Efficient Delivery of heavy equipment

[Khatharr]

I'm starting to try using some heavier parts than I've worked with before. Specifically, I'm trying to set up an ISRU facility on the great plain of Minmus in order to set up a refueling facility.

As a practice run, I've located a uranium deposit on Minmus, and by a combination of luck, save-states, and miraculously strong landing struts, I managed to put a reactor and generator down near the target site (the sensor broke, so I don't yet know if I'm actually inside the hotspot or not). Now I've sent up a craft carrying a pair of ISRU refineries, along with some robotics and KAS parts to connect everything on site.

My problem is that I'm noticing that my delivery for heavy equipment is really pretty bad in terms of efficiency. The methods I've uses for launching countless probes and 1.25m craft isn't working so well when I'm sending up refineries, reactors and generators. In other words, it doesn't seem to scale well. I ended up spending all my transfer fuel on orbital insertion for my ISRU launch, so I sent up a refueling mission, but the fuel was pretty heavy too, so I ended up losing about half the payload on insertion and rendezvous. I managed to launch into a really nice position, but the heavy fuel delivery was more than I'd bargained for. I haven't unlocked the really heavy rocketry node yet (the one with the huge orange 2.5m tank) but I have a bunch of KW parts that I've been using for 2.5m.

I guess what I'm really asking is, is there a trick to launching really heavy payloads like reactors and etc, or do I just have to wait to unlock larger engines?

Edited February 26, 2014 by Khatharr

[helaeon]

You can use cubic octagonal struts to attach lots of engines to those tank bases to get you more thrust (some take this to an extreme and use piles and piles of super efficient engines rather than 1 powerful but not so efficient engine).

I find launching heavy things by using solid rocket boosters in the initial stage helps a lot. They have a very good thrust to weight ratio and can save liquid fuel in your asparagus set up by letting you throttle down (But the SRBs are still going at the power you set them at in the VAB).

Pay attention to the efficiency of whatever engine you use as well. Some make awesome upper stages but suck lower.

Generally my rocket design has a core engine that will have a TWR of 1.1 when it's by itself after losing the initial boosters. then I have 4 to 6 liquid boosters set to drop 2 at a time in an asparagus fashion. Then I may have solid boosters on those boosters either set to drop with that tank or in another separate staging event (timing those through setting thrust in the VAB can be tricky so the booster runs out first, drops, then the liquid tank, then the next SRB, and so on or whatever seems to work best). I want to have my TWR be 1.6-2 as long as possible until I hit those upper stages. Once out of orbit ISP is what matters but I like to have a TWR over about .3 (personal preference).

I've been messing with drop tanks rather than drop boosters. I save quite a bit of weight on engines with that and you can use lots of smaller tanks and drop often though if you get to crazy with that you have lots of mass in decouplers.

What you're running into mostly is rocket equation problems. Heavy things are expensive and difficult to move around. The best space craft is a light one and the bulk of your mass is always fuel. Figure out what your deltaV requirements are for each stage of the flight and that makes things much, much easier to know if you need to add more boosters/fuel or if you can keep it light. Always keep it light and ditch excess weight whenever you can.

Pilots generally have 10-15% more fuel than they think they will need, seems like a good plan to me. For example I usually try to make 5km/s deltaV rockets for launch to orbit around Kerbin.

[Khatharr]

This is the lower assembly I've been using for 1.25m (except for the capsule and MJ, which I just stuck on there so the thing is visible and has stats showing). Up until now it's been getting whatever I put on it into orbit and still having a huge amount of fuel to help shave dV off transfers.

