Radian Aerospace – First Single-Stage-to-Orbit Spaceplane | Hacker News Reader

Radian Aerospace – First Single-Stage-to-Orbit Spaceplane

radianaerospace.com

5 pointsT-A8 comments

Two points:

* Why?

* How?

The degree of optimization & mass-cutting you need to do to get to LEO in an SSTO vehicle with zero payload is absurd, and wings or landing gear sabotage that immediately. And assuming you get there - even very small payloads demands enormous vehicles. Which now you've got to brace for horizontal mass loading, with takeoff from a RUNWAY with some kind of unspecified sea-level propulsion at a mass of what, 500-1000 tons?

What is the special sauce they are pitching to make this worthwhile? Some kind of launch sled?

Taking off at 30 degrees pitch from an as-yet-unbuilt Kilimanjaro Spaceport at just under mach 1 would certainly help an SSTO make its numbers, but is that enough to make the idea competitive? So far, it hasn't been.

We have proven first stage return capability now with more conventional vertical launch rockets, proven a middle-stage hypersonic return capability, and done an early beta test of last-stage return capability from quasi-orbit, and mounting a spaceplane on top of a Falcon 9 first stage for full reusability (or a BFS on a BFR) appears perfectly feasible. Why SSTO? Why now?

I think the only marketable optimization today would be a TSTO flyback booster to simplify recovery. Ala the 1960’s Boeing 832 but without the manned part.

> Why?

The usual: lower cost (no hardware is thrown away, you only pay for fuel and maintenance) and rapid deployment (they are aiming for a turnaround time of 48 hours, so a fleet of only 2 or 3 could launch daily).

> * How?

Sled + wings replace the first stage. The sled lets it leave the ground going at Mach 0.7 with full tanks. The wings give it aerodynamic lift up to (I guess) about 30 km altitude, far more efficient than fighting gravity using the engines alone. That sets it apart from previous SSTO designs like the X-33 [1] (Radian's CTO [2] is an alumnus of that project) which took off vertically and only used aerodynamic lift on the way back, like the Shuttle.

[1] https://en.wikipedia.org/wiki/Lockheed_Martin_X-33

[2] https://www.radianaerospace.com/company

So basically, the questions are: Why attempt a daunting SSTO exercise when we have first stage landings now? How has the situation been improved relative to previous proposals that were judged unfruitful in an era when first stage landings didn't exist? What secret has been unlocked, what novel method has been created? Why is this project different from the last ten projects?

> The wings give it aerodynamic lift up to (I guess) about 30 km altitude, far more efficient than fighting gravity using the engines alone.

Far more efficient at fighting gravity? Gravity losses, aerodynamic losses, and structural mass fraction are tightly coupled tradeoffs, but the longer you spend in atmosphere the more aero losses you accumulate; Glide ratio isn't free. Rockets get up to hypersonic velocity where there's little benefit to trying to take advantage of aerodynamics or utilize control surfaces if it increases drag, very quickly.

> How has the situation been improved relative to previous proposals that were judged unfruitful in an era when first stage landings didn't exist?

Those proposals did not use a sled and aerodynamic lift on the way up, lighter and stronger composite materials are available and rocket engines have evolved.

> Far more efficient at fighting gravity?

Ask anyone who's compared how high and fast you can get a fixed-wing drone powered by one engine and a quad-copter powered by four.

A serious assessment of this design would require detailed specifications and a fair amount of calculation. I am not inclined to summarily dismiss it based on generic hand-waving.

The math is complex, but this approximation is true enough for government work:

You don't need to get high to get to orbit per se. You need to get flying horizontal, much faster than the atmosphere will allow you to sustain; This is the only reason you need to get out of atmosphere. About 10% of the work in conventional spaceflight is attempting to get to >100km above your head, and about 90% of the work is in attempting to get >100km above your head, flying parallel to the Earth's surface at ~8000m/s.

Those wings are useful at mach 0.7 and much less useful at mach 2 and a loss-creating pain in the ass at mach 5. You need to get to mach >20, and the longer it takes you to do so, the more you pay to the aero and gravity gods. Wings with a glide ratio of N at low speed allow a plane to divide what they pay to gravity per unit time by about N, at the cost of paying a bit more to aero losses. But as soon as you get significantly supersonic/hypersonic, the aero costs ramp to impractical levels.

In airplanes, that reduction in gravity losses per unit time translates to lower needed thrust to weight ratio. Most planes fly with a TWR much lower than 1, being incapable of flying straight up. This is useful because jet engines are incredibly expensive, complex pieces of technology compared to rocket engines per unit thrust. Rockets more often fly up at a craft TWR artificially limited to "Whatever the humans and the structure will tolerate without falling apart", which changes as the craft gets lighter.

> Those wings are useful at mach 0.7 and much less useful at mach 2 and a loss-creating pain in the ass at mach 5. You need to get to mach >20, and the longer it takes you to do so, the more you pay to the aero and gravity gods.

If you stay at the same altitude.

You are arguing by analogy with aircraft and hypersonic missiles designed to stay in the atmosphere, not to efficiently climb above most of it.

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