FinalFan

This is my first craft thread, so advice would be welcome if I am in conflict with any conventions. LKO stats have been adjusted for 1.8 aero changes (improved performance for this craft). Big Plane to Anywhere (figuratively) https://steamcommunity.com/sharedfiles/filedetails/?id=1875714661 (Nice try, Bill, but I don't think this is what they meant by "long range ISRU craft"...) Well, it's been a long time coming. I'm mainly a rocket guy, but when I do find myself in the Spaceplane Hangar I very easily get obsessed with fiddling with my designs, more so than with the rockets. Anyway, I think I've finally gotten this to the point where I no longer have to be ashamed to put it in view of the public. Let me know if I'm wrong! If I'm being honest, the series of planes that this is a culmination of probably began life as the concept, "like my first spaceplane, only bigger". But eventually an actual mission materialized: to carry a scientific exploration team to Laythe and (almost) anywhere else with a complete science package and ISRU that wouldn't take months to refuel with. One development I'm a bit proud of was the realization that I could attach radiator panels to the reaction wheel that was adjacent to both of the drills and the refiner, elegantly taking care of all my core heat xfer needs. This development allowed me to finally have a truly streamlined airplane, since I had formerly had medium TCSes attached to the exterior. Two panels and two small TCSes have less than the mass of one of the two medium TCSes, let alone the aerodynamic improvement! The small TCSes (middle of picture) are for actual part cooling, and aren't necessary, but having at least a little active cooling capability is handy from time to time, e.g. cooling off during a multi-orbit aerocapture. (The panels are terrible at non-ISRU cooling in general, and the ones on this vessel in particular are nearly useless at it.) The construction of this spaceplane does not use offsets at all, nor any part clipping aside from the wings tilting into the nacelles. This was a design choice. Table of Contents: Statistics: immediately below the table of contents Atmospheric flight characteristcs: Balance; Lift; Takeoff; Ascent; Reentry; Landing: approach, normal, water, parachute Vacuum flight characteristics: Takeoff; Landing Miscellaneous: Docking; Adjusting balance; Water landing; Water movement; Design choices Edit history Postscript (action groups) Statistics: —Parts: 100 —Mass: 111.35 tons (57.15 dry) —Cost: 401.29 kilocredits —Vacuum Delta-V: 4.6k to 4.7k depending on if you have oxidizer or nothing but liquid fuel. (counting ore tank, since you can refine it into fuel) —Engines: 4xRAPIER, 4xNERV (0.25 TWR on NERVs if the LF/Ox tanks are empty) —Landing gear: 2xLY-60 in rear; LY-60 and LY-35 in front. (35 for better taxiing and runway takeoff; 60 for rougher terrain and lower takeoff speed) —Mining: 2x large drills, large converter, 2x smallest ore tank, M4435 Narrow Band scanner, Surface scanner (Surface scanner can refine M4435's results) —Science: all experiments; 4 places to put science in addition to the lab (Cockpit, probe core, 2x storage unit) —Communications/Control: Communotron 88-88 (normal direct antenna); RA-2 + RC-001S (with one pilot, can control a probe that lacks direct CommNet connection) —Docking: 1.25m only; no RCS. See Miscellaneous for tips. —Airbrakes?: Yes, 4. Retract before landing. —Parachutes?: Yes, 6 plus 2 drogues; positioned to be moderately rear-heavy, but this is adjustable via fuel movement. Enables safe landing in any terrain or water. —RTGs?: Yes, 8; can permanently run lab and SAS at full strength —Fuel cells?: Yes, 3; can permanently run drills and refiner at full capacity —RCS?: Sort of; 6x Vernors under the nose to assist takeoff/landing in airless environments —How much delta-V is left after making LKO?: Even a pilot worse than me should reliably get to LKO with 24-2500m/s remaining before the +203 from refining the ore tanks. My best so far is 2667 (2870) and I seriously doubt this cannot be bested by an actually talented pilot. Test flight landed on Minmus with 1213 dV remaining. Also goes direct to Mun. —Amphibious?: Yes: can land in water on parachutes or aeronautically, and can take off from water (but only below 25% fuel). See Miscellaneous for details. Atmospheric flight characteristics: —Balance: The center of mass is almost exactly on top of the center of lift both full and empty, in the back half of the rear cargo bay. (About 16.3m from the front of the 26.8m craft, or about 61% of the way to the back.) If you want the CoM to move forward after takeoff, you can lock the tank behind the cockpit (this LF should not be needed to make orbit). This also can help counteract the already mild tendency to nose up at high speed (see below). See Miscellaneous for a tip about rebalancing the craft. —Lift: Tendency to nose up or down while SAS is off is not violent at any speed; very slight down at low speed (<200), a bit up at high speed (300-1000), and