Some of my concerns with the 2.5m parts is that they're really quite cramped, and they like to crumble to pieces if there's not a million struts all over the thing. The larger engines have a lot more thrust, but their isp tends to be lower, so I end up carrying more fuel, which harms the TWR. Because of the cramping, I also tend to end up with fewer thrusters as well. When I asparagus a 2.5m, should I use 1.25m for the side-tanks? Using the big radial decoupler I can fit on a ring of 2.5m tanks, but the thing is so fragile that I'm sort of afraid of going beyond that. I managed to get some big SRBs on the one I used to send up the reactor stuff, but I had to make several attempts to get that thing into orbit. Anything less than a perfect g-turn was a disaster in terms of gravity losses, and it was in dire straits when it reached Minmus. Fortunately, the lander stage was able to compensate with RCS, but that made the actual landing something of an event.

I thought about the cubic octagonal strut thing, but I declined it for two reasons: firstly, it feels a little cheaty when there's radial engines; secondly, I haven't unlocked them yet. I think I'll probably look into the radials next and see if I can get better performance that way, but I'll also try using some 1.25m side-stages with a more complex asparagus, since now that I'm thinking about it, I could probably get a much better initial TWR, which seemed to be the root problem with the heavy loads.

I guess, could someone show me an example of a good 2.5m rocket for delivering heavy loads? The biggest one that I'm interested in is delivering a big LFO payload to diverse places inside Kerbin SOI. I think if I can get the hang of that, I'll be able to fiddle from there until I don't need to launch refueling missions anymore. (Until I get to 3.75m -.-...)

Edited February 26, 2014 by Khatharr

[Pecan]

...could someone show me an example of a good 2.5m rocket for delivering heavy loads? The biggest one...

Well yes, lots of people can show you their heavy lifters, but is that really what you want to see? Size (1.25m/2.5m) is not important - what mass are you trying to launch?

[SRV Ron]

Well yes, lots of people can show you their heavy lifters, but is that really what you want to see? Size (1.25m/2.5m) is not important - what mass are you trying to launch?

The payload being launch would determine the lifter. Still, in all cases, stock or mod, Asparagus with booster ring seems to be the most efficient and reliable.

Stock fuel can.

NovaPunch Jool launcher.

Over 100 tons in orbit

[Pecan]

The payload being launch would determine the lifter. Still, in all cases, stock or mod, Asparagus with booster ring seems to be the most efficient and reliable....

Absolutely, and I like your ships. Just didn't want to overwhelm the guy with loads of 100t+ monsters if what he's trying to launch is in the 20-50t range, or whatever.

[Khatharr]

That orange Rockomax looks like a good place to start. I'll try replicating that with the parts I have unlocked and see where I can go with it.

Thank you everyone.

[Pecan]

An orange tube is 36t, which is usually quite a step-up in payload for people who have been doing (~5t?) Minmus/Mun landers with a combined launch-mass vehicle of that size ;-0

I spent ages thinking I'd never be able to get that sort of load off the ground ... and then I did, and now I do. Struts are your friends above ~20t but if you find you're having to put more than one, top-and-bottom, of each booster add a reaction wheel (or two). Then add more struts if you need to. Test, repeat, ...

As I found (to my amazement and dismay, when I couldn't shift 40t) 'heavy' here tends to mean 100t+. Unless you're talking to Whackjob of course (he laughs at such pocket-lint).

Edited February 27, 2014 by Pecan

[SRV Ron]

The Rocomax fuel can is designed for recovery of both the booster and empty can. With a little extra push, it can make Mun high orbit.

BTY, the 100+ton payload was launched with a 850 ton launcher.

Edited February 27, 2014 by SRV Ron

[Khatharr]

I managed to get something similar to the orange can example up there, and it refueled my ISRU delivery perfectly. I took the ISRU to Minmus, missed the reactor by about a kilometer because of derp. I figured I'd just try to scoot it over there with RCS (it had significant RCS thrust for landing without LFO), but first I got out to lock the actual ISRU arms (infernal robotics) into place with some struts (KAS). The Kraken objected to this, apparently. The entire vessel immediately disintegrated and Jeb was literally hurled into orbit. I used his suit RCS to stabilize the orbit, then rebuilt the whole ISRU mission using a modified version of the refueling mission (radial engines added to the asparagus stages helped it a lot). It turned out to be a much better payload as well. I managed to put that one down just 30m from the reactor, scooted it over with RCS and a little LFO, and got the thing up and running, pulling uranium and water from Minmus.

