SSTO's in 0.24

[RidingTheFlow]

Can you land the plane, taxi it a tiny bit off the runway, then not have very far to go with the refueling truck? Or do planes not taxi well on grass?

There are pretty steep slopes around runway. Its possible, but very easy to break something, generally does not make it better than just going to taxiway.

Or do it other way - make supply truck just sit there on the taxiway and going near planes on runway (probably better since it could go faster with electric wheels).

[rkman]

KSP jet engines are actually more like 100x more fuel efficient than rocket engines. The game counts intake air as reaction mass, and as a result, air-breathing engines burn 15-16x less fuel than they should.

Does the game not count oxidizer from fuel tanks as reaction mass? I suppose that explains why fuel consumption of the rapier increases so much when it switches to rocket mode. Always found it weird that it matters so much to that engine where it gets its oxygene from.

At any rate fuel consumption of air breathing engines is much lower than that of rocket engines and they need no oxidizer. The savings of that is not only in the cost of fuel but it significantly reduces the mass of the entire vehicle, meaning fewer, smaller, cheaper parts can be used. Getting a small vessel to ~20km alt that way takes only a few units of fuel, a fraction of what a rocket engine would need.

I think that should be fixed instead of balancing the cost of parts and recovery of rockets vs airbreathing to compensate for the jet engine isp bug.

[LexDavis]

After getting into orbit and coming back down, my space plane was able to be fully refueled and is ready for another flight. the fuel truck probably cost much less than the space plane. if i would have had rover wheels instead of the little engines and landing gear it probably would have cost even less.

[RidingTheFlow]

Fuel truck also could be made very big (e.g. using Kerbodyne tanks), this way it could refuel lot of flights before you have to replace it (and majority of cost will be fuel, so replacement cost will be insignificant in comparison).

[No one]

Does the game not count oxidizer from fuel tanks as reaction mass? I suppose that explains why fuel consumption of the rapier increases so much when it switches to rocket mode. Always found it weird that it matters so much to that engine where it gets its oxygene from.

I'm pretty sure that oxidizer does count as reaction mass. The thing is, you don't have to carry intake air around with you whereas you do have to carry oxidizer around with you.

At any rate fuel consumption of air breathing engines is much lower than that of rocket engines and they need no oxidizer. The savings of that is not only in the cost of fuel but it significantly reduces the mass of the entire vehicle, meaning fewer, smaller, cheaper parts can be used. Getting a small vessel to ~20km alt that way takes only a few units of fuel, a fraction of what a rocket engine would need.

I think that should be fixed instead of balancing the cost of parts and recovery of rockets vs airbreathing to compensate for the jet engine isp bug.

In real life, jet engines have far higher specific impulses than rocket engines.

[magnemoe]

Fuel truck also could be made very big (e.g. using Kerbodyne tanks), this way it could refuel lot of flights before you have to replace it (and majority of cost will be fuel, so replacement cost will be insignificant in comparison).

Has tested a bit with massive Kerbodyne style rovers and the practical limit appearers to be one tank.

Might work with more once you are off the runway or pad, you can use KAS to refuel anyway.

You will refuel truck from static tanks anyway.

[magnemoe]

Tested again and it works, used outriggers with wheels.

5 Kerbodyne tanks, yes it require the large wheels and lots of them, its also uses a lot of power moving.

Main purpose of this would be to refuel larger rocket SSTO.

[LexDavis]

Main purpose of this would be to refuel larger rocket SSTO.

Or the entire planet....

That is massive.

[magnemoe]

Or the entire planet....

That is massive.

Not so very, my standard 18 ton to orbit SSTO, uses 2.5 of these tanks so an 36 ton to orbit launcher would need one of them. That is one orange tank.

[LethalDose]

It could not be as easy as you think. Trying to dock things on wheels is a bit of a pain, since hard to make alignment perfect. I had so much trouble trying assemble bases on ground this way.

To be fair, I think stock KSP should have "refuel" button in addition to "collect" button, and "refuel" should become available when you have craft sitting either on runway or pad.

It's still pretty easy. It just takes a little bit of fiddling to get the tanker's port at the right height.

A "refuel" button or zone would be a better solution, but that depends on SQUAD or a mod implementing it. Trucks are possible in-game with current tech.

[Red Iron Crown]

I would just use the claw on tanker trucks, makes the alignment issue disappear.