very slight up up during final speedrun. —Takeoff: Fully loaded, it can be lifted off the runway at 83m/s (lower on rough terrain due to upslopes) on main wheel (100 for smaller wheel). Consider using the small wheel on runways or flats to gain more speed before liftoff (due to lower angle of attack while on the ground). Left to its own devices it lifts off at the end of the runway at about 99m/s (112 for smaller wheel). —Ascent: After the 1.8 update, it no longer has to stop ascending to break the transonic speed hump. My flight plan, which may or may not be optimal, is: Set at 5° above horizon; it will gradually dip a bit to 2-3° but recover by itself. After going supersonic it will tend to slowly climb in attitude; stay at 5-7° until about 15km altitude, then begin dropping to 2-3° in order to extend final speed run in the 18-22km altitude range. (If you're feeling lazy, you can just let SAS do its thing all the way to 1000 m/s or so.) Activate NERVs when RAPIER thrust falls below 200kN. By the time thrust falls below 60kN you should be around 1500m/s +/-50; switch to rocket mode when jet+NERV acceleration is unacceptably low and aim for the top of the prograde circle (or a little higher or lower, to taste). When oxidizer runs out, drop to prograde lock; the NERVs should be able to maintain or increase your time to apoapsis. —Reentry: It's recommended that fuel be moved foward to the tank behind the cockpit for reentry for extra stability. For example, in testing, given a 100kmX15km LKO reentry orbit and only 500 units of fuel remaining, a neutrally balanced craft struggled to maintain a 40-45° pitch without using airbrakes, while a craft that moved that same small amount of fuel forward had no trouble. It should be noted that the balanced craft had no trouble with a 30-35° reentry profile. 1000 units of fuel (~10%) balanced forward is enough for a radial out pitch to stabilize forward instead of flipping out. Use caution if returning from other celestial bodies; it's not recommended to dig deep into the atmosphere on the first pass without decelerating first. —Landing approach: If airbrakes were used, retract prior to landing as the bottom ones may be destroyed otherwise. Perfect balance and generous control surfaces make it relatively maneuverable for a Mk3 spaceplane. Since it takes off near empty at 53-60 m/s (on big/small front wheels) the stall speed on low fuel should be similarly low unless I'm mistaken. —Landing normally: Try not to touch down at more than 5m/s vertical speed. The fairly wide rear footprint means it should be pretty stable and modest testing has borne this out. —Landing in water: This plane is capable of a safe aeronautic landing in water, which was successfully tested at 25% fuel, but parachutes are recommended for landing in water or very rough terrain. —Landing on parachutes: Even heavily loaded with fuel, this vessel can land safely on its complement of parachutes, but use of engines to help soften the landing is encouraged. The Abort action group opens doors for the parachutes and triggers them. Vacuum flight characteristics: —Takeoff: Fully loaded and on a flat surface, the Vernors are not quite strong enough to lift the nose for vertical takeoff in Munlike gravity. Instead, get forward motion and it should be able to lift off shortly with the help of the thrust attitude, especially if the vehicle runs off the top of a hill. Use of RAPIERs is recommended unless terrain is very flat. —Landing in vacuum: dV can best be conserved by a "reverse takeoff" posture, where final approach retains some horizontal motion while vertical motion is very low, but this is not easy. It is probably more practical to descend on the engines rocket-style, and then transition to horizontal when near/on the ground (fall on the wheels). This has been tested on the Mun. Be careful to keep the plane level (don't let it roll on its side or you may lose a wing). The Vernors can slow the fall of the front end, but this should not be necessary in Munlike gravity. Miscellaneous: —Recommended docking procedure with this vessel is as follows: 1. Rendezvous; 2. Make a close approach (50 meters dock-to-dock* or less), then cancel relative motion; 3. Align to the desired docking port, and accelerate a little for final approach. This technique has been highly successful on larger versions of this vessel docking with still larger vessels. *(Bear in mind that when approaching tail-first the engines are almost 27m closer to the target than the docking port, and flipping tail-to-nose will bring the docking port of this craft when normally balanced 32 or 33 meters closer to the target due to where the vessel's pivot point is. A dock-to-dock separation of 50m would mean a real distance facing away of 23m and 17m after flipping). —When altering the balance of the craft (e.g. to put the CoM more ahead of the CoL), keep in mind that adding fuel to the rear and front LF tanks in a 2:1 ratio will be approximately neutrally balanced, i.e. will keep or return the