PURE rocket SSTO

[TeeGee]

Hi everyone.

Question:

Why don't engineers design a purely rocket ssto?

Step 1) Use an engine with excellent thrust (http://en.wikipedia.org/wiki/RD-171#RD-171)

Step 2) Attach kerolox fuel (highest thrust to volume fuel)

Step 3) Add wings and a cargo bay

Step 4) Launch like a rocket

Step 5) Land like a glider

I've done it with KSP using RF, Stockalike, FAR, engine ignitor (with 1 ignition on my main engine), B9 parts, DR and stock engines.

That way we can get heavy cargo into space without sacrificing thrust.

I know I am not an aerospace engineer but I was wondering what the technical limitations of doing what I am proposing has.

Anybody can comment, I welcome any and all opinions.

[Pursuedtank]

The delta v requirements for reaching orbit around Earth are more than double the requirements for reaching kerbin. it takes ~9500 meters per second of delta v just to reach low earth orbit, and easily a thousand or two more to go anywhere useful. It is not fesable to do a pure rocket SSTO with any reasonable payload.

[luisair]

Well, try your Vehicle with RSS and you will face similar Problems real engineers do.

[TeeGee]

Right but I'm not saying to build a small SSTO, obviously it would be huge but if we use kerolox fuel on a high ISP high thrust engine it should be possible.

[TeeGee]

The delta v requirements for reaching orbit around Earth are more than double the requirements for reaching kerbin. it takes ~9500 meters per second of delta v just to reach low earth orbit, and easily a thousand or two more to go anywhere useful. It is not fesable to do a pure rocket SSTO with any reasonable payload.

How did the OMS engines get the shuttle anywhere? It had a deltav of 300 m/s/.

[R0cketC0der]

As already said, the orbital velocity for earth is about 7.5km/s vs about 2.2 km/s for kerbin. This means that you don't just need a lot more delta-V for launching, but you will also reenter much faster, resulting in much larger heat. Also to my knowledge the engine that you mentioned has no throttling capabilities, meaning that it will create very high g-forces on its payload(probably 10g or even more shortly before burnout).

If you want to build something in ksp to show that it would be reasonable in the real world, download all of the realism overhaul mods and try that again. Even then there will still be a lot of unrealistic things left in ksp, but it will be a lot more realistic than what you currently have.

[R0cketC0der]

How did the OMS engines get the shuttle anywhere? It had a deltav of 300 m/s/.

The orbital manuevering engines weren't for launching the shuttle, they just did the last bit of circularization and orbital maneuvers. The engines that fired at for most of the launch sequence were the boosters and space shuttle main engines.

[Motokid600]

Was there not a few SSTO rockets designed on paper around the Apollo era? It's possible, but the rocket needs to be hilariously large. What would be the point? It'd take a massive, inefficient engine(s).

[sgt_flyer]

Kerolox has not enough isp for a usable SSTO. And the concept you propose already exists and has been researched:

http://en.m.wikipedia.org/wiki/VentureStar

It faced several problems, notably the demonstrator's multilobe fuel tanks . Ultimately - the budget for it got axed and the project was cancelled in 2001. At this time, the x-33 suborbital demonstrator was 85% complete.

Basically, you need a very high isp, at all altitude, and a dry mass as low as possible to be able to take a reasonable payload.

a bell nozzle is designed with specific shapes if you want to use it at sea level or in near vacuum - using a nozzle designed for vacuum at sea level will lower your isp at these altitude.

Edited June 3, 2014 by sgt_flyer

[mdatspace]

Not efficient enough.

[Nuke]

id probibly go with a vtvl design, like the dc-x.

http://en.wikipedia.org/wiki/McDonnell_Douglas_DC-X

[NovaSilisko]

http://www.arcaspace.com/en/haas2c.htm

ARCA is attempting to. But the most they seem to have right now is a mockup and a partially built engine.

Edited June 3, 2014 by NovaSilisko

[TeeGee]

The orbital manuevering engines weren't for launching the shuttle, they just did the last bit of circularization and orbital maneuvers. The engines that fired at for most of the launch sequence were the boosters and space shuttle main engines.