[magnemoe]

I would just use the claw on tanker trucks, makes the alignment issue disappear.

True, reloading a payload is far more problematic. You can reload an rocket with the payload at bottom with an truck however.

KAS helps a lot.

[sdj64]

Something like the designs in the Affordable Space Program challenge would work well to get around the recovery loss but I think you'd need a lot of launches to pay off the increased cost of building one of those vehicles the first time. Jets and intakes are significantly more expensive than rockets for the same thrust.

Plus, you can design smart by allowing the top stage of your lifter rocket to be recoverable. A small-payload launcher would be able to use a Kerbodyne SRB for a first stage that costs only a thousand credits.

We might even see the return of the mainsail in .24, because although it's less efficient, it's much less expensive than the Kerbodyne parts. Though costs of parts in .24 might be rebalanced as well.

[Hodo]

Something like the designs in the Affordable Space Program challenge would work well to get around the recovery loss but I think you'd need a lot of launches to pay off the increased cost of building one of those vehicles the first time. Jets and intakes are significantly more expensive than rockets for the same thrust.

Plus, you can design smart by allowing the top stage of your lifter rocket to be recoverable. A small-payload launcher would be able to use a Kerbodyne SRB for a first stage that costs only a thousand credits.

We might even see the return of the mainsail in .24, because although it's less efficient, it's much less expensive than the Kerbodyne parts. Though costs of parts in .24 might be rebalanced as well.

I loved that challenge. It was one of my top 5 favorites.

I love SSTOs, I think they are surprisingly easy in the stock sized Kerbin but they are still fun. As far as cost vs practical, this is KSP, practical smachtical.

[rkman]

In real life, jet engines have far higher specific impulses than rocket engines.

Not so much that jet engines are the preferred way to get (small) things into orbit.

[Jouni]

Does the game not count oxidizer from fuel tanks as reaction mass? I suppose that explains why fuel consumption of the rapier increases so much when it switches to rocket mode. Always found it weird that it matters so much to that engine where it gets its oxygene from.

KSP counts oxidizer as reaction mass, which is correct. A RAPIER in closed-cycle mode burns fuel just like another rocket engine, as it should.

What's broken are the air-breathing engines. The game incorrectly counts intake air as reaction mass, even though you should only include the mass coming from the ship itself in Isp calculations. As a result, RAPIERs (and basic jets and turbojets) burn way too little fuel in air-breathing mode.

Real-world jet engines are not that efficient compared to rocket engines. The Isp figures for typical rocket engines range from 300 s to 450 s, including oxidizer. For supersonic jet engines, typical figures range from 2000 s to 3000 s, not including oxidizer. Basically, jet engines are ~3x more fuel efficient, with another ~2x factor coming from taking the oxidizer from the atmosphere.

If a future spaceplane/aerodynamics update also fixes the jet engines, we can say goodbye to the current spaceplanes with high payload fractions.

[Laie]

If a future spaceplane/aerodynamics update also fixes the jet engines, we can say goodbye to the current spaceplanes with high payload fractions.

I don't think so.

For one thing, Kerbin looks and feels like earth at low altitudes, but is much smaller in any respect. Everything that works earth-like at low altitudes can get halfway to space by momentum alone.

Second, can you imagine the outcry if you took away so many people's most favorite toy?

[Jouni]

For one thing, Kerbin looks and feels like earth at low altitudes, but is much smaller in any respect. Everything that works earth-like at low altitudes can get halfway to space by momentum alone.

Let's assume that Squad makes jet engines more in line with rocket engines when it fixes the aerodynamics. That means something like 15x higher fuel usage and 2x higher mass for the engine-intake combination.

Assuming that we currently need 50-60 units of fuel per turbojet, we'll need about 800 units or 4 tonnes of fuel per engine after the fix. Add 0.5 tonnes for the fuel tank, 2.5 tonnes for the engine-intake combination, and 0.5 tonnes for aerodynamics, and we'll be at 7.5 tonnes. Assuming 50% average thrust and 2500 s Isp, that 4 tonnes of fuel should be enough for around 15 minutes in atmosphere.