ship to close to its starting balance. —Water landing (via parachute) was tested on Kerbin at 75% fuel. If your splashdown attitude is close to vertical, your docking port may be at risk of destruction as you rotate down, but this can be prevented by using the Vernors to slow your fall. (Presumably similar precautions should be taken if landing on Tylo.) —The top speed in water is 38.7 m/s when full. The top speed on low fuel is uncertain because during testing the plane unintentionally took off. In subsequent testing, the vessel was able to intentionally lift off when top speed in water was about 70 m/s, when fuel fell to between 20 and 25%. (Highest speeds were obtained when fuel was moved forward and SAS set to prograde.) —Why the precoolers? Although the shock cones more than meet the needs of normal flight, I like the idea of having strong static airflow for those, ahem, "off prograde" situations. They are also handsome. —Why the everything? This design was not arrived at minimalistically; I'm sure there is a lot you could cut, starting with the rear parachutes and airbrakes. But I wanted a feature-rich craft, and those are features, and it is Minmus capable, so there. Having said that, I do still welcome criticism if you think anything is too blatantly unnecessary. —Why no offset? I just didn't want to use offset; a little bit to neaten up the often messy intersections of aerodynamic parts is okay but I often see it used in a way I think of as exploitative. So in a way the only minimalistic thing about this design is the offset and part clipping, which was a design choice. —After the 1.8 atmospheric changes, this plane can actually fly on a 3R+3N configuration, but taking away two nacelles worth of LF completely negates the benefit. It's slightly less dV, significantly less TWR, and a more annoying ascent path; why bother? Well, I could probably just yank one jet off the regular design (keeping 4 NERVs) and limp to orbit, but the dV gain (112 I think) is, while noticeable, quite modest and I would be nervous about sending it to Laythe untested with that much less atmospheric performance. So, how do you like the plane? How do you like the post? Comments and questions welcome. Edit history: 1.1—9/28/19: Added vacuum landing notes. Added "Miscellaneous" section. Made major changes to vehicle (reduced RAPIERs from 6 to 4, replacing two Mk3 side pods with four Mk1 side pods). Edited text accordingly. 9/29/19: Added Minmus picture. Edits to vacuum landing notes and other things. 9/30/19: Swapped location of RAPIERs and NERVs (reducing tailstrike risk); minor text edits. 1.2—10/4/19: Minor adjustment to front wing AOI and location; small but critical adjustment to rear wheels to eliminate drift on takeoff. Added location of CoM to "Balance". Added "Takeoff"; edits to "Ascent" and "Landing Characteristics".) Finally changed top picture to reflect new version of craft. 1.3—10/5/19: Added "Communications"; added more detail to CoM location, "Lift", and "Takeoff"; added RA-2 to vehicle and replaced bottom pair of shock cones with NCS tanks. (Reduced cost by 3300, increased wet mass by 0.91, dry mass by 0.11, part count by 3) 1.4—10/10/19: Updated "Landing in vacuum" to reflect finally testing Mun capability; added "Takeoff in vacuum". Removed 1 fuel cell (3 remain) in the belief that this was enough for even maximum ore concentrations, subject to review; please let me know if this is found to be wrong. 10/11/19: Added water landing/movement/takeoff details. 10/13/19: Added postscript. 10/15/19: Added table of contents. Reorganized flight/landing characteristics. Added detail to docking. Updated photos (10/5 changes), minor text changes. 10/16/19: Added "Reentry", minor edit to "Ascent". 10/22/19: Altered commentary on small TCSes. 10/23/19: Removed Z-4K battery after finding a way to mount the drills on the reaction wheel and still fit them through the cargo doors. Added small ore tank for symmetry in new location. (Cost reduced by 4.2k, mass increased by 0.675t wet but reduced by 0.075 dry. Battery capacity reduced from 5710 to 1710.) Updated cargo hold picture. 10/26/19: updated "Ascent" to reflect 1.8 aerodynamic changes. Redid Minmus test—new photo. Postscript: the action groups: 1: RAPIER toggle (on/off) 2: NERV toggle (on/off) 3: RAPIER mode swap & air intake toggle (open/shut) 4: All cargo bay doors toggle (open/shut) & small thermal control system* toggle (deploy/retract) 5: --- (nothing) 6: Drill toggle (deploy/retract) 7: Surface harvester toggle (on/off) 8: Fuel cell toggle (on/off) 9: Obtain all possible science, including crew report 0: AIRBRAKES toggle (deploy/retract) Lights: default (all lights turn on or off) Landing gear: default (all landing gear extends or retracts—note that this is NOT a toggle) Brakes: default Abort: The top cargo doors open and all parachutes deploy. It's possible that some parachutes won't deploy if the doors aren't already open; just press the button again. *(Note: the interior thermal panels are always on by default)

