Trying to make a super heavy ssto to mun

[GradientOGames]

I'm quite knowledgeable in rockets, but what's always stumped me, is heavy planes (and fuel cross feeding but that's for another question), whether its space planes, or sstos, they're just so... difficult. I can sort make  mk2 and mk1 planes, but never mk3 style planes. I'm trying to make an ssto to go to the Mun for recovery missions and I want to have at least 20t capacity in its cargo hold. I tried replicating Matt Lowne's ssto  to the Mun but no luck. Here it is:

image

I'll send craft file if necessary.

[Leganeski]

Welcome to the forums, @GradientOGames!

From what I can tell in the picture, the plane needs a lot more wing area. When I built a Mk3 SSTO that was somewhat smaller than your plane, it had five pairs of Big-S Delta Wings, and even then, it could only barely take off from the runway or land on the polar ice caps.

[GradientOGames]

1 hour ago, Leganeski said:

Welcome to the forums, @GradientOGames!

From what I can tell in the picture, the plane needs a lot more wing area. When I built a Mk3 SSTO that was somewhat smaller than your plane, it had five pairs of Big-S Delta Wings, and even then, it could only barely take off from the runway or land on the polar ice caps.

so more wings or bigger wings? Also, I forgot to mention, a iteration of it did manage to get into orbit, although doing so was very hard and took many attempts, getting off the runway isnt the problem, its just delta V and stability. Ill try more wingspan when I get back on.

[Leganeski]

1 hour ago, GradientOGames said:

more wings or bigger wings

Either. You're already using large wings, though, so more wings would be easier to practically accomplish.

1 hour ago, GradientOGames said:

delta V

This is a tough one. Spaceplanes have an inherent tradeoff between Δv and flyability, although limiting the amount of oxidizer definitely helps.

1 hour ago, GradientOGames said:

stability

Stability issues are caused by improperly positioned centers of lift and drag relative to the center of mass. These centers are not visible in your picture, but make sure that your center of lift is just behind your center of mass, not only when the fuel tanks are full but also when you've used up some liquid fuel to power the jet engines. I've heard good things about TAC Fuel Balancer for that purpose, although I haven't tried it personally.

[Vanamonde]

Your choice of engines is a big part of the problem. What a spaceplane needs is lift at low altitudes but thrust to achieve orbit speed once it's climbed high into the atmosphere, and the LV-N engines will help with neither. They are very heavy and have relatively low thrust, so their advantage is fuel economy for very long trips, which is not what you need. The Rapiers are actually made for spaceplanes because they will switch from air-breathing to rocket so the one kind of engine will do both jobs, meaning that you don't need to combine 2 engine types in the same plane. That's actually good, but then if you do combine them with another type (as your current ship does), you are defeating the purpose of using Rapiers in the first place. My advice to you would be to either replace the LV-Ns with more Rapiers, or use a combination of normal jet engines during climb and then switch to a normal rocket to go from upper atmo to orbit. 

Good luck.  

[GradientOGames]

1 hour ago, Vanamonde said:

Your choice of engines is a big part of the problem. What a spaceplane needs is lift at low altitudes but thrust to achieve orbit speed once it's climbed high into the atmosphere, and the LV-N engines will help with neither. They are very heavy and have relatively low thrust, so their advantage is fuel economy for very long trips, which is not what you need. The Rapiers are actually made for spaceplanes because they will switch from air-breathing to rocket so the one kind of engine will do both jobs, meaning that you don't need to combine 2 engine types in the same plane. That's actually good, but then if you do combine them with another type (as your current ship does), you are defeating the purpose of using Rapiers in the first place. My advice to you would be to either replace the LV-Ns with more Rapiers, or use a combination of normal jet engines during climb and then switch to a normal rocket to go from upper atmo to orbit. 

Good luck.  

so I should switch my liquid fuel tanks into rocket fuel tanks?

[GradientOGames]

18 hours ago, Leganeski said:

Stability issues are caused by improperly positioned centers of lift and drag relative to the center of mass. These centers are not visible in your picture, but make sure that your center of lift is just behind your center of mass, not only when the fuel tanks are full but also when you've used up some liquid fuel to power the jet engines. I've heard good things about TAC Fuel Balancer for that purpose, although I haven't tried it personally.