GREAT SUCCESS

Now that the test mission is over I'm refactoring the whole design and hopefully I'll be able to deliver the whole refueling station in a single launch.

Thanks again, everyone.

[Northstar1989]

I want to have my TWR be 1.6-2 as long as possible until I hit those upper stages.

Actually, and I say this more for the OP's sake than anyone else, that's a *TERRIBLE* TWR to have.

You ALMOST NEVER want to go *that low* on your TWR. The penalty for doing so, in terms of Delta-V to orbit, is *much* higher than the drag penalty for going up at a TWR that's a bit too high...

I actually wrote quite a bit on this once in an early thread, but the ideal TWR isn't 2 all the way up either...

You want a TWR that is AS HIGH AS SANELY POSSIBLE until you almost reach terminal velocity from liftoff (this doesn't take very long to reach, and stock SRB's last quite a long time- so it's actually best achieved as a compromise, with a liftoff TWR of between 2.2 and 2.4), followed by a TWR a little bit *below* 2 for a period of time as you climb straight up, as any steering losses or tail fins on your rocket will reduce ideal TWR.

TWR should continue to rise again as you gain speed after dumping your initial SRB's (use drop tanks that were attached to the SRB's to accelerate the rise of TWR if possible- a full rocket gains TWR proportionally faster than one with extra tank mass...)

Once you start your gravity turn (which should be started low, and be carried out gently/slowly), ideal TWR begins to drop off again as your path length through the atmosphere lengthens, and your climb rate begins to eventually fall off a bit- but then rises again as the atmosphere becomes thin enough that these factors cease to matter as terminal velocity becomes impossibly high (by a bit over 35,000 meters, terminal velocity is, in fact, higher than orbital velocity at that altitude).

If, AT ANY POINT, your rely on active turbojet or RAPIER engines in atmospheric mode in any part of your launch platform, you ideal TWR goes *DOWN*. This is because these engines have a velocity curve- which means thrust and ISP both decrease with increasing speed- and the air intakes necessary to use them also generate a LOT of drag at high speeds... (their drag coefficient increases with speed, up to a maximum value of 3.0)

If you use ion or NERVA engines (or their more realistic KSP-Interstellar equivalents: called "plasma thrusters" and "thermal rockets", but which are actually the same technology in real life, but more realsitically/accurately balanced in-game) in-atmosphere in order to bring up your rocket's TWR during ascent (by utilizing upper-stage engines early- which is actually a good strategy for decreasing the fuel requirements and increasing the payload fraction of any launch...), then your ideal TWR goes UP, as these engines increase in ISP *very rapidly* as the atmosphere around them thins...

One last thing- and this isn't about rocket TWR. I noticed your design has a *LOT* of Monopropellant aboard... This is generally one of the most common mistakes among KSP players- especially new players (I used to do it all the time myself).

You don't need that much Monopropellent for your missions normally. Ideally, you want your Monopropellent supply to run out just as you are finishing the mission's very last RCS maneuver. I'm guessing you have 7, 8, or even 9 times more RCS fuel than you need in that rocket, unless you are *VERY* inefficient with your use of RCS... You can save a LOT of extra mass by cutting down on RCS- generally a couple radial tanks will suffice.

Also, you don't necessarily need any RCS at all on most rockets- even if it is going to dock with a fuel tanker (I prefer use of KAS instead, as I can almost always get close enough with tiny rocket engine maneuvers alone to use the winches for fuel transfer- eliminating my need for RCS thrusters and Monopropellent entirely...)

