sevenperforce

Falcon 9 re-enters tail-first. The entry burn does double duty of reducing entry speed and insulating the engines from re-entry heat. Once deep in the atmosphere, the grid fins provide enough control authority to give the booster a 1:1 glide ratio, but that is (a) still not broadside, and (b) at extremely low speeds relative to the 8+ km/s that the SLS core is booking when it hits the atmosphere on the opposite end of the globe. Which is itself an issue. How exactly do you imagine we'll land a 212-foot building off the eastern coast of Australia and then get it back to the Cape? And grid fins wouldn't work. Not even titanium ones. The only control authority that the grid fins would provide at the SLS core re-entry speeds would be to act as ablative thrusters as they were rapidly vaporized. SMART reuse wouldn't be that terribly difficult. By which I mean it would be INTENSELY difficult, but within the realm of achievability. Of course it would significantly cut into payload but that's beside the point.

Sensors don't need a multi-billion dollar human-rated platform, they can be in a small (cheap) satellite. Yep. Gateway does nothing for human spaceflight that sending somebody on a lunar free-return wouldn’t do anyway. Don’t get me wrong; I’d love to see the moon up close too. But humans aren’t needed to do anything at Gateway that they couldn’t do at ISS. It’s like if you were a marine biologist and you were planning a research excursion to the Great Barrier Reef and someone said “well instead of taking a boat straight there, let’s build a floating research platform half a mile past it and have a boat take you there and you can practice some open water snorkeling and then we’ll send a different boat to take you from the platform to the reef.” And you ask “why can’t I practice snorkeling right here in the bay?” and they say “well the risk of shark attack will be higher at the floating platform so that’s something different.”

It is tremendously difficult to contemplate an air-startable RS-25. It requires at least one gee of acceleration for the prop downweight to allow the start mechanism to actually function. It would not only be difficult to create an RS-25 that starts in space; it would be a fundamentally different engine. And that fundamentally different engine wouldn’t even come CLOSE to being useful for a landing, because the minimum thrust level is an order of magnitude too high to permit a successful landing burn. And you’d never get to “successful landing burn” territory because the SLS core is already going fast enough that the engines and superstructure melt to slag the moment they get anywhere close to the atmosphere. You’d need a 5 km/s entry/braking burn to even approach survivable speeds. Which requires roughly 200 tonnes of reserve propellant JUST for the braking burn.

Making it “as powerful as a nuke” really isn’t the goal. As the article points out, if you need higher thrust, you simply fire pulse units faster. Basically everything proposed in that article is Z-pinch or a variation thereon. The one exception is a continuous fusion accelerator combustor powered by a nuclear reactor. He cites proposals for HE-initiated pulsed fusion, but none of them are small enough to avoid using a pusher plate approach. And scaling them up increases their efficiency, so bigger is better. But it exceeds the weight-per-pulse of the smallest possible fission-fusion Orion long before it approaches the specific energy of a fission-fusion Orion. So it’s basically just a wimpy Orion. The only reason to use pulsed pure fusion propulsion is if you can’t get your hands on enough fissile material to build a proper Orion. Z-pinch can be scaled down very small, small enough to produce pulses which (combined with a magnetic nozzle) can exceed the specific impulse of even ion thrusters. But the specific energies involved are still VASTLY lower than the specific energies you can get from fission-fusion Orion. And even if you multiplied the energy efficiency by 100x with advanced tech you’d still be better off with an ordinary Orion.

It’s not really a question of “simpler” because energy-positive pure fusion isn’t actually something we can do at all, in pulse form or in continuous form. It’s like saying “personal energy shields are simpler than lightsabers” when we have the technology to make neither. But we can’t do either. If there was a way to get pure fusion pulses then there would presumably also be a way to get continuous fusion energy. In tokamak fusion designs the fusion is continuous by nature; there is no pulse at all. Well no, not really. That was tongue-in-cheek, to illustrate that you don’t need pulses at all. If it’s happening in a combustion chamber then it’s not in the atmosphere, is it? That’s the point of a combustion chamber: it’s a place where the heat and pressure are concentrated so they can expand out the nozzle to produce thrust. The engine doesn’t particularly care what happens once the exhaust has left the nozzle. Sure, you’ll need some sort of propellant to absorb the heat of the fusion reaction and expand into exhaust, but whether there is or isn’t an atmosphere present doesn’t really matter.