I've done my research a while ago about COL and COM, for rockets its easy, have the col below the com. For smaller planes, I always have it just a bit behind the com. But for a super heavy plane, how far behind do I put the col. I also heard about having the sol slightly below the com, should I do that?

[GradientOGames]

update: I've tried switching the engines all to rapiers, haven't added more wings yet, but the plane is incredible stable now for some reason. however the stability doesn't last for long, as soon as I reach 400-500 metres per second, the plane flips. This shouldn't  be happening, my col is well behind the com. I've read about the col going closer forward while reaching super sonic speed, but I thought this amount would be enough. 

image

[Vanamonde]

Fuel tanks are a whole additional question. LV-Ns use only fuel and no oxidizer, while most KSP fuel tanks include both. If you use most normal tanks with LV-Ns you will be carrying around dead weight in the form of unused oxidizer. (I know you removed those. I'm just saying. ) 

For the Rapiers, they use only fuel in jet mode then switch to using fuel and oxidizer in rocket mode. Balancing the two can be tricky. Although I don't generally use mods, for this reason I do use one to change the contents of the tanks when I am trying to build spaceplanes. 

Okay, as for stability. As you go higher and the air gets thinner, fins and control surfaces have less effect. I would suggest moving the canards (forward wings) farther toward the front where they will apply more leverage. Another issue could be that the center of balance is moving as you use up fuel. That is very hard to account for,  but in the VAB you can r-click on the fuel tanks to change the amount they are holding to see how much that causes the balance to change. 

If this seems like a lot of different things to keep in mind and work around, that's because it is. Space planes are possibly the hardest things to make in KSP because of all the different factors you have to account for, including engine types, fuel types, balance while full, balance while empty, lift, thrust, and more. By the way, is there anything in the cargo bay while you're making these test flights? If you're using a fuel tank as a test payload, remember that the engines will suck fuel from it, changing the balance of the craft, unless you do something to stop that. 

[18Watt]

6 hours ago, Vanamonde said:

the LV-N engines will help with neither.

Amen.  Even one Nuke engine on a spaceplane is a waste.  Let alone 6 or more of them.  The first thing I would do to improve your spaceplane is get rid of ALL the nuke engines.

[Lt_Duckweed]

1 hour ago, 18Watt said:

Amen.  Even one Nuke engine on a spaceplane is a waste.  Let alone 6 or more of them.  The first thing I would do to improve your spaceplane is get rid of ALL the nuke engines.

This is incredibly false.  Like, bafflingly so.

The most efficient spaceplanes for LKO all use only liquid fuel, swapping directly from airbreathing rapier to nerv powered flight.  While that is a bit beyond the current skill level of OP, having a few nerv engines to help reduce the amount of oxidizer they use on ascent and circularization, and more importantly, for the burn to the Mun, and as much of the Mun landing, and later ascent from the Mun and return to Kerbin, will vastly reduce the amount of fuel they need to carry.

The number of nervs they have right now is overkill, but so is the number of Rapiers.

@OP, I highly recommend watching this video to get a decent handle on how to build large planes: 

Once you can successfully get a large plane like this to orbit with the engine ratios mentioned in the video, then you can work on extending its range to the Mun (just think of the nervs and extra fuel needed as payload that you need to get to LKO first)

Edited November 27, 2022 by Lt_Duckweed

[18Watt]

4 minutes ago, Lt_Duckweed said:

The most efficient spaceplanes for LKO all use only liquid fuel

Well, every plane in the video you linked uses oxidizer.  And exactly zero nuke engines.  

[Lt_Duckweed]

Just now, 18Watt said:

Well, every plane in the video you linked uses oxidizer.  And exactly zero nuke engines.  

That's is because it is a basic advanced spaceplane tutorial.  It's intended to be as simple as possible.  But the craft in the tutorial could be even more efficient if it used nerv engines and didn't have oxidizer.