It is almost always more advantageous and fuel-efficient to dock the lighter craft with the heavier craft during docking- and in 90% of cases your fuel tanker should be the lighter craft of the two... (it should contain, at most, just enough fuel to refill your mission vessel, plus a tiny bit of extra fuel to de-orbit it so you don't leave debris...) Your tanker should be much lighter due to its lack of any payload, so it should do all the maneuvering to get into position for docking, while your mission vessel remains entirely motionless (other than possibly some gently torquing of your mission vessel with SAS wheels...)

Regards,

Northstar

P.S. I'm glad some of these players were already to help you get your payloads to orbit. Always remember to give "Reputation" points to players who gave answers to your questions you found helpful... (when they took the time to answer your "Question" thread) You can do this with the star-shaped button in the lower-left corner of a player's post...

Edited May 18, 2014 by Northstar1989

[Francesco]

Once you start your gravity turn (which should be started low, and be carried out gently/slowly), ideal TWR begins to drop off again as your path length through the atmosphere lengthens, and your climb rate begins to eventually fall off a bit- but then rises again as the atmosphere becomes thin enough that these factors cease to matter as terminal velocity becomes impossibly high (by a bit over 35,000 meters, terminal velocity is, in fact, higher than orbital velocity at that altitude).

To me, it's actually the opposite:

the typical player uses a flight profile where you pitch down to 45° once you reach 10km, therefore, you want a higher TWR at that point since you now have the horizontal element of gravity dragging you down.

(By the way, I think generally you should start the gravity turn sooner, at 8km, and make it more gradual e.g. not pitching down to 45° in a matter of seconds.)

Once you've expended around 3500 m/s of delta-v and your apoapsis is at 55-60 km, you can use an upper stage with a very low twr of 1 or less, slowly gaining horizontal speed while keeping a 10° rate of vertical ascent.

Having a too high TWR in the final part of the ascent likely means that you'll set your apoapsis above the atmosphere too soon, and you'll have to cut the engines with still a lot (1000 m/s+) of speed left to reach orbital velocity.

[Jouni]

You ALMOST NEVER want to go *that low* on your TWR. The penalty for doing so, in terms of Delta-V to orbit, is *much* higher than the drag penalty for going up at a TWR that's a bit too high...

That's an exaggeration. The effect of TWR and ascent path on the delta-v required to reach orbit is negligible. It takes deliberate effort to build a rocket that requires 5000 m/s to reach orbit, and even that's pretty close to optimal.

Besides, minimizing delta-v usage to orbit is a bad idea. If that's your goal, you obviously want to use inefficient engines in the upper stage. The faster you burn fuel, the faster your TWR rises, and the less delta-v you lose to gravity in the upper atmosphere. Minimizing fuel usage, maximizing the payload fraction, and maximizing the payload for a given set of engines are better goals, because you don't have to make the rocket deliberately worse for them.

[TeeGee]

I've lifted even more than this... I placed my Duna refuel station into orbit like this, fully fueled and ready to go.

[Northstar1989]

the typical player uses a flight profile where you pitch down to 45° once you reach 10km, therefore, you want a higher TWR at that point since you now have the horizontal element of gravity dragging you down.

I know. And that's not a gravity turn. It's just... stupid.

A *REAL* gravity turn should be begun much more gradually, and increase in small increments as the rocket climbs: i.e. 90 degrees up --> 86 degrees --> 80 degrees --> 72 degrees --> 60 degrees --> 48 degrees --> 34 degrees --> 20 degrees --> 10 degrees --> 2 degrees etc...

Note the DECREASE in the rate of turn at the very end. This should *always* be performed towards to the end to avoid overshooting the horizontal, and aiming too far below the horizon...

(By the way, I think generally you should start the gravity turn sooner, at 8km, and make it more gradual e.g. not pitching down to 45° in a matter of seconds.)

Indeed.

