No, a Hohmann orbit is one where the perihelion is at the distance of your destination, and the aphelion is the at the distance of your starting point (or vice versa). If your destination is the Sun itself, then in that case, a Hohmann orbit would just barely touch the surface (which is only slightly different from the “fall straight in” orbit), but it’s more commonly used with two planets as the endpoints.
To my mind, the key piece of counterintuitivity–the thing that’s most different from life on Earth–is this:
On the ground, it’s straightforward to change position. You just sorta point yourself in the direction you want to go and move there. It’s a continuous process: you get closer to your desired destination as long as you’re making an effort to go there, and if you change your mind you can just turn and go somewhere else.
In space, the difference is that to a first approximation, you can’t change your position in the now. You can only change your velocity (by firing the rocket). You have only changed your position some time in the future.
Furthermore, if you are sticking to closed orbits (ellipses around a single body), you will come back to your current position over and over again. Nothing you can do now can prevent you from coming back to the same place.
So to completely change orbits, you must do so in discrete steps. You fire the thrusters now to change where you’ll be in the future–and then at that future point, you fire the thrusters again to change where you were (and would come back to if you did nothing). This is the basic principle of a Hohmann transfer–fire once to change your current orbit to intersect with the final orbit (but not match it), then wait for the intersection, then fire again to transform the current orbit to the final one.
As others have said, Kerbal Space Program is great training. It’s really quite intuitive once you get past a few mental roadblocks. I won’t say it’s easy–especially orbital rendezvous, where you need a keen sense of timing if you don’t depend on more advanced tools–but you at least know what went wrong.
If I want to go to the Sun, why can’t I point myself towards it and move there as long as I am making an effort to go there?
If you had an infinitely efficient engine, you could. You would have to make some allowances for gravity pulling you off course, but you could.
Real-life rockets are nowhere close to being efficient enough for that approach, though. So you have to make the transfer itself efficient, which means performing short burns at specific places/times.
There’s a bit of a complication with low-thrust engines (ion engines), which can’t be treated as “infinitely” fast. That’s a different discussion, though.
Let’s say you want to go to the Luxor Hotel in Las Vegas (you really want to try out those slanty elevators). Simple, point yourself toward the beacon and start moving towards it.
However, imagine that you are now on a mode of conveyance that is going past Las Vegas at 67,000 miles an hour. You can’t just point yourself at the beacon and start walking, you have to get off this thing first.
That’s the tricky part of this whole process. You can theoretically just run really fast backward and jump off, but you need to get up to 67,000 mph to make that work, and you can’t do that. Or, the way Munroe suggests, is something like directing your conveyance up a hill until gravity slows it down enough for you to comfortably step off. Then you walk the 3 billion miles back to Vegas. It may take a VERY long time, but you will eventually get there, using technology you actually have available to you.
Maybe his professor was the Sphinx from Mystery Men.
Yes.
A proposed alternative to intercontinental ballistic missiles used that exact approach. Launch a nuclear-armed missile into an orbit that is guaranteed to intersect the Earth’s surface at your intended target point. That means you can even boost “in the wrong direction” and come at your target from the opposite side. They’ll never see it coming.
That’s not really so much an alternative to an ICBM, as just an ICBM that’s launched in a different way.
The differences make all the differences, though.
An ICBM’s flight is ballistic. In other words, parabolic. Non-orbital.
FOBS is truly orbital, but an orbit designed with a perigee that guarantees intersection with the Earth’s surface at a planned point.
An intercontinental ballistic flight path reaches a maximum altitude of about 1,000 kilometers. A FOBS flight path maximum altitude is much lower, about 150 km. However, the same delta-v for both allows a ballistic missile to carry about 2-3 times the payload.
FOBS is different from a ballistic trajectory for much the same reasons that it’s easier to get to the Sun by thrusting away from it: the intuitive understanding of the alternatives is misleading.
An ICBM’s flight is ballistic. In other words, orbital. Elliptical. “Ballistic” is, in fact, an exact synonym for “orbital”.
All projectile flight is ballistic. Since that’s the definition of the word.
But “orbital” implies that the path is a closed conic section – an ellipse. An ICBM’s flight path, given Earth’s gravity and the missile’s mass and delta-v, is parabolic. Not elliptical. Not orbital.
If an ICBM’s flight path were parabolic, it’d never return to Earth. It’s an ellipse.
Actually, all projectile motion that stays near the surface of the Earth is elliptical, but on a small enough scale, a parabola is a good approximation to the relevant portion of the ellipse. Intercontinental is not a small enough scale.
Right, parabolic ballistic paths only happen when the gravity field is uniformly in one direction, like on an infinite plane. All ballistic paths near a spherical planet are ellipitical.