sevenperforce

Then you do the same thing as with a liquid rocket. Let the rocket burn for a few minutes, see how much fuel was burned, and divide total fuel by burn time to get the mass flow rate. Then divide thrust by mass flow rate to get isp.

Much taller than New Glenn, and you get into stability problems due to fineness ratio. Not to mention not having enough surface area on the base for engines. I doubt they'd do a New Glenn Heavy...or if they did, I doubt they'd call it a New Armstrong.

Unlike in KSP, real-world solid rockets do not have constant thrust. Their thrust, flow rate, and even specific impulse can all change over the course of the burn.

Not quite. "Axis of engine exhaust divided by fuel flow" doesn't tell anyone anything. Forget that. Now, if you happen to know the average velocity of the exhaust molecules, you can use that to calculate specific impulse. But it's not exactly an easy thing to measure, so let's forget it, too. "thrust (lbs) divided amount of propellant (lbs) x burn time" isn't quite there, either. To begin with, you need to know the propellant flow rate. The propellant flow rate tells you how fast your rocket engine (or any other engine) uses fuel. A big engine probably uses fuel quickly; a small engine probably uses fuel more slowly. An efficient engine uses fuel more slowly than an inefficient engine of the same size. The propellant flow rate is very easy to calculate. Run your engine for a few minutes, then shut it off. How much fuel did you use? If your engine used 10 gallons of fuel in 5 minutes, then the flow rate is 2 gallons per minute. If it used 20 gallons of fuel in 5 minutes, then the flow rate is 4 gallons per minute. Assuming you know the density of your fuel, it should be trivial to convert gallons into pounds and minutes into seconds. If your fuel has approximately the same density as water, then 4 gallons per minute is the same as 0.53 pounds per second. Once you know your propellant flow rate, then dividing your total thrust by your flow rate will tell you your specific impulse.

Obviously, haha. I meant, is the NA going to be a megacluster?

No, I wouldn't call NG a megacluster. BFR's 31 engines definitely qualifies it as a single-stick megacluster, though. Obviously the N-1 was the original megacluster single-stick rocket, which is why I referred to it before. For NA, Blue Origin can either bundle a few dozen BE-4s or they could build an even bigger engine altogether. I'm leaning toward the former as well. Speaking of the N-1, the new 5-meter parts finally allow a pretty-looking stock N-1.

No, nothing like that. I'm more saying, are we going to see a megacluster rocket with even more BE-4s?

New Armstrong will presumably be bigger than New Glenn. Not a lot of space between NG's seven meters and BFR's nine.

As I've mentioned in a few other threads... The stats on the Skiff and the Wolfhound are swapped. The Skiff is the analogue of the J-2 and the Wolfhound is the analogue of the Apollo CSM SPS. In real life, the J-2 was much heavier than the SPS and had an isp of around 420, while the SPS had twice the TWR of the J-2 but had a lower isp due to its use of pressure-fed hypergolics. In Kerbal, however, the Wolfhound is much heavier than the Skiff and has an isp of around 420, while the Skiff has twice the TWR of the Wolfhound but much lower isp. I think this is probably a coding error. Great overview, though! I will have to play around with the Bobcat a bit. It is definitely disappointing that the Kodiak didn't have some sort of additional flair to make it more useful than a Reliant. For example, it could have had a better vacuum isp and a smaller vertical size.

At least that behavior is correct.

New Glenn is already a seven-meter monster. The BFR will be nine meters. How much bigger will New Armstrong need to be? Are we talking about another N-1?

I see Tweakscaling like clipping. Fine to do it for structural parts, wings, or cosmetics, but not for engines or tanks.

Why is Tweakscale disallowed? Tweakscale doesn't break anything or make the challenge less challenging. Just gives a few more options.

It was already possible to do an Atlas-style LV if you invert a decoupler, attach it to the bottom node of the center engine, and offset it up above the engine. Gives you a terminal, jettisonable attachment node for a circular shroud. I always used the smallest Mk3 cargo bay but these new circular structures are nicer.