Once you've expended around 3500 m/s of delta-v and your apoapsis is at 55-60 km, you can use an upper stage with a very low twr of 1 or less, slowly gaining horizontal speed while keeping a 10° rate of vertical ascent.

You *could*. But it's more worthwhile at this point to continue using the sustainment engine from the middle stage than to drop that stage... You want to have your upper stage be as light as possible- which means small engines AND fuel tanks...

Most players (including even myself at times) make the mistake of having an under-engineered sustainment stage, and an overly-large upper stage relative to the size of their rocket... The MAJOR exception to this rule being if the upper stage is an orbital tug for something much heavier, or an Asteroid Redirect Mission...

Having a too high TWR in the final part of the ascent likely means that you'll set your apoapsis above the atmosphere too soon, and you'll have to cut the engines with still a lot (1000 m/s+) of speed left to reach orbital velocity.

*OR*, you could just make the final part of the gravity turn quickly and early enough.

Just as most players begin their gravity turn much too late and much too sharply, they also make the final part of their gravity turn much too late and don't level out quickly enough. By about 38000 meters, you should be burning straight horizontally.

It should also be noted that there is often significant advantage in aiming for a higher orbit than 72 km x 72 km or something like that... While it will take more Delta-V *from the launchpad* to reach a higher orbit (say 250 or 350 km), it leaves room for phasing orbits for orbital rendezvous *UNDERNEATH* whatever heavy payload you're launching to orbit (space station, interplanetary mission, etc.), rather than above- which enables fuel tankers to reach your payload without having to waste a large amount of fuel phasing at a higher orbit...

It also takes less Delta-V from the parknig orbit for interplanetary transfers *IF YOU USE A MUNAR GRAVITY ASSIST*, if you depart from a slightly higher orbit... That's because your Delta-V for a transfer hits a "sweet spot" based on the size of the ejection burn. If the ejection burn is shorter (less Delta-V) due to use of a Munar gravity-assist, then your parking orbit should be higher. A higher parknig orbit also safely lets you make an accurate/precise ejection burn with a lower TWR- which allows you to save on engine mass (thus increasing your rocket's Delta-V budget).

Regards,

Northstar

Edited May 16, 2014 by Northstar1989

[Northstar1989]

http://i.imgur.com/1Dgn3jt.png

http://i.imgur.com/nl6yWgW.png

http://i.imgur.com/U3wqTzX.png

I've lifted even more than this... I placed my Duna refuel station into orbit like this, fully fueled and ready to go.

Do you have *ANY IDEA* how expensive a rocket like that would be, in real life?

Jet engines are MUCH more expensive than rocket engines, for ones of equal size and comparable thrust... That's why we tend not to rely on jet engines for rocket ascents in real life- despite the potential fuel savings (spaceplane designs are a different story, since their engines are a fall smaller portion of their total cost- as wings allow them to get by with far lower TWR and cost money in themselves, and they would tend to spend LONG periods of time in-atmosphere before kicking out to orbit, compared to traditional rockets...)

Regards,

Northstar

[Jouni]

A *REAL* gravity turn should be begun much more gradually, and increase in small increments as the rocket climbs: i.e. 90 degrees up --> 86 degrees --> 80 degrees --> 72 degrees --> 60 degrees --> 48 degrees --> 34 degrees --> 20 degrees --> 10 degrees --> 2 degrees etc...

In a real gravity turn, you just head prograde after the initial pitchover, and let gravity do the turning. It's not always the most efficient way to reach orbit, but it tends to minimize the aerodynamic loads.

You *could*. But it's more worthwhile at this point to continue using the sustainment engine from the middle stage than to drop that stage... You want to have your upper stage be as light as possible- which means small engines AND fuel tanks...

Most players (including even myself at times) make the mistake of having an under-engineered sustainment stage, and an overly-large upper stage relative to the size of their rocket... The MAJOR exception to this rule being if the upper stage is an orbital tug for something much heavier, or an Asteroid Redirect Mission...