Well, the Skiff's vacuum Isp isn't just a little better, it's a lot better. So the Skiff is a good choice for anything in a certain size range that you light up off the launchpad but still in atmosphere, or even, like the OP said, if it's a long-burning center stage that lifts off the pad aided by boosters. You probably wouldn't do that with a Poodle or Wolfhound! [edit: I mean, let's be honest, the launchpad Isp of 265, which is unchanged, already suggested it was never meant to be best in class during the actual liftoff. Its specialty is having the best vacuum Isp among engines that aren't total garbage on the pad. In other words, mid-atmospheric performance.]

Chalk it up to miscommunication from me to you. Allow me to clarify: your 99% proposal was such a blatant end run around (what I perceived to be) OP's clear intent that I did not think you, an apparently intelligent person, could in good faith actually think a mission using that technique would be accepted as a challenge entry.

Okay, I will assume good faith here. I had thought you were feigning a lack of understanding to highlight your argument, so I'm sorry for misunderstanding. The way I see it is based on intent: you are forbidden to use ions when your motive for the maneuver is to exit the SOI to go to another one. So if your reason for the periapsis kick is to rendezvous with something else in the body's SOI, it is not a violation; if your reason is to leave the SOI, that's an inappropriate use according to the rule. This includes the first of a set of 100 periapsis kicks where the goal is a completed transfer burn. Admittedly, this leaves a gray area where you could use ions for some in-SOI purpose that "just so happens" to leave you in a much higher orbit when you begin your burn. In other words, cheating. There are two answers to this that don't require a blanket ban: one, simply the honor system; two, a modification to the rule saying ion use in-SOI should not leave you in an orbit more than 200km high, or more than 100km in altitude different from before ions were used, or something similar. Would such a rule modification clear up your questions regarding what is and isn't permissible ion usage? (Please note that this is a hypothetical situation, since my interpretation isn't necessarily 100% the OP's intent.) [edit: bayesian_acolyte did a better job than me.]

Well, from context I think it's pretty clear that you would not be allowed to use your 99% idea, as that's directly against the explained motive for the rule. And I also think you're perfectly aware of this. But if someone wanted to use ions for some other purpose unrelated to the main transfer burn it's allowed. I don't see why the ban should be broader than necessary. Wording it so as to be unassailable by the pickiest rules lawyer would indeed be a pain, but in a small contest where the judge is the Creator and available to answer questions I don't see a real problem.

I agree with your decision to avoid individualized bonuses for each achievement, but a totally flat bonus might be going too far in the other direction. Maybe two or three classes of rewards? For instance, Ares I sounds like a fun little touch to add, but I imagine it's much less difficult than Water Park. On the other hand, Beach Bum is the easiest of all with a landing altitude requirement of five hundred kilometers

I suspect that part substitution would end up being one of those ideas that sounds simple but is actually very problematic. Locking, though, I have hope could be done. (Toggle the travel settings instead of changing the part.)

