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Comment by Tuxer | Hacker News Reader

Space isn't like a highway where you can move from A to B at minimal expense. Switching orbits, both orbit heights but mainly orbit inclinations, comes at a very high delta-V expense and therefore propellant expense, which this craft simply doesn't have the volume to hold.

This cannot shuttle between LEO stations and switch orbits to imitate multiple observation sats unless those sats and stations are very close in terms of orbits.

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The U.S. Air Force's former top civilian, Heather Wilson, said it dips into the atmosphere and uses its aerodynamic surfaces to change its orbit.

Ref: https://www.military.com/daily-news/2019/07/23/former-secaf-...

There's no free lunch in orbit. If it dips into the atmosphere enough for control surfaces to bite then it has to boost back up again to avoid orbital decay. Either way it can't possibly carry enough fuel to do much of that.

> it has to boost back up again to avoid orbital decay

It doesn’t have to return to orbit. Switching from an orbital trajectory to a parabolic intercept path would be feasible, and open valuable tactical space.

As long as it can travel back to a US-friendly airfield. Which is probably a large number around the world although few which would support classified vehicles?

It uses an oblong orbit to gain a small window where the next orbit cannot be perfectly predicted by an adversary that can only watch half the sky.

You can't gain delta-v by inducing additional drag on the craft.

If a spacecraft is traveling at 4,000 m/s and it plows headlong into a planet, it has gained 4,000 m/s of delta v.

Delta v a measurement of a vector, not a scalar. The fact that its kinetic energy with respect to Earth is decreasing does not mean it isn't gaining delta v. In addition to the obvious (lowering your apogee) you can also use this delta v to change your inclination. It's not "free" of course- you need to do other engine burns to both put the craft into the atmosphere and then another burn to reestablish orbit once you're out the other side. But it isn't clear to me that this is less efficient than just doing the burns without the atmospheric assists.

> If a spacecraft is traveling at 4,000 m/s and it plows headlong into a planet, it has gained 4,000 m/s of delta v.

Yes and no. You've changed your velocity by 4km/s. You didn't gain usable ∆v, and you've just lost your spacecraft.

∆v means few related things. One, the amount of velocity change your mission plan requires. Second, the velocity change your craft is capable of using its own propulsion. Say you're plotting Mars orbit insertion. It requires (via [0]) 1.5 km/s ∆v. But you're a cheater, and decided to use aerobraking to bleed off (guessing a number) 1km/s of that velocity. This means you can complete this maneuver with a craft that only has 500m/s ∆v at the point of insertion. This does not mean the craft has gained ∆v.

[0] - https://www.reddit.com/r/space/comments/29cxi6/i_made_a_delt...

I'm absolutely certain the air force is willing to give up some efficiency to gain some obfuscation.

You can certainly get delta-v with drag. You’ll keep bleeding energy from orbital (kinetic plus potential) to atmospheric heat, but that’s an entirely different thing.

Yes, orbital mechanics are very non-intuitive.

Thinking "you're in space so can go anywhere" makes as much sense as thinking it's easy to go from Canada to the top of the Everest just because they're both on land.

Definitely, and in fact that's probably the first epiphany one reaches when one starts playing KSP.

Yup. You learn that you can easily go anywhere you want on a curved line, as long as you're willing to wait long enough. But making that line point towards where you want to go is a stupidly fuel-expensive process.

people who haven't played kerbal space program (with mechjeb for data) usually don't fully understand how expensive, in terms of delta V, a large inclination change maneuver is.

I think the idea is that you plunge into the atmosphere, use lift to perform the orbital inclination change and then you raise your periapsis again.

I think you only need very little delta V to plunge back into the atmosphere from LEO.

The question is how much lift vs drag + delta V to lower and the raise you periapsis.

I think the space shuttle only had about 250m/s delta v once the yellow tank was exhausted, so those 250 m/s were enogh to complete both rasing the periapsis and then lowering it to de-orbit.

Think of a space-glider with one tip of its wings pointed straight down and the other straight up, if it has very little drag and a lot of lift, it could use its wings to maneuver north-south, change its orbital inclination, then when it gets out of the atmosphere again, once it reaches apoapsis it will spend some fuel to raise its periapsis again above the atmosphere, but KSP intuition is that if drag is low enough and lift is high enough, it could be better than to do an inclination change in vacuum.

I have so far resisted mechjeb (and other mods). Should I give it a try?

Yes. Some mods like kerbal engineer and machine everywhere makes thing less annoying, not easier. It’s the engines, fuel tanks, and science payloads that can break the balance.

As for mechjeb in particular, I never found it all that useful except when calculating rendezvous. I could do it myself, but I don’t want to bother with a protractor and calculator.

Mechjeb is also invaluable when doing preprogrammed burns on remote probes with a high light lag like you'll see in Realism Overhaul.

Timing a 30s burn when the probe takes 15m to get the signal is rather irritating.

Mechjeb can help you with the boring parts. First couple dozen launches or orbital intercepts are super-fun. But once your goals shift from "let's land on the Mun" to "let's send a 250 ton ship to Duna", assembling it in orbit from a dozen launches starts to feel like a chore.

Mechjeb was far more useful/necessary 3-5 years ago. The stock game has added so many features at this point that I really didn't feel the need for any mods at all when I picked it up again a few weeks ago.

This thing has changed orbits many many times over its several missions:

https://www.space.com/9000-secret-37b-space-plane-changed-or...

http://spaceflight101.com/x-37b-space-plane-maneuvers-into-l...

No, it isn't a highway and you couldn't have a shuttle up there for months bouncing around from place to place.

You could have a few shuttles up there idling in various orbits ready to do a small number of bigger maneuvers to achieve a mission.

Alternatively you could have shuttles doing a whole lot rather slowly with electric propulsion.

Could it "scoop" matter out of the atmosphere that could be accelerated by solar or nuclear powered ion drive to use as propellant?

Alternatively, thinking about the Earth's magnetic field, what if it had a big current loop that could be activated to generate a force and change orbit.

I realize that the magnitude of these effects are probably too small to be significant, but could you in principle use them to repeatedly change orbit around earth, provided you have an abundant energy source?

scoop? no.

magnetic field? yes. - https://en.wikipedia.org/wiki/Electrodynamic_tether

Unlikely it can scoop material and use it for propulsion just yet, though I have seen some articles about ion engines accelerating the rarified upper atmosphere being studied.

The ESA is developing an air-breathing electric thruster which sounds what you described:

http://www.esa.int/Our_Activities/Space_Engineering_Technolo...

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