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SSTO Help


The Interplanetary SSTO Rapier Dilemma  

7 members have voted

  1. 1. More Rapiers, More Fuel, Less Rapiers, less fuel

    • More Rapiers, Less Fuel
      0
    • Less Rapiers, More Fuel
      4
    • More Fuel, More Rapiers
      3


Question

Dear Everyone,

On the topic of SSTOs does more rapiers help provide more delta V? Cause more rapiers the faster you travel through the atmosphere and the more speed you can gain in the upper atmosphere, but then again if you are making that SSTO a interplanetary craft the more rapiers the more wasted delta V you will end up having, but then again the less rapiers the more fuel you have to burn at sea level to gain speed to climb effectively! So which is better, more fuel less rapiers or less fuel more Rapiers OR more fuel more rapiers, I hope this question can help me and possibly others in the future.

Sincerely, Mk3 Maniac:wink:

(P.S. if you chose to do the poll keep in mind that I look for which one gives the most Delta V>)

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I prefer Rapier + Nerv for maximum dV. You can use the airbreathing mode longer, and carrying a bit of oxidizer lets you use the rocket mode of the Rapier to supplement the nuclear thrust during ascent. 

More generally: Adding engines usually decreases delta-v, as they are dry mass. You want just enough engine to do the job and no more. 

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I generally go with one jet engine per 30 tons of gross takeoff weight, a mix of Whiplash and Rapier .   The Whiplash fatten out the bottom end of the power curve making it easier to get supersonic.  The max top end speed you can get is soft capped by the engine velocity curve , meaning that adding more engines to get faster than 1400 m/s air breathing runs into diminishing returns hard

400px-CR-7_R.A.P.I.E.R._Engine_velocity_

If your craft does not have any part attachment issues causing excess drag you should be able to get through the sound barrier easily.  Generally I use up to 20% of my fuel getting to air breathing top speed.       From 1400m/s,   you need another 900m/s velocity to make low orbit.   My designs have a low thrust-weight ratio so gravity/drag losses mean my delta V (as reported by Kerbal Engineer System) has fallen by  up to 1200 or so by the time we reach orbit, but you shouldn't see losses worse than that.

Nukes,  I generally bring one per 15 tons if I'm making an oxidizer free ship.  A minimal amount of oxidizer allows you to halve that number of nuke engines.

Wings - I prefer adding more wing to my designs, but a lot of it is down to individual preference.

After takeoff,  I prefer to climb to a higher altitude where drag is less, before levelling off and attempting to penetrate the sound barrier.   However, as the air gets thinner you need more wing to support the craft at the comparatively low speed, without having to yank the nose more than 5 degrees above the prograde vector, which generates drag.     At a minimum, you want enough wing area that you can fly around at 7km altitude , subsonic (under 250m/s) without having to yank the nose more than 5 degrees above prograde.

I did try the other method of getting supersonic, by just doing it at sea level , flying level after takeoff till over 440 m/s with tiny sub wings.   But I found this used a lot of fuel,  drag is huge at zero altitude, it's partly disguised by the massive amount of power your engines have down there but the fuel flow rates are scary.

Liquid fuel only SSTO need even more wing area in my opinion,  motor-glider levels of lift:drag ratio help a lot when getting a  craft to orbit with relatively low thrust  

(this is a low tech panther/nerv ssto that can go to the surface fo the mun and back)

sOK1Q7w.jpg

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KIckback+Dawn, single stage to orbit , multi stage from orbit. A 'small inconvenience' its that it seriously limits where you can land (specially if you want return from where you landed).

"What give most deltaV? " Is a question that misses several points:

1st: deltaV you don't have a use for is just dead-weight you carry around needless. It hinders you: maneuvers takes more time,  you have less control and so on.

2nd: What is efficient in a situation can be very inefficient in another. You can have a lot of deltaV and still be unable to reach several destinations. Its also worthless to have a huge deltaV to waste everything landing(/lifting) with pitiful TWR in a high gravity body (e.g. Tylo)

3rd:It worth nothing to have a theoretical higher deltaV if in pratice you lose that to steering and gravity. It make you no good to have X% more fuel and consume (X+Y)%.

4th:deltaV budget alone will not define what a craft can do and that is what really matter. A small science probe  and a large tourist transport may both have the same deltaV butt their uses are enterily different.

 

to sum up:

1.Decide what you want to do (e.g. explore Laythe).

2.Define the phases required for the mission (e.g 1 Kerbin Orbit>transfer to Jool> capture at Laythe> Land at Spot A > Land at Spot B > ...)

3.Design the vessel (or vessels)  for the mission. Consider what is needed for each part and try to do it with the least expenditure of resources.

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22 hours ago, Red Iron Crown said:

I prefer Rapier + Nerv for maximum dV. You can use the airbreathing mode longer, and carrying a bit of oxidizer lets you use the rocket mode of the Rapier to supplement the nuclear thrust during ascent. 

More generally: Adding engines usually decreases delta-v, as they are dry mass. You want just enough engine to do the job and no more. 

Too add to this, most space plane designs I build incorporate a tank that carries oxidizer.  So more reason to use rapier to take advantage of all fuel stores.

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19 hours ago, ForScience6686 said:

Too add to this, most space plane designs I build incorporate a tank that carries oxidizer.  So more reason to use rapier to take advantage of all fuel stores.

Likewise. The oxidiser capacity of a Mk2 dual engine mount (and sometimes a Mk2 -> 1.25m piece for the nose) is usually just enough to boost the apoapsis up to the point where the nukes can finish the job at zero AoA and low drag. Nuke + RAPIER with minimal oxidiser makes a very nice synergistic combo.

The oxidiser comes in handy for Vernor VTOL setups, too; very useful for thin- or no-atmosphere landing.

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In my experience, any gains from using the Whiplash over the Rapier is cancelled out by the inferior falloff of thrust at high speeds by the Whiplash, exactly when you need to get the most speed out of your air-breathing engines. Additionally, having more engines (Nerva/Whiplash) means more frontal area, which increases drag and degrades overall performance. A very aerodynamically slick vehicle with Rapiers can perform very well, especially since drag losses are minimized in the upper atmosphere, where you must use rockets.

 

I've never settled on a liquid-fuel only design that is aerodynamic, practical, and has sufficient thrust throughout the entire flight regime while carrying any appreciable payload. Better to lift your space-optimized vessel into orbit with the SSTO and have the same SSTO grab returning missions for a safe landing with 100% returns.

Edited by Raideur Ng
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