Turbopump spool-up rate

[TheDestroyer111]

Well, most liquid fueled rocket engines use turbopumps to get fuel into the combustion chamber. And the turbopump is essentially a tiny jet engine that pushes fuel. And in KSP, jet engines take a long time to spool up, while rockets do so in an instant. So if the turbopump is a tiny jet, shouldn't it take a few seconds for rocket engines to get full thrust?

[Bill Phil]

Yeah. But KSP isn't known for being realistic. 

Also, it's not really like a jet engine, but a centrifugal or axial compressor/pump, which is really only a component of a turbojet. 

Also, rockets are technically jet engines.

Edited July 15, 2016 by Bill Phil

[DDE]

Furthermore, most rocket engines cannot throttle down as easily, and cannot be fired more than once. And then we have ullage and lOx boil-off.

We have it really easy in KSP.

[wumpus]

As far as I know, realism overhaul deals with a number of these issues, but I'm not at all sure about turbopump spool-up rate.  I think it models that one, as well.  I haven't downloaded it (I have enough trouble with near-stock), but I think I've seen you-tube vids with spool-up, ignition, and then liftoff.

[Camacha]

7 hours ago, Bill Phil said:

Also, rockets are technically jet engines.

How so? Due to the pump? It is just a fuel pump. Some rocket engines work through other methods, but I would consider neither to make the rocket engine a jet engine.

[Brotoro]

A jet engine is a reaction engine that produces thrust by emitting a jet of matter. Rocket engines are a subset of jet engines because they work by this principle. When most people think of jet engines, they are probably thinking only of the subset of jet engines that are some form of internal combustion air-breathing jet engine (such as turbojets, ramjets, etc.).

Edited July 16, 2016 by Brotoro
Typo

[Van Disaster]

Rockets provide their own reaction mass, I think that's the quantifiable difference.

[TheDestroyer111]

3 hours ago, Van Disaster said:

Rockets provide their own reaction mass, I think that's the quantifiable difference.

As do airbreathing jets. Only propellers use completely external reaction mass (but they may use fuel for power). Airbreathing jet engines have their reaction mass from fuel and air combined. Rockets, however, carry oxidizer internally and don't use air for reaction mass at all.

[Camacha]

On 16/07/2016 at 6:52 AM, Brotoro said:

A jet engine is a reaction engine that produces thrust by emitting a jet of matter.

By that definition, a propeller or duct fan aircraft is jet powered too. Air is matter, after all.

[Van Disaster]

11 hours ago, TheDestroyer111 said:

As do airbreathing jets. Only propellers use completely external reaction mass (but they may use fuel for power). Airbreathing jet engines have their reaction mass from fuel and air combined. Rockets, however, carry oxidizer internally and don't use air for reaction mass at all.

Unless it's an air-augmented rocket, just to confuse matters more. Jet engines can't work in a vacuum ( other than a pointless mechanical reaction from spraying fuel ) even if you fed self-combusting fuel because they need external working mass. It's only ramjets that use all their energy as reaction, most of a turbo engine's work downstream of combustion goes into driving the compressor.

[Bill Phil]

On July 15, 2016 at 9:54 PM, Camacha said:

How so? Due to the pump? It is just a fuel pump. Some rocket engines work through other methods, but I would consider neither to make the rocket engine a jet engine.

A jet is a stream of fluid projected into the surrounding medium, as per Wikipedia. Propellers use that medium, so it's not a jet.

Even pressure fed rocket engines are jet engines. Because the reaction mass is expelled as a jet.

[TheDestroyer111]

10 hours ago, Van Disaster said:

Unless it's an air-augmented rocket, just to confuse matters more. Jet engines can't work in a vacuum ( other than a pointless mechanical reaction from spraying fuel ) even if you fed self-combusting fuel because they need external working mass. It's only ramjets that use all their energy as reaction, most of a turbo engine's work downstream of combustion goes into driving the compressor.