@Zhetaan Wow! Thank you for the extremely educational post. I had been secretly wondering what the harmonic mean was. Referring to the original question, you did answer it. I believe I had been misled by reading too much into my three example data points that had multiple variables. Clearly the biggest difference between my oversimplified model and the correct one is illustrated in extreme cases that expose the difference in limits that are approached, 2a in one case and b/2 in the other. The most non-intuitive effect (for me) of doing things the correct way comes in if the high thrust and high Isp engines are the same, because I don't instinctively consider fuel flow and as conditions get more extreme the fuel flow on low Isp (higher fuel flow) engines will tend to overwhelm the high Isp (lower fuel flow) engine even if it's the lower thrust one. But I suppose this is what causes the upper limit on combined Isp in the first place. I read your post at work, thought about it from time to time, and I'm back home now. I can't remember the last time I rewrote a post so many times based on increased understanding, even if it's partly due to divided attention.

Wow! I'll have to remember that somehow. But is it fair to say that the error (produced by doing it the wrong way) increases dramatically, perhaps exponentially, as the Isp difference increases? I compared the same engines (Terrier + Reliant) using the method I had formerly thought was accurate and came up with 317 (vs. 316, tiny difference). Comparing a Mastodon + Wolfhound setup seems to yield about 310 versus 316.5 the wrong way. (6.5 is still relatively tiny compared to 90.4 difference between NERV/Terrier calculations despite the Isp difference being over 25% as large)

Sorry, but you presume incorrectly. The Cubs are much smaller and narrower than the Thuds, allowing three to be crammed in approximately the same space as a single Thud. Maybe I'll try it again with dead weight to make up the difference; the 3-Cub formation is actually at a small launchpad thrust disadvantage versus a Thud, so it'll be interesting to see how that goes.

Thank you very much. That makes much more sense than what I was imagining. I was thinking of the old "hang things off the side" construction even though it's obviously not necessary with radially attachable engines. Three tries were not enough for me to replicate your success, no doubt due to pilot error. I also had terrible issues with wobble from the upper portion. I tried a version using a 1.875m payload and it seemed to be flyable, but I still did not immediately succeed in orbiting without help from the payload. My "what about Cubs instead?" version was easily able to do it with 1.875m and 2.5m payloads, replacing the 3rd, 4th, and 5th stage Thumpers with groups of 3 Cubs per Thumper. Fins were not necessary; these tests produced some of the smoothest no-control gravity turns I've ever done. (I discovered this when trying to counteract SAS oversteering; the Cubs have really outrageous gimbal for this vessel.) Unfortunately uncontrolled gravity turns tended to not be shallow enough for best fuel efficiency so my successful tests were manually controlled. In the end I set all the Cubs to 50% gimbal. Initial tests made orbit with between 14% (1.875m) and 19% (2.5m) fuel remaining in the last launch stage. However, there was a problem: My 13.88t payload vessel had less launch weight than your vessel, biasing the test in favor of the Cubs. I then adjusted it for a 76.323t total weight (14.683t payload), with 2.16t difference due to the Cubs weighing less than the Thumpers they replaced and 0.001t lost due to inaccuracy. It made orbit with 13.5% fuel remaining. The Cubs' weight advantage may have helped, but the biggest difference was probably the Isp advantage.

I have a lot of trouble picturing your craft. My attempt to recreate it did not have the right tonnage. In any case, I think you may have misunderstood my comment about the Skipper. Clearly the Cub has much less raw thrust, but their performance relative to size is pretty similar: —Engine TWR ASL/VAC is 19.33/22.09 versus 19.12/22.66 —Isp ASL/VAC is 280/320 versus 270/320 In fact, you can slap 16 of them around a 2.5m stack and have almost the equivalent of a Skipper (with extreme vectoring) ... for 16,000 funds instead of 5,300, but c'est la vie. If cost is no object, I believe Cubs would be better than Thuds outside of the bottom stages. There are two main difference between the Cub and the Thud: you need 3 Cubs to be as strong as 1 Thud; and the Cub has decent vacuum Isp. The Cub also has much higher engine TWR. 3 Cubs also have the same VAC thrust but about 7 less ASL; and Isp that is a bit worse ASL but evens out at just 2400m.

Aren't the aerodynamic issues an even bigger reason? It's true there aren't all that many radial engines but you can get reasonable quality out of the ones that are there: the Cub from MH has comparable performance to the Skipper in TWR and Isp.