The Firefly design is ridiculously cool, but unfortunately it does not actually work. To get off the ground on two engines, the engines need to be located at the center of mass, but that means they have no actual pitch authority. Maybe the inertial/gravitational dampeners provide pitch authority? I don’t know. Also, if you have a super awesome fusion plasma rocket which produces a massive explosion upon reaction with oxygen, then do that at smaller thrust levels inside a simple duct and boom, air breather. Otherwise you’re leaving energy on the table. Then again, maybe that’s precisely how the rotating engines operate. I mean, this is tautological. If you decide to use giant explosion pulses in your engine for no reason then your engine will have giant explosion pulses in it. That seems implied.

Why does it need to be pulsed? Why does it need to be pulsed at all? There is no reason to use pulsed propulsion if you can simply do the same thing with continuous thrust. What is the obsession with using pulse propulsion? Does your story need a pulsed propulsion system as a plot point? If so, then just propose a mechanism for creating pure fusion which has some minimum size associated with it, smaller than nukes. You will never need both ablative and active cooling. One is plenty. And if you are talking about using a film of oil as your ablator, then really that is just another form of active cooling since the oil will need to be replenished. Instead of using oil, just use the propellant that you already have. That’s what active cooling is, fundamentally. Are you familiar with the Firefly universe? In the series, the Serenity is a cargo ship which bears a lot of resemblance to what you have described. It is a single stage spaceship which takes off and lands vertically as a belly lander, and it has two airbreathing chemical engines in rotating nacelles which allow it to transition between horizontal and vertical flight. Not unlike the proposals you’ve made, the ship has an internal gravity cancellation field which reduces the effect of gravity and thus the amount of thrust that the chemical engines need to produce to get off the ground and into orbit. The main engine is powered by fusion and releases a controlled plasma explosion which, along with an inertial dampener, is enough to propel the ship between planets with a single pulse. The main engine cannot be used in the atmosphere or the plasma will ignite with the oxygen in the air and produce a nuclear-level explosion. Or you could just use multiple engines. Wouldn’t that be easier?

The core absolutely cannot be landed. Those engines each have a mass of over 3 tonnes. The center of mass is much too far aft for any sort of controlled biaxial entry. Bereft of its upper stage and the gimbal on its firing RS-25s, the SLS core is a lawn dart. And the SLS is jettisoned while in an actual elliptical earth orbit; it hits the atmosphere faster than a re-entering Crew Dragon. The engines will melt to slag immediately. What you could do, though, is give it wings. And control surfaces. That way it can re-enter on its side and shield the engines from re-entry heat. The heat shield will be very heavy though. Oh wait, that’s the Shuttle.

The concepts which had Orion providing a portion of the launch dV typically used massive disposable boosters (notionally a ring of 12-16 Shuttle-sized SRBs) to loft Orion out of the atmosphere first. There were some concepts which lit that candle on the ground but it never would have worked. Regardless, make the story about your story, not about the minutiae of your ship. Like Firefly.

Newton’s laws of motion say (paraphrased) that the faster your reaction mass flies away from you, the more impulse it provides to you. Equal and opposite reactions and all that. Rocket engines produce enormously high pressures and temperatures. But high pressure alone isn’t particularly efficient. You want your exhaust gas to leave your engine as fast as possible, because the faster the exhaust leaves, the harder it pushes you in the opposite direction. And in order to get the exhaust gas moving at maximum speed, you need to find a way to force it to expand. The best way to get exhaust gas to expand fast is to choke it — create a region of maximum pressure where the gas flow becomes locally supersonic. Since pressure waves cannot flow backwards in a supersonic flow, this directly converts pressure into speed.

You are describing a completely new spacecraft. Putting massive propellant tanks on this new Frankenorion would not only require a complete redesign of the capsule, but it wouldn’t fit within the existing OML. And even if it did fit, it would make Orion vastly heavier, which in turn means you need more thrust on the LAS system, which means more propellant, and so on and so forth. The existing escape tower is sized only for pulling the capsule away, not the service module too. Also real-world engines have minimum throttle ranges, so you would still need a separate service propulsion system somewhere for OMS. dV to enter or exit lunar orbit from an Earth trajectory is 900-930 m/s. SLS can’t fly more than once a year at most so using it to send cargo to the moon is a non-starter.

Let me help you out. Ordinarily when we do challenges like this, we say something like “No use of part mods or physics-altering mods.” This communicates that you want a fair challenge with equal competition.

Technically, putting LOI props on the command module is slightly more efficient, unless your descent module uses cryogenics, in which case the higher isp cancels out the poor architecture choices.

Finally, swapping Jeb out for a probe core and a solar panel allows me to reach a T/W ratio of roughly 7 which gives me a Kerbol escape trajectory at T+3:13.

