KSP vs. Real world

[MrMisu]

Hello guys,

I know that a lot of guys that work in the space/aeronautic industry play KSP, so mabey some one can give me an answer on the following:

What technology, materials, procedures etc. are we missing in the real world to be able to build a space plane? For me this seems the easyest (most efficient) way to get things into orbit? Use jet engines to get to upper atmospere then use rokets.

Thanks,

Misu

[Nibb31]

Hello guys,

I know that a lot of guys that work in the space/aeronautic industry play KSP, so mabey some one can give me an answer on the following:

What technology, materials, procedures etc. are we missing in the real world to be able to build a space plane? For me this seems the easyest (most efficient) way to get things into orbit? Use jet engines to get to upper atmospere then use rokets.

Thanks,

Misu

This is where KSP gives the impression that reaching orbit is easy, when it actually is insanely hard. KSP has the balance of spaceplanes completely off.

We are missing a lot of things to make an SSTO spaceplane, but the biggest roadblock is weight.

A conventional rocket is basically engines+propellant+tankage+payload. The payload fraction is about 0.5% of the takeoff mass of the rocket, which is achieved by throwing away empty tankage along the way.

A spaceplane is engines+propellant+tankage+wings+landing gear+heatshield+hydraulics+payload. That extra stuff that you are carrying all the way to orbit and back is much heavier than a 0.5% payload fraction, meaning that you won't reach orbit with any significant payload.

Your particular proposal involves carrying (at least) two sets of engines, which means even more mass. Those jet engines are also going to have to survive hypersonic re-entry, which requires some sort of shielding or fold-away contraption, making it more complex and heavy. Even if those jet engines take you Mach 5, that's still only 20% of the acceleration you need to reach orbital speed (which is ~27000 km/h).

There are 3 tricks to make an actual reusable SSTO spaceplane:

- Carry less propellant by making the engines more efficient. This is hard, because the same engine has to propel the vehicle from subsonic to hypersonic, both in and out of the atmosphere.

- Make the airframe lighter. For example, using a lifting bodies is a way to combine tankage with "wings", but there are physical limits to how light the airframe can be.

- Find a business model that makes a reusable spaceplane viable. This is probably the hardest bit because there is no market to absorb the high flight rate that a reusable SSTO is designed for.

Edited December 1, 2014 by Nibb31

[kerbiloid]

First of all, we miss a planet with 2.6 km/s orbital speed.

[zekes]

Another big problem is aerodynamics - the atmosphere is like pea soup

[Starwhip]

Another big problem is aerodynamics - the atmosphere is like pea soup

"Souposphere", not atmosphere.

[Megalodon 720]

I actually like the current atmosphere.

[sal_vager]

It's easier to get to orbit with either a rocket or a spaceplane with the FAR addon, just saying

Though preventing your plane from flipping out on the way up, and slowing it on the way down? Nah

Fun though.

[astropapi1]

In normal KSP, you can see Kerbin's curvature from the launchpad. That might have something to do with it, don't you think?

If you want to understand why we don't have SSTOs in real life, playing RSS might be a good start.

[Yarbrough08]

@Nibb31: Although I agree with you completely, I want to point out that you are oversimplifying your answer a little in a biased way. Heatshield is required by both, albeit a larger one for a spaceplane. Landing gear is offset at least a little bit by a lack of full parachutes- although SPs could include drag 'chutes. Hydraulics and equipment to move the control surfaces though....

@MrMisu: I would also like to throw in expenses. It is suppose to be cheaper, but it will rarely turn out that way..

The shuttle was originally purposed to cost in the mid $50 million per launch (you can check this out in NASAs archives btw, I did a paper about this recently for a class), but instead cost between $300 - $600 million per launch. That price range will also send a mission to the moon (probe or rover style mission if I remember correctly?), which is of course something that the shuttle could not do..

As an American, I feel like the shuttle program was a failure. It was costly, could not meet its launch quota (org proposed 24 a year - best was in 80s @ 11), could not carry as much weight as planned (although I thought is was descent), could not take us beyond LEO limiting what we could accomplish. Then there were those tiles.. I DO NOT, feel like it is NASAs fault though. Congress approved the program as the budget for NASA was falling through the floor.

