Rapier (SABRE) but no VASIMIR?

[Jonfliesgoats]

Hey, KSP folks.  How come we have untested engines like the Rapier, but we don't have PDE engines or VASIMIR engines?  While I'd like to ditch the Rapier until the SABRE at least does some bench tests, I know that's not feasible.  Can we at least get more engines that have actually been tested?

[tater]

True, they're all just as fanciful as each other. But we have SABRE because spaceplanes. 

[Temeter]

Because you'd need a whole set of mechanics to balance out vasimirs.

And stuff like the mars mission vasimir needs high performance nuclear reactors producing enough electricity to support a small city.

 

In terms of performance itself, they aren't all that different from the ion engine.

[tater]

And ion engines already don't work properly in KSP. Without constant-thrust trajectories, they are sort of pointless, IMHO.

[p1t1o]

Rapier is *not* sabre, rapier is a KSP invention that is both a jet engine and a rocket. The special thermodynamic cycle which is the unique part of SABER is not simulated, its just a decent jet and a decent rocket in one package to simulate near-future possibilities.

We dont have vasimir either, but we do have "Ion" engines that are far and away more powerful than real ion engines, ie: probably a fairly close approximation of VASIMIR - a high-thrust electric engine.

 

 

[Shpaget]

Guys, it's VASIMR, not VASIMIR or VASMIR. It's all capital letters.

[Wjolcz]

1 hour ago, p1t1o said:

Rapier is *not* sabre, rapier is a KSP invention that is both a jet engine and a rocket. The special thermodynamic cycle which is the unique part of SABER is not simulated, its just a decent jet and a decent rocket in one package to simulate near-future possibilities.

I feel like it's kind of a bad argument acutally. Many things aren't simulated in KSP. Where are the turbopumps or helium tanks? Why does the NTR work fine with aerozine? And I think the "oh, but it's a game, it doesn't need all that complexity" and "because it's kerbal" arguments don't really apply here, do they? As much as I agree that engines don't need to be that complicated, RAPIER still is a SABRE-like hybrid engine, whether you want to believe it or not. We don't even have electric propellers for oxygenless atmospheric flight, even though these things were tested and functioned perfectly fine IRL.

Edited November 23, 2016 by Veeltch

[p1t1o]

15 hours ago, Veeltch said:

I feel like it's kind of a bad argument acutally. Many things aren't simulated in KSP. Where are the turbopumps or helium tanks? Why does the NTR work fine with aerozine? And I think the "oh, but it's a game, it doesn't need all that complexity" and "because it's kerbal" arguments don't really apply here, do they? As much as I agree that engines don't need to be that complicated, RAPIER still is a SABRE-like hybrid engine, whether you want to believe it or not. We don't even have electric propellers for oxygenless atmospheric flight, even though these things were tested and functioned perfectly fine IRL.

Fair enough I suppose, this is my point of view:

If Rapier is a simplified SABRE, then the ion engines are still a simplified VASIMR.

According to a quick google, a contemporary VASIMR produces approx. 5N for 200kW. The current KSP "Dawn" electric engine produces 2,000N for 3.6E/s.

So if you really want a true VASIMR replica, its going to be absolutely useless, even if you improve it by a factor of 10. Or you tweak it to be useful, in which case you'd end up with something pretty close to what we have.

(I dunno if you are into mods at all, but it almost goes without saying there are some excellent part mods for extra engines like these - with stats useful for KSP though.)

[KSK]

16 hours ago, Veeltch said:

Why does the NTR work fine with aerozine?

I can't think of a reason why you couldn't use aerozine with a nuclear thermal engine. At least in principle - I'm not sure about the exact materials science involved.  You wouldn't get KSP levels of ISP though without using hydrogen as a propellant. But KSP engines are all over the place anyway when it comes to propellants and performance.

[Wjolcz]

2 hours ago, p1t1o said:

Fair enough I suppose, this is my point of view:

If Rapier is a simplified SABRE, then the ion engines are still a simplified VASIMR.

According to a quick google, a contemporary VASIMR produces approx. 5N for 200kW. The current KSP "Dawn" electric engine produces 2,000N for 3.6E/s.

So if you really want a true VASIMR replica, its going to be absolutely useless, even if you improve it by a factor of 10. Or you tweak it to be useful, in which case you'd end up with something pretty close to what we have.

(I dunno if you are into mods at all, but it almost goes without saying there are some excellent part mods for extra engines like these - with stats useful for KSP though.)

My point isn't really that I want a VASIMR in the game, but more of a question of why the heck do we have RAPIER (AKA SABRE) if it's never been tested nor built before.

As much as I would love to see the actual Skylon fly, there are other engines that already work and exist IRL and they could be included in the game. Yet they aren't, for some odd reason.