Uhh yeah I know that. I was making the point that shuttle OMS circularized with less than 300 m/s delta v in its tanks.

[TeeGee]

Kerolox has not enough isp for a usable SSTO. And the concept you propose already exists and has been researched:

http://en.m.wikipedia.org/wiki/VentureStar

It faced several problems, notably the demonstrator's multilobe fuel tanks . Ultimately - the budget for it got axed and the project was cancelled in 2001. At this time, the x-33 suborbital demonstrator was 85% complete.

Basically, you need a very high isp, at all altitude, and a dry mass as low as possible to be able to take a reasonable payload.

a bell nozzle is designed with specific shapes if you want to use it at sea level or in near vacuum - using a nozzle designed for vacuum at sea level will lower your isp at these altitude.

Kerolox would allow engineers to build a smaller spacecraft. Look how massive the skylon will be just to put up 33 tons. All of that extra volume is used to house liquid hydrogen.

ANYWAYS, how can the venture star be the only attempt at building an ssto?

Here is a stupid system I used in my heavy payload ssto rocket launches:

1) for heavy payloads I attach SRB's to the spaceplane and launch it without activating the 1 time ignition liquid rocket

2) wait until I hit the max apoapsis with the srbs then jettison them, ignite the liquid and continue up. That buys me a lot of delta V

I know that's not an ssto anymore, but I've been able to get payloads up into LKO with just rocket on winged aircraft using ferram aerodynamics and real fuels etc.

After I did this my mind started wondering why this wasn't done before.

All of this designing came from my successful building of a space shuttle with a similar design. It was such a pain in the butt however I finally created a functioning shuttle. She had TONS of delta v after reaching 200 km apoapsis and thought that I could prob match that orbit with a single stage craft, and I did so. Most of this spacecraft is payload bay and kerolox fuel. The last 2 tanks are hypergolics for the oms.

I dunno, maybe I should give RSS another go.

[cantab]

Uhh yeah I know that. I was making the point that shuttle OMS circularized with less than 300 m/s delta v in its tanks.

With a flat enough ascent, the circularisation doesn't take much. Remember the main engines can freely raise periapsis to at or even above Earth's surface.

Anyway, the reason rocket-powered aircraft aren't common is that rockets are inefficient at low speeds. Thus it's not very sensible to build a winged rocket-powered SSTO. A pure SSTO rocket, on the other hand, is possible, it's just you need too much rocket for too small payload. Even though it means extra engines, staging works out better value.

[Iskierka]

Kerolox would allow engineers to build a smaller spacecraft. Look how massive the skylon will be just to put up 33 tons. All of that extra volume is used to house liquid hydrogen.

That "massive" Skylon is actually very narrow, so while it's very long, its volume is quite low, comparable to rockets in its payload category. Which is actually 15 tonnes, though it can do 33 if the vehicle itself does not reach orbit and launches a second stage at the peak of its parabola.

Going to the rocket equation around earth, we find that for a traditional vertical-launch LH2/LOx rocket, the required mass ratio for the delta-v is 8.3 or so, leaving no spare fuel. (A rare occurrence)

Thanks to the fancy-pants engine, we find that Skylon drastically reduces its dv to launch, and the mass ratio, including spare fuel, is just under 5.1. Additionally, because of its fancy engines and use of the wings on ascent, its initial TWR can be less than one, drastically reducing empty weight of the vehicle.

Going to a traditional vertical-launch RP-1/LOx rocket, which is the design you are suggesting, minus extra weight for wings, we find that a mass ratio of 15.1 is required. Again, with no spare fuel, you can probably make that 16 or 17 accounting for that.

Now, for the RP-1/LOx rocket, we have certain requirements. Firstly, its TWR needs to be greater than 1. Secondly, we are looking at at least three times the mass, due to that ratio. So the engines need to be likely 5-6 times as powerful versus the Skylon designed there, at a modest estimate. Skylon's engines are quite heavy, with a TWR of only 14 in airbreathing mode, but 5-6 times more and even RP-1 rockets struggle to match that overall weight.