Now a re-useable CM might be worth it without the additional weight of those extra flight systems and would probably be far cheaper to maintain. Don't get me wrong btw, the shuttle was a marvel (especially for the time). Which is why I think it captures peoples imagination, but imagine if we had kept the Saturn V and only used it to go to LEO.

[RobotsAndSpaceships]

I actually like the current atmosphere.

Love me some atmo-soup.

[Guest]

Technically, there's nothing stopping us from developing a spaceplane, there's nothing missing. So we are making one. That said, it'll be very different from garden variety KSP SSTO. It's called the Skylon.

Take a good look at Skylon. It's our best shot at a true spaceplane. Large tanks, very efficient, state of art engines (thanks to hydrogen fuel, which is the reason for the tanks) and not all that much payload to orbit. It does seem like it could make it economically, even, though it'd have a hard time competing with rockets. I suppose you could shrink it's airframe if you converted it to use kerosene (less efficient, but a lot more dense, which would mean less heatshielding and structure), cut down on the payload and used a kerosene-burning SABRE. I was fooling with that concept in RSS, but found the Mk2 set lacks some parts I wanted to have. That said, it'd essentially be an orbital business jet, so we're not going to make such a thing unless there's a market for those (there isn't).

[cantab]

The big one I think is the jet engine. It needs to operate at something above Mach 5 to make enough of a dent in delta-V requirements that rockets can propel the plane to orbit, that's tough. It needs to also operate from a standstill and that's really tough. The Reaction Engines SABRE (that inspired KSP's RAPIER) is one of the few realistic efforts at this, but it's some way off ever propelling a vehicle.

The SABRE, incidentally, requires a cryogenic fuel; it's used to chill the incoming air before being burnt. A kerosene-burning SABRE would be impossible. Even liquid methane wouldn't be able to offer the same performance in the precooler.

[Yarbrough08]

It needs to also operate from a standstill and that's really tough.

You know, this got me thinking... Theoretically, you could have a platform the took it from the runway to a higher altitude and then launch off of it.. Similar to the what was done for most of the old X-# series. Now I know this could not technically be called an SSTO, but here is the interesting thing: the SP could after reentry re-dock inflight with the same platform for landing, or perhaps, even for refueling... The inflight docking would not much more difficult (I would think) than inflight refueling that is already performed...

This could save weight, while allowing the same crew to come back to earth and re-launch into space without ever even leaving the vehicle...

But of course this would probably have a tiny payload fraction..

[Bill Phil]

Well, KSP isn't 100% accurate... But it does teach you more about orbits than battlefield at shooting.

I think the main problem is that in KSP you just put stuff together in a Lego like way. It's not like that in real life. It take months to design and possibly years to build. Not to mention the constant threat of budget cuts.

[razark]

What ... are we missing in the real world to be able to build a space plane?

Money and desire.

If someone has enough money and the desire to do it, we could do it with today's technology.

Money tends to be the limiting factor in many space enterprises. There's nothing that says we can't start building a generational starship to take us to other star systems using the technology we have today. The problem is that we don't have the money for it, and we would need to strip mine half the earth and pollute the hell out of the other half to do it. And it seems no one desires a generational starship that much.

[Nibb31]

You know, this got me thinking... Theoretically, you could have a platform the took it from the runway to a higher altitude and then launch off of it..

Air launch is nothing new (see Stratolaunch, Pegasus, Spiral/BOR, Lockheed D-21, X-15, SpaceShipOne, etc...).

Getting to space is all about hitting orbital speed, not altitude. Subsonic air launch doesn't gain you much dV, it only acts as a rather poor first stage.

Hypersonic air launch has never been done, because it's hard to get to those flight regimes with anything bigger than a missile. Separation manoeuvers at such high speed are likely to end up badly (like the A-12/D-21 fiasco)

[cantab]

An air launch would serve to "start" a ramjet. But then you can just do that from the runway with JATO boosters.

[MrMisu]

Wow, first of all thanks to all of you, I have a lot of stuff, from your answers, to google in the next days.

I know that KSP isn't a realistic depiction of space flight (especially from surface to LEO), but from what I got from your answers, engine inefficiency/dual behavior needed (atmosphere/space) is what keeps us from building a space plane.

Misu