[p1t1o]

33 minutes ago, Veeltch said:

My point isn't really that I want a VASIMR in the game, but more of a question of why the heck do we have RAPIER (AKA SABRE) if it's never been tested nor built before.

As much as I would love to see the actual Skylon fly, there are other engines that already work and exist IRL and they could be included in the game. Yet they aren't, for some odd reason.

Well my answer would still have to be "because it isn't SABRE".

There is no "odd reason" that VASIMR doesn't have a direct analogue - the reason is very simple: the thrust level, if realistically portrayed, would be useless.

[Wjolcz]

17 minutes ago, p1t1o said:

Well my answer would still have to be "because it isn't SABRE".

But if you say that RAPIER is not SABRE then the LV-N is not a nuclear engine, because it's heating is all wrong and it doesn't use hydrogen. Then why the hell is it so long and so heavy and looks like a nuclear engine? Well, the asnwer is: because it's a nuclear engine. By that logic RAPIER is SABRE, because it's hybrid. You can't say it isn't because that's simply not true.

Edited November 24, 2016 by Veeltch

[p1t1o]

26 minutes ago, Veeltch said:

But if you say that RAPIER is not SABRE then the LV-N is not a nuclear engine, because it's heating is all wrong and it doesn't use hydrogen. Then why the hell is it so long and so heavy and looks like a nuclear engine? Well, the asnwer is: because it's a nuclear engine. By that logic RAPIER is SABRE, because it's hybrid. You can't say it isn't because that's simply not true.

LV-N is a nuclear engine, it just isn't a *specific, real-world* nuclear engine. Reaction engines did not invent the jet/rocket hybrid engine with SABRE, they invented *a* hybrid engine of particular design.

If Rapier is a SABRE, then it is at the same time an RB545 as well.

Either way, the only way to have a VASIMR be of any use would be to make it arbitrarily similar to the current KSP ion engines.

[Streetwind]

@Veeltch - KSP is a game, made to be fun. Spaceplanes are fun for a lot of people. The game needed a good spaceplane engine. Real life has few, if any, inspirational sources for spaceplane engines. The SABRE engine was new and cool and applicable. Harvester is a space geek, and it was cool enough to him to put in. And it made spaceplanes fun.

Sometimes the answers are just that simple. There's no need to look for sense where there is none beyond one person's whim.

[Wjolcz]

14 minutes ago, p1t1o said:

LV-N is a nuclear engine, it just isn't a *specific, real-world* nuclear engine. Reaction engines did not invent the jet/rocket hybrid engine with SABRE, they invented *a* hybrid engine of particular design.

If Rapier is a SABRE, then it is at the same time an RB545 as well.

Either way, the only way to have a VASIMR be of any use would be to make it arbitrarily similar to the current KSP ion engines.

Fair enough. It's still a working hybrid engine that has never been built and functioned IRL (not yet at least).

I think a nice compromise to have a VASIMR in the game would be to simply intorduce a nuclear reactor. 

1 minute ago, Streetwind said:

@Veeltch - KSP is a game, made to be fun. Spaceplanes are fun for a lot of people. The game needed a good spaceplane engine. Real life has few, if any, inspirational sources for spaceplane engines. The SABRE engine was new and cool and applicable. Harvester is a space geek, and it was cool enough to him to put in. And it made spaceplanes fun.

Sometimes the answers are just that simple. There's no need to look for sense where there is none beyond one person's whim.

Not true. The linear aerospike is also a good SSTO engine. Example: linear aerospike built for X-33. It never flew, but it was actually built and tested, unlike SABRE.

[DoctorDavinci]

I don't want a nuclear reactor in stock KSP .... We need to pollute the atmosphere of Kerbin so our little green buddies can thrive in a carbon dioxide rich atmosphere since they live through photosynthesis 

Go clean coal 

[Red Iron Crown]

6 hours ago, KSK said:

But KSP engines are all over the place anyway when it comes to propellants and performance.

The chemical rocket engines are pretty consistent, with performance that is pretty similar to Aerozine 50/NTO in various combustion cycles, chamber pressures, and nozzle shapes. 

The others are mostly gameplay concessions. The LV-N is a bit simplified so the game doesn't need another propellant resource and associated tanks, the ion has unrealistically high thrust to compensate for the lack of high timewarp thrusting, and the airbreathers are at the extreme upper end of the realistic range of performance to make spaceplanes more feasible. 

[Gaarst]

53 minutes ago, Red Iron Crown said:

and nozzle shapes. 

The Poodle and Terrier make me sad each time I think about this.

[tater]

People in the "it's kerbal" camp tend to forget that complexity can make the game better, because design choices make the game more interesting. When everything uses the same propellant, for example, it homogenizes design. It makes a big difference if your I_sp trade off includes lower-density propellants that boil off, for example. Or something we'd never see, reliability (one less thing to go wrong when you use hypergolics).