I have been using the Mun because it's a shorter trip than Minmus, but that was predicated on the moon in question being the refueling point. (And really, Mun orbit seems to this noob like an all-around more sensible staging point than Minmus orbit for interplanetary operations with its shorter orbital period and lack of inclination.) The extra fuel spent shuttling the miners to and from the surface wasn't a big loss for me since they weren't exactly operating at capacity anyway. However, if we're talking about filling a tanker that goes to LKO with no tight schedule, then clearly Minmus operations are better. I haven't been doing that—not systematically, anyway—but it seems like a good plan. Time to start shifting gears in my operations.

re: 3. I think your pedantry is slipping! "SSTO to wherever" is simply a further restriction; it not only gets T.O. but also to elsewhere. If you really want to go down that road, SSTO is arguably not a thing but a property of a thing, i.e. you can have an SSTO rocket, and that rocket will be SSTO, but you arguably don't have "an SSTO" per se. However, SSTO also meaning a rocket or spaceplane that is SSTO is good enough for me.

I understand the story you're telling. The moral is, "if you've got oxidizer, burn it." I agree—I think I did the math back when I first sent a spaceplane beyond LKO. But I don't see how it's in any way relevant to the OP.

Well, now you've got me curious, @Rayder. Two 2.5m ore tanks and one full Mk1 liquid tank makes the same 32 tons of fuel, but 4.25t dry weight instead of 4. For fun, let's put an empty T400 on there to match the NERV's liquid tank, and with both engines that's 8t. But there's also the converter—a 250, of course, to avoid wastage—but since we didn't calculate probe cores or anything for the original vessel I'll ignore cooling now. So 12.25t dry and 44.25t wet. Plug it in and... Just over 10,000 delta-V. If we had ignored the different weight ratio of ore tanks and the need to add a converter, then it would have been extremely simple to figure out, and 7700 would be obviously way too low: if the ONLY difference was 800 isp versus 345 isp, then we'd have 800/345 times the Terrier's ~5,600 m/s, over twice as much. Periapsis kicking on a burn that's already less than two minutes from periapsis on each side? Not my flavor of crazy. It's come in handy, though, most notably when my first ion probe shipped out. (Thankfully, all it has to do is hit a circular solar orbit for an asteroid detecting contract: a relatively easy target to hit.) I like that fuel tanker idea, though. It's certainly better than sending the tanker after a ship that's already outbound, which I did once.

Everything you said seems basically true (I got slightly but not significantly different delta-V numbers when I checked you), but it's a very different situation than either my actual situation or my hypothetical NERV + other scenario. The reason that burning both engines (or even the Terrier alone!) is better than NERV-only in your scenario is, as you said, because you eliminate a bunch of mass in the form of oxidizer. But in the scenario, the question was whether ore should be refined exclusively into liquid for the NERV or partially into oxidizer for the LFO engine as well. There should be no oxidizer left over since you simply wouldn't refine any that you weren't planning to use, and thus both should end at the same dry weight. And in my actual scenario, of course, there was no issue of deadweight unusable fuel at all. In both of these cases, clearly the efficient engine will always win out unless there is some other factor at play like a TWR requirement (such as on Tylo) or the Oberth effect as in the OP. I'm sure you were aware of the following, but it's worth noting that while the NERV and Terrier in your example make for a very simple average Isp (since they have the same thrust, you just add them up and divide by two), it can mislead readers into thinking it always works this simply. In fact, you have to weight them by thrust. For example, one Rhino (2000 thrust, 340 Isp) and one Vector (1000/315) average to ((2000 x 340)+(1000 x 315))/3000 = 331.67 Isp for the whole 3000-thrust combination. [edit: @Zhetaan's post below gives the real, somewhat more involved, way to calculate average Isp. The struck-through method will give you an approximation that I think should be fairly close as long as the engines' Isp isn't too radically different (i.e. Dawn or NERV vs. anything other than themselves).]

Did a little math, and I reckon that, with the difference in Isp, the less efficient engines "wasted" enough fuel to run the efficient engines for about 1.875 seconds. So the question becomes "is the difference in Oberth effect worth more than 1.875 seconds of free thrust?" Based on my foggy recollection of the maneuver's total cost, and given Brikoleur's "single digit" estimate ... no, engine efficiency was better unless I completely screwed up my math or Brikoleur is off by a lot. I calculated close to 20m/s so a minor mistake in the maneuver dV doesn't change the outcome. But there were a lot of moving parts in that calculation and it's not inconceivable that mistakes accumulated or my logic was flawed. Or Brikoleur could be wrong. Is there any relatively easy Oberth estimator around? Either way, thanks for inspiring me to think about this. [edit: got home and I think I messed up the fuel flow difference. I think it's more like over 4 seconds of free thrust.]