They already have a pretty good hydrolox engine, so I anticipate that being their workhorse for cislunar space. Their ignition system currently uses consumable solids, I believe, but that can be changed to use spark ignition. The BE-4 will use autogenous pressurization. Does the BE-3 also use autogenous pressurization? If so, then they should be able to set up propellant transfer pretty darn effectively. Hydrolox can conceivably perform EOI burns, allowing orbital refueling reuse without aerocapture.

Link is broken.

I know that R changes symmetry modes, between radial and mirror, but is there a key to cycle through symmetry value? Like, if you're in 2-way radial symmetry, you'd press this key to go up to 3-way, and again to 4-way, and again to 6-way and two more times to cycle back to none. Was the SM-18 intended to serve as the core of the descent module for the LM? I wasn't sure what it was to be used for. A single Mk16 parachute fits quite neatly inside each of the four chambers inside the SM-6A. Of course it doesn't surface attach, so you do have to place a cubic strut and offset it down, which is suboptimal. But the finished product looks very neat, and the Mk16s have much longer line lengths than the Mk2-R, which looks good. I put three Mk16s in three chambers and put a pair of surface-attached radial drogues in the fourth chamber. You can use an action group to cut the drogues at the same time that you deploy the mains. I, for one, am really bummed that Making History didn't include any particularly-elegant way to clone an N-1.

Yeah, it is def too long. The stats on the Skiff and the Wolfhound are swapped. Use a single Skiff on the CSM; use five Wolfhounds on the Saturn second stage and one on Saturn's third stage.

Suppose Alice has an engine which produces 200 pounds of thrust while consuming 2 pounds of propellant every second, while Bob has an engine which produces 100 pounds of thrust while consuming 2 pounds of propellant every second. Which engine is more efficient? Obviously, Alice's engine is more efficient, because they both consume the same amount of propellant every second but Alice's engine gets twice as much thrust. But what if you add Carol? Carol's engine produces 100 pounds of thrust while consuming 1 pound of propellant every second. Now her engine doesn't produce as much thrust as Alice's...but it does use less fuel. So which one is more efficient? To answer this question, you ask a new question. "How much thrust (or 'impulse') is produced for each pound of fuel burned every second?" In other words, what is the impulse at a specific fuel consumption rate? That's a question you can answer. Carol's engine produces 100 pounds of thrust while burning 1 pound of fuel per second; Alice's engine produces 200 pounds of thrust while burning 2 pounds of fuel per second. 100 pounds / 1 pound per second comes to 100 seconds, and 200 pounds / 2 pounds per second comes to 100 seconds. So the specific impulse of both engines is 100 seconds, so they have the exact same efficiency. Note that you can't just drop "seconds" here. What if Dave comes along and his engine produces 50 pounds of thrust and consumes 2 pounds of propellant per minute? If you divide 50 by 2, you get 25...but if you follow the units, that's a specific impulse of 25 minutes, which converts to 1,500 seconds, far more efficient than the engines of Alice, Bob, and Carol. So the units are important. And like I said before, the units are meaningful. If Dave's engine produces 50 pounds of thrust and has a specific impulse of 25 minutes, then you know it would take him 25 minutes to burn 50 pounds of propellant, or 50 minutes to burn 100 pounds of propellant, or 5 minutes to burn 10 pounds of propellant. Alice's engine would take 100 seconds to burn 200 pounds of propellant. Carol's engine would take 100 seconds to burn 100 pounds of propellant.

Sure, lemme just refly the mission and I'll take a few. Fairings still work quite well for most purposes (I used a fairing for the upper structural cylinder of Mercury-Atlas so I wouldn't have to worry about decoupling it) but if you need pass-through, the cylinders are beast. I'm also anticipating using the cylinders for some stock bearings.

