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Q&A

Centripetal Burn in Orbit

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Imagine a spacecraft in a highly eccentric orbit. Say the craft burns at periapsis towards the planet such that its orbital eccentricity does not change and the craft's periapsis moves with the ship so the two stay at the same point at all times. This would result in a circular orbit, but a faster orbit than would normally occur at such an attitude.

Aside from this being horribly wasteful and generally impractical:

My first question is whether this maneuver has a name. I would guess not because it's an extremely wasteful maneuver in most cases, and wouldn't be worth any benefit you could get by performing it.

One possible use case, however improbable the scenario, would be to accelerate an orbit. Imagine a spacecraft in low-earth orbit moving at 7.5km/s. If the craft were to burn prograde until it were moving at 15km/s (doubling its velocity) and then continuously burn towards the planet as I described, it could effectively halve its orbital period by doubling the centripetal force keeping the craft in orbit (half of the acceleration is due to gravity, half due to the constant burn).

Is there a more efficient way than this to accelerate your orbit beyond the orbital rate that can be achieved just above the Carmen line (assuming we don't want to drop below the line)?

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3 comments

@Mr Bumble OP updated. jnhyatt‭ 5 months ago

The scenario seems self-contradictory to me: if the spacecraft is in a highly eccentric orbit and burns "such that its orbital eccentricity does not change", how does it end up in a circular orbit? Peter Taylor‭ 5 months ago

@Peter Taylor The craft is at periapsis in a highly eccentric burn. If it continues along its path, it will certainly move away from the planet. If, however, it burns directly towards the planet at the right acceleration, the periapsis will move with the spacecraft, but the eccentricity will stay the same. That is to say, the altitude at apoapsis and periapsis will remain constant, but the periapsis will "orbit" the planet at the same rate as the craft. jnhyatt‭ 5 months ago

1 answer

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It appears you want to go around a planet significantly faster than at the speed of a normal inertial orbit. That's gonna cost one way or another.

Something has to create the downward force to keep the craft from escaping into the elliptical orbit (or even hyperbolic escape) indicated by the speed. Your suggestion is a deliberate burn pointing down. For a free-flying craft in space, that's probably the only option.

You say you want to stay out of the atmosphere, but what about using the very thin outer edges of the atmosphere to provide downward "lift" with wings? The dissipation would be the speed times the drag force. With large enough wings, the resulting heat is more spread out. The efficiency of the wings also gets better, so there is less drag that causes heat relative to the downward force required to keep the craft in the desired circular trajectory. The thrust to keep the system going is now only to overcome the drag, which would be significantly less than the vertical force needed previously.

Off the top of my head it seems that the power required gets smaller and smaller as the wings get bigger. Note that the wings aren't airfoils, just flat things with a slight angle of attack. The real problems are the engineering tradeoffs to make the wings strong enough, large enough, be able to handle heat, but not too heavy.

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