The proper size of the upper stage depends on many things. One important factor is the availability of engines: we can't just select an upper stage engine and scale it to 2.5 meters and 300 kN, but we have to choose from a limited selection of engines. If the best alternative has less thrust than we would like, it makes sense to make the upper stage smaller and the lower stage larger. With more powerful engines, you may want to make the upper stage larger.

Recently I've been building simple rockets with two vertical stages and two boosters. When the nominal payload is just too small to justify switching to a larger engine, the upper stage is usually in the 800-1000 m/s range. In the next lifter class, the upper stage with a larger engine may produce 1600-1800 m/s.

Jet engines are MUCH more expensive than rocket engines, for ones of equal size and comparable thrust... That's why we tend not to rely on jet engines for rocket ascents in real life- despite the potential fuel savings (spaceplane designs are a different story, since their engines are a fall smaller portion of their total cost- as wings allow them to get by with far lower TWR and cost money in themselves, and they would tend to spend LONG periods of time in-atmosphere before kicking out to orbit, compared to traditional rockets...)

In the real world, jet engines are heavy and weak, while rocket engines are light and powerful. In KSP, jet engines are light and powerful, while rocket engines are heavy and weak. Additionally, KSP jet engines allow flying much faster than real jet engines, while the difference between speeds achievable by jet engines and orbital speeds are much lower than in real life. As a result, if you care about fuel efficiency or similar things, rocket-based first stages are pretty much obsolete for payloads less than 100 tonnes.

[Pecan]

It should also be noted that there is often significant advantage in aiming for a higher orbit than 72 km x 72 km or something like that... While it will take more Delta-V *from the launchpad* to reach a higher orbit (say 250 or 350 km), it leaves room for phasing orbits for orbital rendezvous *UNDERNEATH* whatever heavy payload you're launching to orbit (space station, interplanetary mission, etc.), rather than above- which enables fuel tankers to reach your payload without having to waste a large amount of fuel phasing at a higher orbit...

I think there is much that is arguable in your posts here but for now I'll stick with this one.

Since this is a "heavy equipment" thread I'm assuming that 'this' load is the heavy one and anything else is therefore lighter. It is always preferable to manoeuvre the lightest vehicle(s) during any operation as they will need least fuel for any acceleration, unless they have very inefficient propulsion. As such a heavy load should be dropped as soon as possible - 70-75km and everything else should do the work of getting to it. However, everything depends on how many other vehicles are going to have to go to that extra effort, in which case you might want to put the heavy load into a a higher parking orbit, as you say (generally, I use 250km for exactly that reason), but you don't launch into that orbit!

Every launch (rules are made to be broken) should be circularised at 70-75km, then perform a Hohmann transfer to the final parking orbit. Using this method you're minimising the flight requirements in thick, draggy, atmosphere and maximising them at optimal orbital-adjustment points. Like a lot of things, the difference isn't much with light vehicles over a short distance but can quickly become significant with large loads across large distances.

[Red Iron Crown]

Besides, minimizing delta-v usage to orbit is a bad idea. If that's your goal, you obviously want to use inefficient engines in the upper stage. The faster you burn fuel, the faster your TWR rises, and the less delta-v you lose to gravity in the upper atmosphere. Minimizing fuel usage, maximizing the payload fraction, and maximizing the payload for a given set of engines are better goals, because you don't have to make the rocket deliberately worse for them.

I agree with this. A high-TWR rocket like Northstar1989 describes may use a few dozen less m/s to attain orbit than a lower one, but it will consume more fuel to do so because of its higher dry mass. I've found that an initial TWR between 1.3 and 1.6 maximizes payload fraction and consumes less fuel to reach orbit for a given payload size.

Jet engines are MUCH more expensive than rocket engines, for ones of equal size and comparable thrust... That's why we tend not to rely on jet engines for rocket ascents in real life.