All of the following SSTOs use no oxidizer at all:

This one does use ox, but only on a technicality (it has to use a small amount to power the fuel cell)

 

[miklkit]

20 tons is a lot of cargo.  What are you trying to do?  Go to the Mun to recover ore and bring it back to Kerbin?  The hardest part of that is landing as it needs a really soft landing or the ore tanks tear loose and all the ore disappears.  Lots and lots of wing helps there.  I am using triplanes now.  

  An ore carrier is actually easy to do because it takes off empty so it pretty light.  This saves on fuel too.  Do you have a refueling solution on the Mun?  That will help too.  Also, getting the fuel tanks laid out so the CoM stays pretty stationary is very important.  The cargo bay should be exactly at the CoM too.  I never get it correct so my MK3s take off very nose heavy and land a little tail heavy.

[camacju]

One thing that I've noticed is that for both Rapier-only ssto craft and Rapier+Nerv craft, I use approximately the same amount of liquid fuel in both the jet and rocket phases. For a Rapier-only craft, that means I consume oxidizer during the rocket phase which eats into payload mass, but also I have a lower dry mass which means more payload mass.

However, the additional dry mass of the nuclear engines is more than compensated by the reduced fuel consumption. This can be seen intuitively, because the mass of oxidizer used is nearly always going to be less than the mass of the engines. (Otherwise you've got way too much engine for your craft's mass).

We can use an example craft to demonstrate:

This plane is 109.5 tons on the runway, and is powered by three Rapier and two Nerv engines.

It has 3800 liquid fuel, nearly all of which is used to place 75.1 tons of ore tanks (can be replaced with other payload) in low Kerbin orbit. Based on the rule of thumb outlined above, this means it uses 1900 liquid fuel in the jet ascent phase and 1900 liquid fuel in the rocket phase.

Basically, the plane itself is 34.4 tons, the payload is 75.1 tons, and this is just about the maximum mass that can take off from the runway for this number of Rapier engines.

Now we can consider the effect of removing the Nerv engines, which are three tons each. This reduces the plane's mass to 28.4 tons. However, now that the 1900 liquid fuel in rocket phase must be combined with oxidizer, we must add 2322 units of oxidizer to the plane, or 11611 kg. And that's not even counting the dry mass of rocket fuel tanks, which is worse than the Mk0 liquid fuel tank.

This extra fuel tank mass will eat into payload fraction and ultimately reduce the capability of this SSTO.

Note: This analysis is purely for payload capacity to low Kerbin orbit. If you plan to go further than this, then Nerv engines are advised, and you might as well use them on the ascent and save on the 11.6 tons of oxidizer.

[GradientOGames]

UPDATEE WOOOOO: I managed to get the ssto into orbit with 1500 delta v remaining! However that's not enough to go to the moon and back, let alone the amount of delta V spent on going to the location to recover Jeb and Val. My new question for ya all:

How to get more delta V without getting rid of stability.

Newest iteration

[Vanamonde]

You have now exceeded the performance of my best ever spaceplane. I have nothing more to teach you. Go forth and prosper. 

[miklkit]

You still have a lot of work left to do.  I still don't think you can go to the Mun and back with Rapiers only.  This one did go to the Mun and Minmus, complete repair rover contracts, and return. 

[Leganeski]

9 hours ago, miklkit said:

This one did go to the Mun and Minmus, complete repair rover contracts, and return.

In case it's hard to tell from the picture, that spaceplane has five NERVs (with their shrouds still attached).

 

20 hours ago, GradientOGames said:

UPDATEE WOOOOO: I managed to get the ssto into orbit with 1500 delta v remaining! However that's not enough to go to the moon and back, let alone the amount of delta V spent on going to the location to recover Jeb and Val. My new question for ya all:

How to get more delta V without getting rid of stability.

Newest iteration

To add to what @camacju and @Lt_Duckweed have already explained: from the picture, it looks like you're getting the 1500 m/s remaining Δv figure in low orbit from closed-cycle RAPIERs alone. If you managed to get the same payload fraction with a NERV vacuum stage instead, you would have over 4000 m/s from low orbit. Of course, the increased dry mass means that you wouldn't actually get as much payload fraction, but it would most likely still be a large increase in Δv.