The added complexity of constant-thrust trajectories would be cool, too, and it would point out problem with ion if there was also life support.

This sorts of decisions are FUN, IMO.

[Jonfliesgoats]

12 hours ago, Streetwind said:

@Veeltch - KSP is a game, made to be fun. Spaceplanes are fun for a lot of people. The game needed a good spaceplane engine. Real life has few, if any, inspirational sources for spaceplane engines. The SABRE engine was new and cool and applicable. Harvester is a space geek, and it was cool enough to him to put in. And it made spaceplanes fun.

Sometimes the answers are just that simple. There's no need to look for sense where there is none beyond one person's whim.

 

9 hours ago, tater said:

People in the "it's kerbal" camp tend to forget that complexity can make the game better, because design choices make the game more interesting. When everything uses the same propellant, for example, it homogenizes design. It makes a big difference if your I_sp trade off includes lower-density propellants that boil off, for example. Or something we'd never see, reliability (one less thing to go wrong when you use hypergolics).

The added complexity of constant-thrust trajectories would be cool, too, and it would point out problem with ion if there was also life support.

This sorts of decisions are FUN, IMO.

There are a lot of good points made here, and I wore out my thumbs liking everyone's posts.  Seriously, I really like the discussion here and if I didn't hit the like button on your post, it's because I missed it with my thumb.  Personally, I get something from the challenges posed by KSP.  Having easy performance or wildly optimistic engines makes the game more accessible but a little less fun.  That's just my take.  

Along those lines the Rapier is a stretch to provide acessibility it seems.  When I use them, it feels like I am cheating on an exam and thus receiving an easy pathway to fake spaceflight which would otherwise be reserved for slow children.  I am not impartial, however.  In the game I really favor simple, rocket launch systems,  I am probably being over critical and yielding to my own bias.

Also, Happy Thanksgiving to everyone in the US.

[MaxL_1023]

There are limits to what can be implemented without severely disrupting gameplay. Spaceplanes are only really feasible to begin with because of the downscaling of the Kerbin system - in stock you can get a better payload fraction with a rocket SSTO (think a Reliant or Vector lifting a pod + a bunch of fuel tanks) than you can get in RSS/RO using multistage rockets. 

The rapier is too good in air-breathing mode to begin with - jet engines can't produce thrust easily at Mach 3.6, and the ones that do are not designed to have oxidizer dumped into the combustion chamber to go "rocket mode." In Stock, jet engines can get you more than halfway to orbit (1500 m/s is quite possible using Rapiers in air breathing mode, with 1000 m/s easy). 

In RSS/RO, 1500 m/s won't really get you out of the atmosphere, and you somehow need to find another ~7 km/s of delta-V to get into LEO (assuming you are following a normal gravity turn when you reach that speed). An air breathing engine would have trouble pushing enough fuel to give that much delta-V - their TWR is much lower than rocket engines (think ~5 vs 100+) even factoring out the lack of oxidizer. 

The technology for Practical SSTO vehicles does not exist and is not really being tested yet - KSP having feasible spaceplanes is already a stretch. 

Edit: The concept venturestar spaceplane uses Hydrolox aerospikes and would mass 1000 tons to put 20 into LEO. Not exactly payload efficient. 

Edited November 25, 2016 by MaxL_1023

[Jonfliesgoats]

Max makes a good point.  We already have a softball, scaled down planet with a lower atmospheric scale height to deal with.  Too bad RSS isn't an option for Xbox.  

 

 

 

 

 

 

[wumpus]

Just how different are Kerbal Ion engines from VASIMR engines anyway?  The key point is that all KSP engines (except SRBs) can be throttled.  Also KSP Ion engines already have wildly more power than any ion engine should (which *still* isn't enough to really work in KSP, they need to allow "physicsless" engines that include thrust (note that they are already accelerating thanks to one gravity source in the SOI, they "just" need to allow slighly different acceleration).

SABRE made sense thanks to KSP's silly small planet.  Making such an engine should be easy for Kerbals, harder for us.

[Red Iron Crown]

21 hours ago, wumpus said:

note that they are already accelerating thanks to one gravity source in the SOI, they "just" need to allow slighly different acceleration

When on rails position and speed are determined by Keplerian math rather than directly applying accelerations, it is not as trivial a problem as it first appears. I doubt is in unsolvable, though.

[Gaarst]

23 minutes ago, Red Iron Crown said:

When on rails position and speed are determined by Keplerian math rather than directly applying accelerations, it is not as trivial a problem as it first appears. I doubt is in unsolvable, though.

Squad can't even properly program burn times. We'll end up with something a) terribly broken, or b) absorbing all your CPU whenever you have more than 2 ships flying. (Logical "or" here: both at the same time is possible)