Hacker News Reader

Comment by klodolph | Hacker News Reader

> From someone who knows little about this -- what is the advantage of this kind of engine?

Take a traditional bell nozzle engine. The shape of the bell is designed to redirect the exhaust in the correct direction, but one of the design parameters for the shape is the ambient air pressure. If you design it to work at a specific altitude, it will be less efficient at other altitudes.

The aerospike is more efficient over a broader range of altitudes. (That is, if you take the average efficiency over a wider range of altitudes, the aerospike wins. If you pick one altitude or a narrow range of altitudes, the traditional nozzle wins.)

View on HN

Piggybacking off this too, if you look at rocket specifications, they’ll often mention the same motor on different stages with a (VAC) behind it. This denotes the different bell shape required to make the motor efficient in vacuum as compared to sea level.

The issue with an aero spike is that the benefits just aren’t there for a multi stage rocket. We do multi stage rockets to balance the needs of high thrust at the launch pad with the need to minimize our final non-payload weight in orbit. Varying the bell design to match flight profile naturally plays well with this approach. If you’re going to ditch parts of your rocket for weight saving during ascent, you might as well tune each stage for the altitude it will actually work at. Motors that need to work at all altitudes are pretty rare, the only ones I can think of are SSTOs in theory, and the space shuttle main engines.

If we could get a falcon 9 with aero spikes it would theoretically be an improvement, but not a huge one. All the gains would be in the edges of various stages where say, the stage 1 motors are flying above their designed altitude. The efficiency gains are there, but they might be completely offset by increased weight, cost, cooling concerns, etc.

Piggy backing off of this- the real net benefit is unblocking an SSTO design (Single Stage to Orbit).

Instead of having a lift stage and an orbital stage, aerospikes can theoretically do both, meaning one cohesive vehicle from launch through orbit.

The weight savings and reusability factors are huge if SSTO setups become feasible.

Fine, except the era of expendable launch vehicles is already drawing to a close.

Reply on news.ycombinator.com