[Citation needed] Jet engines are mass produced, the cost of a jet engine is at least an order of magnitude less than a comparable rocket engine. We don't use jets for getting to orbit because they don't work all that well at the speeds required, and they don't work at all at higher altitudes (can't air hog like in KSP).

[cantab]

It should also be noted that there is often significant advantage in aiming for a higher orbit than 72 km x 72 km or something like that... While it will take more Delta-V *from the launchpad* to reach a higher orbit (say 250 or 350 km), it leaves room for phasing orbits for orbital rendezvous *UNDERNEATH* whatever heavy payload you're launching to orbit (space station, interplanetary mission, etc.), rather than above- which enables fuel tankers to reach your payload without having to waste a large amount of fuel phasing at a higher orbit...

250 km is probably too high, but I do wonder if a very low orbit costs you fuel because you're in the upper atmosphere for long. Establish a 70 km apoapsis and you're experience drag nearly the whole way there; establish a 90 or 100 km apoapsis and you get out of the atmosphere well before it.

In a real gravity turn, you just head prograde after the initial pitchover, and let gravity do the turning. It's not always the most efficient way to reach orbit, but it tends to minimize the aerodynamic loads.

Indeed. This is something I managed once in KSP, with a small probe launched atop a big SRB. No fins, no reaction wheel, canted over five degrees on the pad and it flew pretty much ballistically to an apopasis where the probe circularised.

Since this is a "heavy equipment" thread I'm assuming that 'this' load is the heavy one and anything else is therefore lighter. It is always preferable to manoeuvre the lightest vehicle(s) during any operation as they will need least fuel for any acceleration, unless they have very inefficient propulsion.

Or you're docking two or more similarly heavy things together, which is very likely if you're assembling a big ship in orbit from parts especially if you design the parts to fit a standard launcher's capacity.

Back on to the main topic, in terms of delta-V or fuel usage my standard heavy launcher core is not efficient. And it has a liftoff TWR of something like 1.2. But it's eleven parts in one 3.75 m stack and will get 45 tons into LKO. If I need to lift more I stick two or more cores together - that's why I made it be one stack even though it meant a fuel-spending low launch TWR - and/or add some SRBs.

(From top to bottom: Large ASAS, Rockomax-Kerbodyne adapter, 14400 tank, KR-2L, kerbodyne decoupler, 14400 tank, 14400 tank, cluster engine. Battery and cuboct strut stuck to the ASAS, OKTO2 on the cuboct strut.)

[TeeGee]

Do you have *ANY IDEA* how expensive a rocket like that would be, in real life?

Jet engines are MUCH more expensive than rocket engines, for ones of equal size and comparable thrust... That's why we tend not to rely on jet engines for rocket ascents in real life- despite the potential fuel savings (spaceplane designs are a different story, since their engines are a fall smaller portion of their total cost- as wings allow them to get by with far lower TWR and cost money in themselves, and they would tend to spend LONG periods of time in-atmosphere before kicking out to orbit, compared to traditional rockets...)

Regards,

Northstar

The payload I lifted was almost 200 tons AND the entire lifter was reusable. How much would a rocket based lifter cost to match that payload AND be fully reusable?

[Pecan]

Off topic but .. what mission would you need to lift 200 tons for?

ETA: for RIC's post below - :-) 'what mission benefits from a 200-ton single launch' - And now I ask it properly it answers itself - BIG ROCKETS!

Edited May 16, 2014 by Pecan

[Red Iron Crown]

Need is such a strong word. Big rockets are their own reward.

[cantab]

The more you can launch in one go the less orbital assembly you need to do. Saves you time and can give you a more rigid ship.

[Northstar1989]

The payload I lifted was almost 200 tons AND the entire lifter was reusable. How much would a rocket based lifter cost to match that payload AND be fully reusable?

I see detached boosters flying off in every direction in the final screenshot. How can that possibly be reusable?

Regards,

Northstar