How do you know how many rapiers you need to strap on an SSTO? [rappier:mass question]

[Dr. Kerbal]

How do you know how many rapiers you need to strap on an SSTO? Okay. The reason why I have this question because I'm going to make a very large SSTO to Gilly or Ike (or both). So. What is the rappier to mass thingy? 

[Lt_Duckweed]

25-30 tons is a good starting number, and as you get more skilled at building sstos you can push that higher.

To understand how to build sstos around these numbers, watch this video, it is the best ssto tutorial out there: 

 

If I'm making a "sporty" ssto where I don't care that much about squeezing out every last drop of performance I use about 35-38 tons per rapier.

If I'm going for maximum possible performance I will take off from the KSC flats and use anywhere from 44-52 tons per rapier, depending on the exact mission parameters.

Edited May 4, 2021 by Lt_Duckweed

[Spricigo]

@AeroGavhad a rule of thumbs of one air-breather for 30 ton of craft. Given than people still refer to the plane/spaceplane tutorials  he wrote, I guess it may be a good starting point.

In any case, you will need to design>test>iterate as suggested by the previous answer.

[Incarnation of Chaos]

8 minutes ago, Dr. Kerbal said:

How do you know how many rapiers you need to strap on an SSTO? Okay. The reason why I have this question because I'm going to make a very large SSTO to Gilly or Ike (or both). So. What is the rappier to mass thingy? 

SSTO design is honestly a lot of trial and error.

The first issue is getting something that you can take off, go supersonic, decelerate, and land successfully.

The second issue is then getting that same craft to make orbit, and then have enough DV left over to perform your desired transfer burn OR go to the mun/minmus and mine to refuel.

If the result of solving the second issue is changing the design, then the first issue comes back up....

The third issue is getting this unicorn of a craft to complete reentry gracefully, and maintaining control while doing so.

if the result of solving the third issue is changing the design, you loop back to 1 all over again.

Meanwhile you can take whatever payloads you want, split them up, lob them into orbit and rendezvous with them and send a mothership with a mining lander wherever you want with conventional rockets. This craft can then remain in orbit after the initial mission is complete, and serve as a tour bus or w/e you need it for...while also saving a crap ton of time vs an SSTO.

I'm not saying this to discourage you, just imparting that a SSTO from Kerbin to Duna is going to be a lot more involved than knowing the ratio of X engines for Y amount of tonnage.

[Guest]

On 4/29/2021 at 3:48 AM, Dr. Kerbal said:

How do you know how many rapiers you need to strap on an SSTO? Okay. The reason why I have this question because I'm going to make a very large SSTO to Gilly or Ike (or both). So. What is the rappier to mass thingy? 

Making a spaceplane that takes off from the Kerbin runway and then lands on an airless body is really hard. I've made a couple, and it was one of the most time-consuming, intricate KSP projects I've ever attempted. There are so many requirements it needs to meet that you need to balance against each other.

• It needs aerodynamics to get into LKO and back to Kerbin.
• It needs legs to get to its destination.
• It needs sufficient vacuum TWR to be able to brake to a safe landing speed on an airless body.
• It needs to be able to take off and land vertically in vacuum.
• And it needs to do all this without piling on so much dry mass that it becomes unmanageably huge. 

I've managed this on a craft carrying a crew of three, powered by two RAPIERs and one NERVA, weighing in at just under 30 tons on the runway.

Spoiler

 

FWIW I usually end up with higher RAPIER/mass ratios than the ones listed here on my long-range spaceplanes, which usually have a 2 RAPIER + 1 NERVA combo and weigh in between 25 and 40 tons. In theory they would be able to make do with only one RAPIER, but the problem is that they need a NERVA for deep-space propulsion, and I have yet to figure out a way to place only one of each on a craft in a way that CoT and CoM line up closely enough in each flight mode that thrust torque doesn't cause it to spin out of control.

 

 

 

[fourfa]

On 5/1/2021 at 1:04 AM, Brikoleur said:

Making a spaceplane that takes off from the Kerbin runway and then lands on an airless body is really hard.*

* (unless it's Minmus)

And re: vertical landing, I have seen a number of designs that land horizontally but retrograde (!) on landing gear at fairly high speed, and use both engines and brakes to slow down.  You need a very long flat 'runway' so obviously it's trivial on Minmus, but I've seen examples on both Moho and Tylo (and Duna, which for a lot of spaceplanes might as well be airless).  And you need good scouting from orbit to find the spot.