A reasonable spaceplane going to LKO might need something like 1000 m/s of delta-v from the rocket stage. With 350 s Isp, the mass fraction of the rocket stage should be about 1.33. Assuming that the jet stage has 0.5 tonnes of fuel remaining when the rockets ignite, the payload for the rocket stage is 4 tonnes + the actual payload. Assuming 1 tonne of dry mass for the rocket stage and 6 tonnes of actual payload, we'll need 3.5 tonnes of fuel for the rocket stage.

Overall, we have a 7.5-tonne jet stage, a 4.5-tonne rocket stage, and 6 tonnes of payload, making the payload fraction around 33%. That's slightly better than what we can achieve in FAR with two boosters, a core stage, and no fuel lines, so it's probably roughly in line with good staged rockets.

The biggest caveat is that the plane weights 18 tonnes, while having only a single turbojet. I haven't tried spaceplanes with FAR, so I'm not sure if that's enough power to make the ascent in time.

Second, can you imagine the outcry if you took away so many people's most favorite toy?

For many people, that's going to happen anyway. The only alternative is that Squad never makes any hard decisions and KSP becomes a game that showed great promise but never delivered it.

[LethalDose]

The only alternative is that Squad never makes any hard decisions and KSP becomes a game that showed great promise but never delivered it.

IMO, they're kinda headed this direction anyway.

We might even see the return of the mainsail in .24, because although it's less efficient, it's much less expensive than the Kerbodyne parts. Though costs of parts in .24 might be rebalanced as well.

HarvesteR's already said he's changed the performance of the Mainsail to be more efficient than the LFB:

<p><strong>Felipe (HarvesteR)</strong><span><strong>:</strong> [...] Several parts have had some stats tweaked, but most importantly, I’ve tweaked the costs and Isp values on the Mainsail and the Kerbodyne LF Booster. The Mainsail is now the more efficient one, so it should be a more appealing option now, when comparing the two. </span></p>

[CaptRobau]

I think the current setup as it's been communicated to us will work pretty well. If you build a disposable rocket and recover just the command pod plus the parachutes then you just some money back for those parts. This way returning is easy, but because you won't get as much money back you'll have to work harder to get the extra cash to do other things. The SSTO on the other hand will get you a lot of money back as you'll get a lot of it back to Kerbin, but the return flight will take extra time and effort. This makes other things easier.

One thing I would like to see is a distance modifier. I want to be rewarded for managing to land right on top of the space center.

[magnemoe]

I think the current setup as it's been communicated to us will work pretty well. If you build a disposable rocket and recover just the command pod plus the parachutes then you just some money back for those parts. This way returning is easy, but because you won't get as much money back you'll have to work harder to get the extra cash to do other things. The SSTO on the other hand will get you a lot of money back as you'll get a lot of it back to Kerbin, but the return flight will take extra time and effort. This makes other things easier.

One thing I would like to see is a distance modifier. I want to be rewarded for managing to land right on top of the space center.

Yes an distance modifier would be nice and pretty realistic still it should not be too large as its not that expensive to move parts with truck or boat.

It all boils down to the recover faction. if its 90% you would rarely bothering refueling with the exception on crew transport.

if its 50% an SSTO don't make sense unless you reload and refuel it.

[magnemoe]

This rocket takes 20 ton to LKO,

Height of cargo is one orange tank, including docking ports, another version can simply increase this length.

Yes an disposable rocket taking 20 ton to LKO would be smaller, the major benefits of most disposable rockets is that you usually don't have to take them to orbit. Most heavy payloads will go other places so simply increase the mass on the transfer stage and you don't need more than 4000 m/s to get payload into orbit,

Say 20 ton including a LV-N, 180 liter extra fuel gives you 700 m/s and mainsail two orange tanks and two of the large SRB gives you 3900 m/s

[CitizenAerospace]

Problem with recovering rockets is that the render distance shuts that idea down pretty quickly. Unless you can multitask like a complete boss, and have wizard powers.

[rkman]

Second, can you imagine the outcry if you took away so many people's most favorite toy?

I hope Squad won't not fix certain bugs just because a part of the community got used to those bugs.

[CaptRobau]

Problem with recovering rockets is that the render distance shuts that idea down pretty quickly. Unless you can multitask like a complete boss, and have wizard powers.

Only for lower stage rockets. The upper stage rocket that helps get your spacecraft into orbit would be quite easy to recover if you stick some landing legs, some extra landing fuel or parachute and a large probe body.