new engine for that including a plugin, I pressume.

Not really, most plugins extend the original engine since it\'s hard to rewrite engines from scratch (and causes MechJeb to ignore them).

Look in the plugin-powered add-on forum, kellven made an engine that has lower fuel consumption out of the atmosphere.

It originally used LOX/LH2, but that caused bugs in .15, I think.

[Braun]

Not really, most plugins extend the original engine since it\'s hard to rewrite engines from scratch (and causes MechJeb to ignore them).

Look in the plugin-powered add-on forum, kellven made an engine that has lower fuel consumption out of the atmosphere.

It originally used LOX/LH2, but that caused bugs in .15, I think.

That\'d be nice, if I\'d knew what the heck I should do with DLLs.

[princessjane]

An engine that could switch from being a jet engine to a rocket would not be pointless. A jet engine gets its oxidizer from the surrounding atmosphere, which means that your vehicle doesn\'t need to carry that oxidizer with it. This makes a smaller rocket for the same payload. The increased mass of the exhaust vs. the input fuel means you can lower the speed of the exhaust without sacrificing force, so the effective Isp is larger for a jet engine than for an equivalent rocket engine using the same reactants. This, again, raises delta-V.

If the two engines are separate, in order to get both the advantages of a rocket (operates in space) and a jet engine (operates on free ambient oxidizer), you need two engines — one for atmospheric flight, and one for space flight. That\'s a weight penalty, resulting in a lower delta-V. If you have an engine that can switch between these two modes, air breathing in an atmosphere, a rocket in space, then you don\'t pay that weight penalty.

+1

would be even better if they were 'smart' engines that switched based on the presence of atmo, then it would just be a matter of figuring out how much oxidizer you no longer needed.

[Braun]

would be even better if they were 'smart' engines that switched based on the presence of atmo, then it would just be a matter of figuring out how much oxidizer you no longer needed.

that\'d be easy based on fuel intake and internal pressures.

turbofan gears up and turns faster than normal while compression chamber pressure drops -> low atmosphere, switch to ramjet - funel vents close, valves open to reroute fuel and cone between compression chamber and engine chamber regulates flow of air and fuel.

pressure drops further -> cone is pressed into the connection, oxidizer valves open, engine goes rocket mode.

[kellven]

Good thing I decided to check this subforum for once.

I made a plugin that extends mechjeb\'s rocket engine extension code so that it has an airbreathing mode, and a rocket mode, and switches automagically at a predefined altitude.

It was an attempt to implement functionality for the SABRE engines for the SKYLON project, but the plugin was configurable to whatever specs you put in. Thobewill10 kindly made a model for it, but the code works with whatever model it\'s given.

I haven\'t worked on it in quite a while, and it appears I never repackaged the last fix into a proper plugin, so I\'ll go do that now.

The 0.14 versions ran on special LOX/LH2 tanks, but I think the 0.15 stuff runs on normal fuel, as the fuel system in 0.15 is um...'feature-full'.

The thread is buried here:

[fowlplaychiken]

Oh awesome, *deletes thread asking for sabre engines*

[fowlplaychiken]

Where can we find that plugin?

[Braun]

Good thing I decided to check this subforum for once.

I made a plugin that extends mechjeb\'s rocket engine extension code so that it has an airbreathing mode, and a rocket mode, and switches automagically at a predefined altitude.

It was an attempt to implement functionality for the SABRE engines for the SKYLON project, but the plugin was configurable to whatever specs you put in. Thobewill10 kindly made a model for it, but the code works with whatever model it\'s given.

I haven\'t worked on it in quite a while, and it appears I never repackaged the last fix into a proper plugin, so I\'ll go do that now.

The 0.14 versions ran on special LOX/LH2 tanks, but I think the 0.15 stuff runs on normal fuel, as the fuel system in 0.15 is um...'feature-full'.

The thread is buried here:

great. *looks for a way to close this one*

[shadowfax]

An engine that could switch from being a jet engine to a rocket would not be pointless. A jet engine gets its oxidizer from the surrounding atmosphere, which means that your vehicle doesn\'t need to carry that oxidizer with it. This makes a smaller rocket for the same payload. The increased mass of the exhaust vs. the input fuel means you can lower the speed of the exhaust without sacrificing force, so the effective Isp is larger for a jet engine than for an equivalent rocket engine using the same reactants. This, again, raises delta-V.

If the two engines are separate, in order to get both the advantages of a rocket (operates in space) and a jet engine (operates on free ambient oxidizer), you need two engines — one for atmospheric flight, and one for space flight. That\'s a weight penalty, resulting in a lower delta-V. If you have an engine that can switch between these two modes, air breathing in an atmosphere, a rocket in space, then you don\'t pay that weight penalty.

So meanwhile as you\'re flying around with the jet, you\'re burning fuel. If you burn enough fuel so that you no longer have enough fuel to use up all your oxidizer, then you\'ve just magically added an oxidizer weight penalty to your ship that doesn\'t need to be there.

I should, however, have been more clear - the more efficient design is to have a jet take the ship up to altitude, then separate and go back to the runway while the ship continues on rocket power.

We\'re also assuming that we have a fuel which will play nice both with air as an oxidizer and with the oxidizer you bring with you. There\'s a reason rocket fuel isn\'t gasoline and an oxygen bottle.

[Wyrm]

So meanwhile as you\'re flying around with the jet, you\'re burning fuel. If you burn enough fuel so that you no longer have enough fuel to use up all your oxidizer, then you\'ve just magically added an oxidizer weight penalty to your ship that doesn\'t need to be there.