Standard turbojets can work in vacuum even if fuel is self-combusting. The compressor does NOT push working mass, what the compressor does is increasing air pressure to make combustion more effective. If you analyse a compressor's design, you see that it cannot create thrust by itself even if something rotates it. Jet engines can create thrust without external working mass, but are more efficient when collecting oxidizer and reaction mass from the atmosphere. Really, the only difference between a turbojet and ramjet is the compression method-ramjets compress air by literally ramming it into the engine, while turbojets use an axial or centrifugal compressor. Turbofans and sometimes even turboprops/turboshafts/propfans can create thrust without external reaction mass if provided with self-combusting fuel, but are not that effective as much of their jet thrust is sacrificed to drive the fan/propeller. Turbojets and even ramjets are also less efficient than rockets (in vacuum with self-combusting fuel) as turbojets take some jet thrust to drive the compressor, while ramjets use nozzles that work better in atmosphere and at high speeds.

[Motokid600]

On ‎7‎/‎15‎/‎2016 at 10:39 PM, wumpus said:

As far as I know, realism overhaul deals with a number of these issues, but I'm not at all sure about turbopump spool-up rate.  I think it models that one, as well.  I haven't downloaded it (I have enough trouble with near-stock), but I think I've seen you-tube vids with spool-up, ignition, and then liftoff.

Yea RO does factor in spool-up. I found out the hard way trying to land rocket stages  It makes KoS and knowing how to code a dependency if you want to do it in RSS/RO. And many rockets lost to releasing launch clamps too early. 

 

 

[Brotoro]

22 hours ago, Camacha said:

By that definition, a propeller or duct fan aircraft is jet powered too. Air is matter, after all.

A propeller? No. A 'jet' is a stream of fluid constrained to pass through a nozzle or aperture. A propeller does not produce a constrained jet of fluid.

Ducted fan... I don't mind that one. All of the jet engines that have pushed me around in the sky recently get most of their thrust from their ducted fans parts. Now exactly how much ducting is needed to provide a jet effect is something I will leave up to the aerodynamicists to decide, since it's a gradual change.

but rocket engines clearly all produce a jet of fluid to produce their reaction thrust, so they are clearly jet engines.

9 hours ago, TheDestroyer111 said:

Standard turbojets can work in vacuum even if fuel is self-combusting. The compressor does NOT push working mass, what the compressor does is increasing air pressure to make combustion more effective. If you analyse a compressor's design, you see that it cannot create thrust by itself even if something rotates it. Jet engines can create thrust without external working mass, but are more efficient when collecting oxidizer and reaction mass from the atmosphere. Really, the only difference between a turbojet and ramjet is the compression method-ramjets compress air by literally ramming it into the engine, while turbojets use an axial or centrifugal compressor. Turbofans and sometimes even turboprops/turboshafts/propfans can create thrust without external reaction mass if provided with self-combusting fuel, but are not that effective as much of their jet thrust is sacrificed to drive the fan/propeller. Turbojets and even ramjets are also less efficient than rockets (in vacuum with self-combusting fuel) as turbojets take some jet thrust to drive the compressor, while ramjets use nozzles that work better in atmosphere and at high speeds.

A turbojet burning a monopropellant or bipropellants in a vacuum (without air for the compressor to push into the combustion area) will end up expelling exhaust out of both the rear and FRONT of the engine. The wall of compressed air from the compressor serves as the front of the combustion chamber, which would be missing in a vacuum.

[Camacha]

2 hours ago, Brotoro said:

A propeller? No. A 'jet' is a stream of fluid constrained to pass through a nozzle or aperture. A propeller does not produce a constrained jet of fluid.

The definitions here seem rather a bit contrived, especially since the various definitions of a jet engine do not seem to line up. Whatever floats your boat, it is a semantic discussion anyway.

[Bill Phil]

13 minutes ago, Camacha said:

The definitions here seem rather a bit contrived, especially since the various definitions of a jet engine do not seem to line up. Whatever floats your boat, it is a semantic discussion anyway.