Oh, it certainly wouldn't come back. That's why I suggested orbital propellant transfer. You'd leave the lander in lunar orbit and simply refuel it between sorties. So reuse is not suggested by the cost of the lander, but by the mass of the lander. Every kg you send to lunar orbit is costly; the more infrastructure you can leave in place, the better. Pressure-fed hypergolic drop tanks are probably the nearest-term solution, yes. You don't even have to drop the tanks between burns, necessarily; you just need some check valves. When one tank (or set of tanks) is at 5%, you open the valve on the next tank. Let them both run until the first runs dry, close that valve, drop it, and continue. An electronic turbopump would end up increasing efficiency and cutting down on tank mass, since the tanks only have to hold ullage pressure, not chamber pressure. But that really comes into play when you're talking about using propellant transfer instead of drop tanks. Well, we have two solutions: first, we can test it, because the lander solution would ideally be used first for cargo. Second, we can use multiple engines for redundancy. If the same lander design is used for crew and cargo (reusably or otherwise) then you need extra thrust anyway, to deal with a range of payload masses. So you'd end up with enough engines that you could lose one on crew ascent and still make orbit, albeit perhaps a lower one. If you have a reusable lander vehicle, then you want indefinite loiter time. https://www.nasaspaceflight.com/2018/03/nasa-courts-commercial-options-lunar-landers/

Well this would have made my Minmus mission a great deal easier.

Right, that's what he was saying. Rockets which formerly required fins were making orbit without fins. Rockets were MORE stable than they should be. The reason was increased part drag on the engines due to a programming bug, which makes rockets unduly stable but also makes drag losses skyrocket.

Just thinking ahead. A fully-reusable solution would be a vehicle capable of doing lunar descent and lunar ascent in a single stage, expending only bipropellant, and accepting propellant transfer from an Earth tanker. Using pressure-fed engines is suboptimal for this, for multiple reasons. One, you're going to need large tanks to pack almost 4 km/s onto a single stage, and dry mass of a pressure-fed solution scales really poorly when your tanks get large. Two, pressure-fed engines require that a tanker replenish not only the bipropellant tanks, but also the pressurant tanks, which means the prop transfer gets overcomplicated. Hypergolics are desired for ignition assurance (and because it means fewer consumables), but if you can get 100% reliable ignition without additional consumables, then you can expand beyond hypergolics. That's why I was thinking you could use a resistance heater to vaporize kerosene so it could be reliably spark-ignited. A resistance heater which produces kerosene vapors could also be used for autogenous fuel tank pressurization, and the warm vapor would act to prevent propellant freezing. The oxidizer is the problem; it is much easier to combat freezing than it is combat the boil-off you'd get with LOX. Hence nitric acid, because you could do the same thing as you did with your kerosene.

One thing to keep in mind is that "seconds" in a specific impulse is a physically meaningful measure. If you had a rocket with exactly enough thrust to lift the weight of its own propellant on Earth (though not necessarily dry mass or payload), and you fired it at full throttle, then it would burn until all its propellant is expended. The amount of time it would burn is the specific impulse of the engine. For example, if you had an engine which produces 1,000 lbs of thrust while burning 5 pounds of propellant every second, its specific impulse would be 200 seconds (1000 lbs/5 lbs/sec = 200 sec). If that engine was placed on a rocketplane carrying 1,000 lbs of propellant and the engine was fired at full throttle, the engine burn would last for 200 seconds before running dry. This applies to any sort of engine, actually, and you can employ various transformations to make it more useful. Suppose you have a jet airliner powered by turbofan engines with a specific impulse of 3500 seconds. If this jet reaches cruising altitude with 2,000,000 pounds of fuel, then we know that it would take 3500 seconds to burn all that fuel if the engines produced 2,000,000 pounds of thrust. Of course, they do not produce anywhere near that amount of thrust; they likely produce around a tenth of that, or 200,000 lbs of trust. So this tells us that the engines can burn at cruise for 3500 seconds * 10, or around 9.7 hours.