You can do this with the two-engine design as well, so I don\'t see how it would be an improvement.

I should, however, have been more clear - the more efficient design is to have a jet take the ship up to altitude, then separate and go back to the runway while the ship continues on rocket power.

Sure, but then you don\'t have the jet engine available for a powered landing, which some space plane designs would need. You\'d then have to lug the jet engine along your entire trip and you\'re right back where you started.

We\'re also assuming that we have a fuel which will play nice both with air as an oxidizer and with the oxidizer you bring with you. There\'s a reason rocket fuel isn\'t gasoline and an oxygen bottle.

Yes, a chemical rocket probably isn\'t going to play well with atmospheric oxygen, but a nuclear rocket —where energy production from the fuel has jack all to do with the reaction mass you squirt out the rear— this might be an option.

[kellven]

The thing about fuel/oxidizer loadout mismatch is mostly solved IRL by simple mission planning. Only in KSP do we just strap on some tanks/engines and liftoff.

That said, fuel, at least LH2, is extremely light, as in virtually negligible in a functional rocket design, and again with LOX/LH2, the actual mass of fuel/oxidizer is also tiny, 1 LH2 : 6 LOX is the stoichiometric ratio, which in practice means it\'s the LOX requirement you have to worry about. The tank size (atmo drag), and cryogenic system that make it not so trivial IRL, but KSP doesn\'t really model those well enough to consider.

When I had separate LH2/LOX tanks in KSP, I just setup a lot of 'structural' pieces to hold LH2, since almost all of the mass was the tank itself. In flight, this meant I had to guestimate the amount of LOX tanks I would need in space, add an equal number of matched tanks, and just build the rest of my chassis with 'structural' LH2 tanks, and I never really had to worry about fuel. As long as I brought along enough LOX to get through the space portion, I always had plenty of fuel to joyride through the atmosphere on return, and do a powered landing with fuel still in the tanks.

If you\'re using separate vacuum/atmo engines, then you have to consider engine weight, as atmo engines are heavy, but if you have combined engines, it\'s just a flat engine mass increase offset many, many times over by not having to haul extra oxidizer through the atmosphere in the most demanding stage of the flight.

If you are using separate engines, a powered landing doesn\'t really take that much fuel. The lower atmosphere does almost all of the work, so dumping the atmospheric engines may well give you a better working mass in space, but it also shifts COM, which could cause problems later on. If you want to adjust your landing after de-orbit, however, or if you\'re taking off again, might as well just keep them. Bringing along the jets could also let you do a deorbit burn with just LH2 if you set periapsis in the stratosphere, potentially saving a few orbits.

[Braun]

Guys, stop arguing and read the wikipedia entry for the aerospikes and sabre.

The aerospike engine is a type of rocket engine that maintains its aerodynamic efficiency across a wide range of altitudes through the use of an aerospike nozzle. It is a member of the class of altitude compensating nozzle engines. A vehicle with an aerospike engine uses 25–30% less fuel at low altitudes, where most missions have the greatest need for thrust. Aerospike engines have been studied for a number of years and are the baseline engines for many single-stage-to-orbit (SSTO) designs and were also a strong contender for the Space Shuttle Main Engine. However, no engine is in commercial production. The best large-scale aerospikes are still only in testing phases.[1]

The design comprises a single combined cycle rocket engine with two modes of operation.[2] The air breathing mode combines a turbo-compressor with a lightweight air precooler positioned just behind the inlet cone. At high speeds this precooler cools the hot, ram compressed air leading to an unusually high pressure ratio within the engine. The compressed air is subsequently fed into the rocket combustion chamber where it is ignited with stored liquid hydrogen. The high pressure ratio allows the engine to continue to provide high thrust at very high speeds and altitudes. The low temperature of the air permits light alloy construction to be employed which gives a very lightweight engine — essential for reaching orbit.

[theflyingfish]

So meanwhile as you\'re flying around with the jet, you\'re burning fuel. If you burn enough fuel so that you no longer have enough fuel to use up all your oxidizer, then you\'ve just magically added an oxidizer weight penalty to your ship that doesn\'t need to be there.

That\'s why you bring along only the oxidizer needed for the vacuum portion of your trip. The reason the SABRE engine is so much more efficient than conventional rockets is that it doesn\'t need to bring all that extra oxidizer along, which makes up a significant very large portion of the mass of the ship (because liquid oxygen is so much more dense than liquid hydrogen). The ship would bring along lots of low-mass LH2, but only enough L0X as needed to attain orbital speed and circularize. And those tasks would be made easier because you already gained most of orbital speed in the atmosphere, using the scramjet/ramjet and the oxidizer already prevalent in the atmosphere. It is only in rocket/oxidizer mode for a short period of time.

For a good example of this type of craft, look at Moach\'s G42-200, which uses combination turbofan/ramjet/scramjet/rocket engines fed by LH2 and LOX, and an RCS OMS (Orbital Maneuvering System).

[Wyrm]

To be fair, the attraction of an aerospike has nothing to do with being able to use atmospheric oxygen, but rather compensating for the effect of ambient atmospheric pressure on exhaust expansion. A rocket bell has to be designed for a specific altitude to work most efficiently. Operate too high and the gas expands past the bell, lowering efficiency; operate too low and you\'re dragging weight of unused bell, as well as having the potential for dangerous oscillations.

The sabre engine is an example of an engine that can switch between air breathing and sucking in oxygen. Thanks, Braun.