So not only do we need a lot more fuel, but our engines are heavier. Meaning now we need more fuel. Plus the wings need to be bigger to carry those big engines. Which means more fuel to get it into orbit. And because of all this fuel weight, the structure needs to be much stronger - especially when all this weight has to survive multiple G's during ascent! So now we add more fuel.

At this point, you're using four times as much fuel as Skylon by mass, easy. At this size, Skylon will still have slightly larger LH2 tanks than your proposed RP-1 tanks, but Skylon's LOx tanks have nothing on yours. Which have to be built really strong and heavy and cause all sorts of difficulties. While Skylon's can sacrifice much strength, become very light, and then make it really easy to reenter, as the vehicle is practically light as a feather for its size, meaning its deceleration on reentry starts long before things heat up. Meanwhile you have a smaller, very, very heavy vehicle to try protect from heat ... Good luck.

This is also ignoring that overall rocket mass ratios are quite similar for LH2 and RP-1. RP-1 does get the better overall in general, but not by as great a margin as LH2's required mass ratio, meaning LH2 has the greater payload capacity, if you care to build the rocket large enough. Most people don't due to the cost of building something that big repeatedly, which would be no issue on a reusable vehicle like Skylon.

I dunno, maybe I should give RSS another go.

Yes, you should. Then you might realise this design won't work

A pure-rocket SSTO was tried with the much more likely LH2 fuel, and it's been pointed out earlier in the thread how well THAT went. It was so difficult to manufacture to LH2's less demanding spec that they lost funding, they never would have gotten anywhere with RP-1.

[Redjoker]

a bell nozzle is designed with specific shapes if you want to use it at sea level or in near vacuum - using a nozzle designed for vacuum at sea level will lower your isp at these altitude.

For a rocket ssto, a bell nozzle would likely not be used, since you could use an aerospike engine, like the one planned for the x-33 venture star, which would deliver an isp that would be more consistent across a range of altitudes.

http://en.wikipedia.org/wiki/Aerospike_engine

[Armchair Rocket Scientist]

Well, the required mass ratios have already been mentioned, but:

en.wikipedia.org/wiki/Single-stage-to-orbit

Basically, rocket-only SSTOs are known to be possible, and a few existing lower stages, including the Titan II and possibly the Saturn V and Falcon 9 could be turned into SSTOs. The problem is, you get crappy payload fractions and no hope of reusability, so unless you went with a total Big Dumb Booster approach, you couldn't compete with the launch cost of multistage rockets.

Also, interestingly, according to the Wikipedia article hydrogen performs about the same as dense fuels in an SSTO as far as payload fraction is concerned. However, as Iskierka mentioned, a hydrogen-fueled vehicle would be very low density with its fuel drained, which would make reentry easier. However, the use of an inflatable heat shield could make that not a problem.

I think the biggest problem with a reusable rocket-only SSTO, besides the bit where it might not actually be possible, is that it would be just as hard to build as either a multi-stage reusable rocket or an air-breathing SSTO, but wouldn't be able to compete with either of them on payload fraction.

The one other option sometimes considered is a nuclear rocket-only SSTO. This would be heavily dependent on how much thrust we can actually get out of a nuclear engine, but in theory such a vehicle could manage some decent payload fractions. The problem is, the extreme safety standards a nuclear-powered vehicle would need to meet to be allowed to fly would make refurbishing the space shuttle seem like a walk in the park.

[sgt_flyer]

For a rocket ssto, a bell nozzle would likely not be used, since you could use an aerospike engine, like the one planned for the x-33 venture star, which would deliver an isp that would be more consistent across a range of altitudes.

http://en.wikipedia.org/wiki/Aerospike_engine

I know about aerospikes - but they are only prototypes. They have been never used on a complete rocket for the moment be it toroidal or linear - only test firings have been made

Mainly i think, because they are too costly to develop for expandable rockets:)

Edited June 4, 2014 by sgt_flyer

[jwenting]

Uhh yeah I know that. I was making the point that shuttle OMS circularized with less than 300 m/s delta v in its tanks.

so what? If you've spent 9900 dv using other engines and calculated things correctly, it may be all you need.