The horizontal takeoff is easier, with some taxiing around to get pointed at a hill to serve as a launch ramp

[Guest]

I recently explored this question with a craft I was trying to build -- something that gets from KSC runway to Laythe's ocean and back without refueling. I ended up with a spaceplane that weighs a hair short of 50 tons and runs on two RAPIERs, i.e. with 25 tons per RAPIER. It's fairly easy to fly to orbit too, basically just point it up at 5 degrees and wait.

https://kerbalx.com/Brikoleur/Billbo

I tried making bigger ones up to 60 tons or so, but I didn't gain any vacuum dV that was worth mentioning, and of course it was significantly slower and more tedious to get into orbit. I could get to 1500 m/s on airbreathers, but at that point it was going almost straight and level. The single NERV didn't have sufficient thrust, so I needed a significantly longer closed-cycle burn on the RAPIERs to get to orbital speed. That ate up almost all of the extra fuel I was able to carry, leaving me with more dry mass (=empty tanks) to haul to Laythe and back, which wiped out the rest of my gains.

So for a small SSTO running on twin RAPIERs and a single NERV, I think the practical limit is around 25 tons / RAPIER. However, I think I'll continue to use somewhat lower ratios simply because these craft are more enjoyable to operate.

I think 30 tons / RAPIER would work on a bigger craft where you could optimise the aerodynamics better; Billbo has rather more wing than it needs as I used the Big-S ones for their internal fuel storage (and high temperature tolerance), and carries a small underslung payload which introduces some drag too. 

On 4/29/2021 at 8:34 PM, Lt_Duckweed said:

If I'm making a "sporty" ssto where I don't care that much about squeezing out every last drop of performance I use about 35-38 tons per rapier.

If I'm going for maximum possible performance I will take off from the KSC flats and use anywhere from 44-52 tons per rapier, depending on the exact mission parameters.

Do you have links to some of these craft files? I'm really curious to see what a 50 ton/RAPIER craft looks like and what it's like to operate. That's double the ratio I ended up with!

Edited May 8, 2021 by Guest

[Lt_Duckweed]

21 hours ago, Brikoleur said:

I recently explored this question with a craft I was trying to build -- something that gets from KSC runway to Laythe's ocean and back without refueling. I ended up with a spaceplane that weighs a hair short of 50 tons and runs on two RAPIERs, i.e. with 25 tons per RAPIER. It's fairly easy to fly to orbit too, basically just point it up at 5 degrees and wait.

https://kerbalx.com/Brikoleur/Billbo

I tried making bigger ones up to 60 tons or so, but I didn't gain any vacuum dV that was worth mentioning, and of course it was significantly slower and more tedious to get into orbit. I could get to 1500 m/s on airbreathers, but at that point it was going almost straight and level. The single NERV didn't have sufficient thrust, so I needed a significantly longer closed-cycle burn on the RAPIERs to get to orbital speed. That ate up almost all of the extra fuel I was able to carry, leaving me with more dry mass (=empty tanks) to haul to Laythe and back, which wiped out the rest of my gains.

So for a small SSTO running on twin RAPIERs and a single NERV, I think the practical limit is around 25 tons / RAPIER. However, I think I'll continue to use somewhat lower ratios simply because these craft are more enjoyable to operate.

I think 30 tons / RAPIER would work on a bigger craft where you could optimise the aerodynamics better; Billbo has rather more wing than it needs as I used the Big-S ones for their internal fuel storage (and high temperature tolerance), and carries a small underslung payload which introduces some drag too. 

Do you have links to some of these craft files? I'm really curious to see what a 50 ton/RAPIER craft looks like and what it's like to operate. That's double the ratio I ended up with!

I don't have any of them uploaded, but Brad uses a very similar ratio of rapiers and nervs in this video: 

In terms of how it is to fly.... really damn hard.  You accelerate really, really slow, and need quite a high takeoff speed, and once you get in the air, you spend a while with very little excess thrust.

Edited May 9, 2021 by Lt_Duckweed