In terms of stability, replacing three RAPIERs with two NERVs won't really change much.

[miklkit]

Yes.  I found that a light MK3 can land on the Mun with 4 NERVs but the heavier ones need 5.  Rapiers use too much fuel and are hard to control. 

[GradientOGames]

Another update: Sorry I took so long, I've just attached 4 nerves on sides (might put 1 more in centre replacing rapier) and I can get to orbit with rapiers. Once I'm in orbit, I switch to my nervs only via an action group and that leaves me with 2000 approx delta V remaining. After checking my delta V map, I can confidently say...

Not 

enough

delta

V.

But I was thinking, what if I used the nerves to not only get to the mun, but also help me get into orbit,

On 11/29/2022 at 4:13 PM, Leganeski said:

To add to what @camacju and @Lt_Duckweed have already explained: from the picture, it looks like you're getting the 1500 m/s remaining Δv figure in low orbit from closed-cycle RAPIERs alone. If you managed to get the same payload fraction with a NERV vacuum stage instead, you would have over 4000 m/s from low orbit. Of course, the increased dry mass means that you wouldn't actually get as much payload fraction, but it would most likely still be a large increase in Δv.

I'm going to try it, but if it doesn't work out, I need more delta V somehow. Any pro tips?

 

Edit: forgot to mention, my 4 nervs is only getting about 2mps, is that good for landing or should I get more nervs?

Edited December 3, 2022 by GradientOGames
forgot to mention something

[Leganeski]

1 hour ago, GradientOGames said:

But I was thinking, what if I used the nerves to not only get to the mun, but also help me get into orbit,

Yes, this is critical for saving Δv because it lets you reduce the amount of oxidizer. Every kilogram of oxidizer (and oxidizer storage!) is wasted mass unless you absolutely need it to get into orbit.

1 hour ago, GradientOGames said:

Edit: forgot to mention, my 4 nervs is only getting about 2mps, is that good for landing or should I get more nervs?

At 2.0 m/s², a Mun landing is very difficult but not impossible. However, your TWR will increase by 25% or more from LKO to the final landing burn, and even at 2.5 m/s², landing is much more manageable. It's still not easy to do with a plane, though, and a fifth NERV would make landing much easier at the cost of some Δv.

Also, if you manage to reduce the amount of oxidizer storage by using NERVs for more of the ascent, that would improve TWR as well.

[GradientOGames]

37 minutes ago, Leganeski said:

Yes, this is critical for saving Δv because it lets you reduce the amount of oxidizer. Every kilogram of oxidizer (and oxidizer storage!) is wasted mass unless you absolutely need it to get into orbit.

At 2.0 m/s², a Mun landing is very difficult but not impossible. However, your TWR will increase by 25% or more from LKO to the final landing burn, and even at 2.5 m/s², landing is much more manageable. It's still not easy to do with a plane, though, and a fifth NERV would make landing much easier at the cost of some Δv.

Also, if you manage to reduce the amount of oxidizer storage by using NERVs for more of the ascent, that would improve TWR as well.

So getting into orbit, I have both rapiers and nervs on at the same time? or nerves alone

[Leganeski]

3 minutes ago, GradientOGames said:

So getting into orbit, I have both rapiers and nervs on at the same time? or nerves alone

Start with both, but turn off the RAPIERs as soon as the TWR you need to continue circularizing drops below what the NERVs provide.

Optimally, it would be "run out of oxidizer" rather than "turn off the RAPIERs", but it takes quite a bit of testing to get that to happen at the right time.

[GradientOGames]

10 minutes ago, Leganeski said:

Start with both, but turn off the RAPIERs as soon as the TWR you need to continue circularizing drops below what the NERVs provide.

Optimally, it would be "run out of oxidizer" rather than "turn off the RAPIERs", but it takes quite a bit of testing to get that to happen at the right time.

so use nerves during launch, or at like, 27k altitude?