Propellers work by creating a disturbance in the medium. It's somewhere in between, yes. But a jet is usually considered to be something that comes out of a hole of some sort. Rocket engines are commonly referred to as jet engines, as they are such.

[Camacha]

38 minutes ago, Bill Phil said:

 as they are such.

I think this was the nature of the discussion. Even if technically correct, anyone referring to a jet engine in common and even most technical speak will refer to a turbojet engine powered craft when they use the term jet engine. When you stricly adhere to the an opening through which something gets pushed definition, you run into trouble sooner or later, as there are ducted fans and enclosed propellors that provide a gradient without boundaries. That makes it a fully semantic discussion.

[Bill Phil]

2 hours ago, Camacha said:

I think this was the nature of the discussion. Even if technically correct, anyone referring to a jet engine in common and even most technical speak will refer to a turbojet engine powered craft when they use the term jet engine. When you stricly adhere to the an opening through which something gets pushed definition, you run into trouble sooner or later, as there are ducted fans and enclosed propellors that provide a gradient without boundaries. That makes it a fully semantic discussion.

The jet refers only to the exhaust.

In the common word, a jet can be many things. An airplane, an engine, water shooting out of holes in a pool... and so on.

It is semantics, but effective communication requires that both parties know what each other's words mean.

Many jet engines are not turbojets, but other kinds of air breathing jet engines. And "jet" is also a term for jet plane. There's a lot of grey areas.

[wumpus]

13 hours ago, Brotoro said:

A propeller? No. A 'jet' is a stream of fluid constrained to pass through a nozzle or aperture. A propeller does not produce a constrained jet of fluid.

Ducted fan... I don't mind that one. All of the jet engines that have pushed me around in the sky recently get most of their thrust from their ducted fans parts. Now exactly how much ducting is needed to provide a jet effect is something I will leave up to the aerodynamicists to decide, since it's a gradual change.

The problem is that as far as I know, when a "jet" pulls up to your gate you will probably see a [mostly] ducted fan.  While there might be a jet engine somewhere under all that, a large amount of air is bypassed (I suspect to be later heated by the exhaust).  Pure jet engines were a development step of the 1940s, but might still exist in afterburners (and I'd expect "pure" jets to be roughly as inefficient).

I'm sure this is mostly wrong (but still slightly more accurate than calling modern jets "jets"), so any corrections would be appreciated.

[Brotoro]

2 hours ago, wumpus said:

The problem is that as far as I know, when a "jet" pulls up to your gate you will probably see a [mostly] ducted fan.  While there might be a jet engine somewhere under all that, a large amount of air is bypassed (I suspect to be later heated by the exhaust).  Pure jet engines were a development step of the 1940s, but might still exist in afterburners (and I'd expect "pure" jets to be roughly as inefficient).

I'm sure this is mostly wrong (but still slightly more accurate than calling modern jets "jets"), so any corrections would be appreciated.

Why is that a problem? That's what I Was just refrring to... that engines on modern jet aircraft get most of their thrust from the flow through the fan and out the cold nozzle of the engine, not through the gas turbine core... so I have no problem with calling ducted fan engines a form of jet propulsion. One could design an ultra-high-bypass engine that extracts as much energy as possible from the gas turbine's exhaust stream to put all that energy into the ducted fan, so that almost all of the engine thrust comes from the jet of air moving through the fan and out the cold nozzle.

As long as the engine produces a jet of expelled fluid that provides the thrust by reaction force, the engine meets the definition of a jet engine. 

And, back to the original point of this tangential conversation, a rocket engine by definition derives its propulsive force by emitting a jet of fluid, so it is absolutely a subset of the family of jet engines.

[WestAir]

To add to the semantics, air is a fluid.

[TheDestroyer111]

On 7/18/2016 at 5:48 AM, Brotoro said:

A turbojet burning a monopropellant or bipropellants in a vacuum (without air for the compressor to push into the combustion area) will end up expelling exhaust out of both the rear and FRONT of the engine. The wall of compressed air from the compressor serves as the front of the combustion chamber, which would be missing in a vacuum.

Thanks for clarifying some things to me.