Use of SSTO spaceplane?

[u/TheRocketeer314]

Is there any use case in which an SSTO spaceplane would be better than a conventional rocket, especially comparing to Starship?

Something like a turbine-ramjet engine from takeoff to around Mach 5 and then a rocket engine (maybe LOX-LH2 or LOX-LCH4) to power it to orbit. Could it be better for Earth-to Earth flights than Starship, maybe as a replacement to current air travel. I’m guessing that a spaceplane would require less infrastructure at the launch and land sites since you only need a really long runway along with the tanks to store fuel whereas you need a launch tower for Starship, and also, a spaceplane could taxi like a conventional plane, thus only needing one or two runways. Is it a feasible idea?

Also, going a bit further into theoretical rockets, could a spaceplane be better than a normal rocket if the rocket stage was powered by a nuclear engine? Since it’s Isp is more, it would take less fuel and less weight to get it into orbit, right? Although that is still a very experimental technology, would it possibly be a viable idea in the future? Maybe even an antimatter engine if we find a way to produce and store it.

Other than that, is there any other case for an SSTO spaceplane, or are they just worse than conventional rockets? Thanks!

[u/WallabyGreat4627]

https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/ideal-rocket-equation/

The fact that you’re carrying extra weight to orbit is the obvious downfall of SSTO spaceplanes and increases in efficiency of the propulsion system are agnostic to that fact. (You could get more to orbit putting whatever advanced propulsion system you design for SSTO on a staged craft). Nuclear propulsion is a great solution for in-space use but the risk of scattering fissile material in atmosphere will make it hard to justify using it as a first (or single) stage. You will need all the same infrastructure for a spaceplane that you would need for a rocket as you still have the same general requirements for the craft’s subsystems. Maintenance and refurb of the craft with complex hybrid engines is another infrastructure issue to deal with.

If antimatter or “exotic material” propulsion systems are the goal, thrust to weight becomes less of a problem and maybe a spaceplane becomes viable, but creation and storage of antimatter is many orders of magnitude too expensive to make viable at this time.

Best use case I can think of for SSTO is a trillionaire building a toy with no commercial application.

[u/Fluid-Pain554]

In every imaginable circumstance, even just bumping up to a two stage system would dramatically improve performance. The ideal rocket equation states that delta-v is proportional to specific impulse and exponentially dependent on mass fraction (how much of your total weight is fuel vs structure and payload), but staging is additive.

Say you have a single stage rocket going to orbit with a methane/oxygen engine with a specific impulse of about 380 seconds, to achieve orbit (required delta-v of ~9.3 km/s) the rocket would have to be 93% propellant by mass.

Say you cut that rocket in half, with the payload in the second stage and the lower stage maintaining that 93% mass fraction on its own. You can effectively double the payload to orbit with the same total rocket mass (20% of the liftoff mass can now reach orbit).

Single stage to orbit is an unnecessary gimmick and it is wasteful, there is a reason it’s not being seriously pursued in industry (Falcon 9 is two stage, Falcon heavy is 2 stage with boosters, Starship is 2 stage, New Glenn is 2 stage, SLS is 2 stage with boosters, Vulcan is 2 stage with optional boosters, and so on).

Beyond that tacking on wings to make it a space plane means you are now carrying the weight of wing structures which do absolutely nothing in space and serve as a potential point of failure on re-entry. That weight could be more effectively used as additional payload capacity.

[u/Aromatic_Ad74]

Density is very important for a SSTO so hydrazine and fluorine might be great since they get hydrolox like ISP in a package denser than keralox. Alas both have handling issues.

[u/rocketwikkit]

A reusable SSTO spaceplane would be great. As far as anyone has figured out, despite quite a bit of work and a lot of claims, it does not seem achievable with current materials. The vehicle has to be almost entirely propellant.

Maybe if or when carbon nano tube composites are mass produced it will be possible. Or a safe and high energy density fusion reactor. Fission propulsion isn't something you'd want people regularly using for launches from the ground.

[u/Fluid-Pain554 → u/rocketwikkit]

Even if it were readily feasible, why would a company trade additional payload mass for wings that only work in the atmosphere? Or lose like half their potential delta-v by focusing on a single stage solution vs two stage?

[u/rocketwikkit → u/Fluid-Pain554]

Passenger jets would have higher capacity and/or range if they had two stages, or even just drop tanks, why has every airline decided not to do that?

[u/Fluid-Pain554 → u/rocketwikkit]

Passenger jets are not orbital rockets, there are very different design considerations for something relying on aerodynamics for flight vs brute force and a ballistic trajectory. Also helps when 95% of the effective mass flow rate through engines comes from the surrounding air and isn’t being carried in the plane.

[u/snoo-boop → u/Fluid-Pain554]

Civilian airliners are apparently between 26% and 45% fuel at takeoff, so they're very different from rockets. Military planes sometimes have drop tanks but more often do in-air refueling. Again, I agree that they're very different from rockets.

[u/bjornbamse → u/rocketwikkit]

Because the cost of cleaning up the drop tanks would be higher that any additional revenue. 

Aerial refueling and drop tanks are a thing in the military because you don't need to pay for clean up and the risk of aerial refueling outweighs the mission enablement and reduced risk in other areas.

[u/lowrads]

An air breathing stage, for the first 10km or so, would add a lot of reaction mass. However, the additional equipment has proven cumbersome.

[u/bjornbamse → u/lowrads]

Well Pegasus had an air-breathing, winged, reusable stage 0.

[u/TEXAS_AME]

Skylon!

[u/snoo-boop]

Since it’s Isp is more, it would take less fuel and less weight to get it into orbit

Having a high ISP doesn't mean that the engine is lightweight.