[magnemoe]

An reusable rocked SSTO might be possible if you build it very large as in larger than a staturn 5.

Problem here is that the development and building costs will be huge.

You get more for your money making an two stage craft, this also increased flexibility, use a manned upper stage, think mergine the dragon pod and second stage on falcon 9, for satellites you use an reusable upper stage.

an reusable upper stage has the benefit that its far smaller and easier to heat shield and recover than a ssto,

[NathanKell]

First: Size matters not. Mass ratio does. If you make a rocket ten times the size of the Saturn V, assuming it still has the same wet:dry ratio, it will have the same delta V.

Now. Aerospikes aren't magic pixie dust. They're less efficient than a bell nozzle at the bell nozzle's design pressure, and they at most save you the 15% or so nozzle losses you incur when not firing at optimum pressure. They certainly don't, as they do in KSP, give you equal Isp at sea level in vacuum. And they are heavier for a given thrust.

Further, while hydrolox is much better than kerolox in vacuum (and indeed is pretty necessary for SSTOs) it's relatively worse than kerolox at sea level, so you don't even get the full hydrolox "bonus" Isp.

Consider, if you will, the J-2S-derived aerospike engine to be used on the X-33, the XRS-2200. Note that it has a specific impulse of 439s in vacuum and 339s at sea level; slightly better than the J-2S (it is, after all, a 90s, not 60s, derivative) in vacuum and much (vs. 200s) better at sea level. But also note that it has a lower sea level specific impulse than the SSME (363s). Chamber pressure matters much more than nozzle design here.

As for air-breathers, here is a calculator (from e of pi, IIRC) for mixed-mode (Skylon-like) SSTOs. Follow the directions, guess a dry mass fraction, and go.

You can also, of course, use it for single-mode SSTO, just set changeover velocity and height each to 0. It will overestimate ground-start rockets, however, since it doesn't take pressure losses (lower sea level Isp) into account.

[TeeGee]

These are all fantastic comments, I'm learning a lot. I guess the point of the skylon sabre engine is to cut out the initial delta v from standing to mach 5 at a thinner atmosphere than at SL before switching to rockets, but even so, the payload fraction in comparison to weight is super small. Rockets will ALWAYS have a better payload weight than sstos BUT rockets are disposable, sstos are not. If we were to build a station in space using smaller modules, I think it would be more cost effective to use an ssto to build it bit by bit rather than use 1 large rocket to push a larger payload up all at once.

All we have to worry about is fuel cost and maintenance.

But that's besides the point.

The issue with airbreathers is that they are altitude and speed limited. Assuming the sabre engines perform as they should, that will be amazing achievment for merely an airbreathing engine to do. As far as I know, only rocket technology has been able to break the mach 3 barrier, unlike the much slower airbreathing engines. And the more complex we build these airbreathers to go faster, the heavier they become. So I thought to cut out the airbreather altogether and use heavy lift rockets with great ISP at SL.

IF I/you/we can build an ssto using pure rocket technology in RSS and show you guys it can be done, maybe we can settle this idea. If anybody has done so already please post your craft!

[shynung]

As far as I know, only rocket technology has been able to break the mach 3 barrier, unlike the much slower airbreathing engines.

The Lockheed SR-71 Blackbird reconnaissance plane has a cruise speed of mach 3.2, yet it only needs 2 turbojet/ramjet hybrid engines (P&W J58), rather than rockets. If further research on scramjets yields something airworthy, we could get mach 5+ airbreathing engines that is more efficient than even the SSME.

Also, you might want to look at Aquarius rockets.

Edited June 4, 2014 by shynung

[R0cketC0der]

You seems to be confusing SSTO with reusable SSTO. As already mentioned, various rocket first stages could be used as SSTO with a ridiculously small payload, but they wouldn't be able to return because they have no heatshielding and some of them aren't even able to reignite their engines.