It is fairly significant. As far as I know, keeping combustion chambers from melting has a small limit on the Isp of chemical rockets (true at least for shuttle engines, and they have amazing Isp). It is a significant problem, yes, but mass is always the primary problem. My comment is aimed at oft-expressed sentiments like "we can't make science-fiction spaceships too large because it's impossible to get that much thrust without melting the engine" which doesn't really make sense. Active cooling of engines is one of the less complicated aspects of rocket science.

The lowered propellant capacity was originally because Orion was supposed to be paired with Altair and so it only needed to come back from LLO in the first place. Making the service module twice as long, which adds about four meters to the height of the stack. And even that wouldn't get what you needed. Let's suppose a direct TLI to LLO, and let's generously double the propellant but only increase the dry mass by 50%. Since the ascent module wouldn't have to go back to NRHO they could probably get it down to 7 tonnes, and since the ascent module weighs less the descent module could probably get by at 13 tonnes. So let's imagine a 20-tonne two-stage lander that magically fits inside this tiny fairing. Current Orion's injected mass to TLI is 26.52 tonnes. Double the propellant and grow the dry mass of the ESM by 50% and that brings us up to 45.41 tonnes. So the launch vehicle needs to throw 66 tonnes to TLI. Even so, getting the 900 m/s of dV you need to go from TLI to LLO is going to require the ESM to burn 16.9 tonnes of propellant. Suppose the lander descends to the surface and then returns. Orion now has just 300 kg of propellant, giving it a measly 32.3 m/s of dV when it tries to make the return to Earth.

Update. T+ ten hours, and I've passed Duna and I'm moving at 0.3% of the speed of light relative to Kerbol.

I only learned today that you can use a KAL-9000 to set the expansion size of an inflatable airlock, even though there is no tweakable for the airlock. I might be able to key that to a throttle in order to create a throttleable airlock drive. Can also be useful to anyone who wants to create inflatable airbags or who needs the airlock to be physically smaller.

Interesting, can you show the vessel? Here you go. You should be able to see most of the details here. Reaction wheel, two docking port Jrs, an RTG, a single battery, and three little grip pads for landing legs. In retrospect I could have swapped the RTG for a solar panel and it would have had a higher TWR. I could also add more docking port pairs for more thrust, but it's nice with just one because I can use the Docking Acquire Force tweakable as a throttle. Unfortunately that tweakable can't be keyed directly to the throttle, even using a KAL-9000. I've been letting it run for the past half hour realtime and I'm now at 6 days to Jool.

I think you may underestimate the power of a Brachistochrone trajectory. For the record I just slapped together two docking ports, a reaction wheel, and a command chair, and Jeb was on a Kerbol escape trajectory 5 minutes and 57 seconds after launch. Passed the Mun at T+18:29. Passed Minmus at T+34:07. Left Kerbin's SOI at T+44:41. If I turned off the engine now and coasted the rest of the way, I would pass Jool in about 9 days.

No, it is not. If you use a constant-thrust engine rather than pulsing, then yes, you can dump the heat into the propellant as it leaves the nozzle. If you have a pure fusion engine then just use your pure fusion engine to take off and land. All other inquiries are unnecessary.

I do not understand why you think the primary problem in space flight is melting.

If you can use DLC parts then that’s hard but not TOO hard, and if you can’t use DLC parts that’s impossible, right?

Yes, it appears ions are too OP. I suppose I could make it a payload fraction challenge rather than a straight-up dV challenge; that would nerf the ions appropriately. Would that make this challenge better overall? I am moderately concerned that what we are beginning to see is not a vertical launch with a gravity turn, but a vertical launch with an immediate tipover and airbreathing ascent. I'm not sure how to fix this. Maybe make airbreathers a separate category?

[snip] You can build an SSTO at any size if your fictional situation requires it. Up to a point, the square-cube law is going to favor larger and larger vehicles. If we had a financially-compelling reason to launch truly massive payloads into orbit, then Saturn V would look like a bottle rocket. The fact is that we don't have a need to launch really ridiculously big payloads into orbit. If we had pure-fusion-based energy at a reasonably affordable we could and would build all of our launch vehicles around pure fusion-based energy. If we had a reason to lift massive payloads into orbit we would use fusion for that. If we needed an SSTO spaceplane we would use fusion for that. But even with pure fusion rockets, the utility of staging remains attractive. SSTOs have never been glorified shuttles. SSTO designs were based on the theory of reusability. But a TSTO will always be more efficient and more readily reusable than an SSTO using the same engines. If you want a science fiction spaceship that can take a small crew to Mars or Jupiter or wherever else and back to Earth without refueling, cool. Write one. Give it pusher plates if necessary. But there's no use case for Project Orion.