Like I said in my reply to Brikoleur, I'll need it at the destination. The extra engines are limited application but higher TWR compared to the main engines which are heavier but more efficient. If we take as a given the single stage nature of the craft, is there still a trick I'm missing?

Thanks everybody for all the answers! It sounds like someone in low orbit (as opposed to a flyby) has considerable leeway on either side of periapsis in terms of taking advantage of the Oberth effect, so it probably helped to keep the less efficient engines turned off. Brikoleur, the less efficient engines were added for extra punch on the Tylo landing I wanted this vessel to be capable of. It should have a local TWR of 1.45 fully loaded as I recall. Tylo and Laythe are the only moons I'd need that kind of thrust for. That said, I am very silly and the thing is a gigantic beast. Its name is "Admiral Perry" because of the muscular exploration it's meant to do. It's designed to deliver up to two dozen kerbals in style to anywhere short of Eve within its 5,500 deltaV reach and carries enough ISRU to do it all over again without having to wait a year. The more and less efficient engines in question are Rhinos and Vectors. @Rayder, I guess I was insufficiently clear when I said "transfer burn from Kerbin to Jool". What I meant was the actual burn that made me leave Kerbin and cross Jool's path. But your information was very helpful. I will certainly keep it in mind for my future maneuvers around Jool's orbit and other places.

I just did a transfer burn from Kerbin to Jool with a vessel that has different LFO engines. I used only the ones with better Isp but I was wondering if I would have done better to fire the other ones as well, counting on the increased Oberth effect to overwhelm the loss in fuel (burned less efficiently). In this case I could have shortened a 3m27s burn to 2m35s. My instinct is that the amount you shorten the burn by doesn't matter a lot for this decision because the cost in fuel efficiency scales in proportion to how much thrust you are adding. Is this instinct misguided? Is it always worth it when the engines are in the same general class? (e.g. LFO generally gets 300-350 or 290-412 for the extreme cases) If so, is it always worth it, period? (e.g. let's say hypothetically I have NERVS, LFO engines, and ore I can refine. Should I make oxidizer?)

If I'm not mistaken, it has 37% the Mammoth's launchpad thrust and 40% of its weight with worse Isp and equal engine gimbal at 33% of the cost. The comparison to the Twin Boar hardly bears mentioning. What is totally OP about it? Or, alternatively, what is the big difference in what it does?

That's an amazing story. Thank you for sharing it. I love that your first orbit was basically "boost straight up until you find geocentric stationary orbit"

That's true; I haven't been appreciating that its TWR has a slight edge over even the mighty Mammoth. That may possibly outweigh the extra weight in fuel burned on a short lived initial stage. (On the launchpad, three Mastodons have the same weight as a Mammoth and 4.4% more thrust, while the Mammoth has 5.4% better Isp.) And while the Isp gets much worse, it never entirely loses its edge in TWR. And the Mastodon's gimbal is a healthy 5°, while the Rhino (4°) is the only other engine in its weight class to beat 2°. In fact, the Vector (10.5°) is the only bottom-mounted engine that beats it on that score, and only the much smaller Bobcat matches it. So I have to take back what I said about the Mastodon not having any stand-out features. That leaves the glaringly obvious cost issue. At 22k a pop, the cost when clustering quickly becomes ridiculous. The Vector (18k) has a similar problem but obviously it, unlike the Mastodon, has the Mammoth. But I think pricing it the same as the Mainsail would go too far in the other direction. Given the engine's qualities and the inherent versatility that the Mammoth by its nature completely lacks, I think pricing them at exact parity (comparing one Mammoth to 3 Mastodons) would be selling the Mastodon at a significant discount. (Now that you've opened my eyes to seeing its virtues instead of fixating solely on its glaring performance flaw.) Just off the top of my head, 15,000 seems appropriate. It means you pay a premium versus the one trick pony Mammoth but the price isn't completely out of whack with everything else ... even the Vector.