[0.23] Tangent Rockets 16% Payload Fraction Lifters

[mhoram]

Version 0.3 2013-10-22 for Kerbal Space Project V 0.22.

Download on Dropbox https://www.dropbox.com/sh/573rc6a1jtnmkow/jM_6Pq1X8f

For the next generation of Tangent rockets, head over to the new thread.

MISSION STATEMENT

When you launch a rocket into space, your first step is to reach a low

Kerbin orbit and your second step is to bring your payload whereever

it needs to be. So you basically use engines for liftoff and separate

engines for the spacetravel of the payload.

Here is the question: Can these payload-engines be used for the

liftoff from Kerbin? Tangent Rockets answers this question by

providing rockets that have the following properties:

- Bring payload and engines into a 75km Kerbin orbit

- Some nuclear engines stay with the payload that provide a TWR of at

least 0,5

- Payload fraction of at least 16%

- Orbit mass fraction of at least 20% (part of the rocket that stays in

orbit)

- Self sufficient power supply even during night

- Enough RCS fuel to deorbit the final stage of the propellant for a

debris free orbit

- Flat surface on top of the nuclear engines for building the payload

- Support payload that provides no own rotational control

- Stock parts only

- MechJeb compatible

Since in this setup some parts that stay in orbit help during the

liftoff, we need to clarify some terms.

- Propellant: part that does not reach/stay in orbit

- Engine Deck: part that helps during liftoff and stays in orbit

- Payload: part that stays in orbit and does not help during liftoff

The following scetch shows these notions.

DESIGN

The reason for selecting nuclear engines is that they provide the best

fuel usage in vacuum, as they provide more than twice the efficiency

comared with all other engines. A minimum TWR of 0.5 for the maximum

payload was chosen because a significant smaller value has little

impact on liftoff and maneuvers should even in space be doable in a

reasonable amount of time.

75km seem to be a reasonable low kerbin orbit with a bit of margin

above the atmosphere.

Most rockets in this series have a first stage without attached

engine. This is usually a timer for startig the nuclear engines

because their fuel efficiency is incredibly bad at sealevel. So when

starting manually, be aware that there is no audible and visual

notification of the end of the first stage - non-stock-parts can help

with this.

RCS as an emergency deorbiting method for the final propellant stage

was introduced for cases where you push the orbit too far and run out

of fuel, so a debris free orbit is guaranteed.

ROCKETS

Currently this set consists of the Tangent 2, 3, 6, 12 and 18. The

download contains .craft files with predefined payloads and

subassemblies.

The numeral indicates the number of nuclear engines that stay in

orbit.

A rough estimate of the admissible payload is 'numeral * 10'.

So Tangent 18 can lift payloads below 180 ton.

Now for the detailled specs

The rockets were tested with differently formed payloads. The folder

'example-payloads' in the package contains payloads that were

successfully brought to orbit manually and with MechJeb.

USAGE

Here are some general guidelines for the design of the payload

- Keep the payload mass below the maximal admissible payload mass

- Keep it wobbling-free

- Keep it near the engine deck

- Don't put struts from the payload to the propellant, rather connect

it with the engine deck

- Ensure that no fuel from the payload is used until the propellant is

separated.

On manual mode I roughly follow these instructions for the gravity

turn

- start the gravity turn at about 10km heading to east

- as a general rule, always try to keep the orientation near prograde

in the eastern direction

- while the pitch is between 45° and 90°, keep the orientation a

little bit nearer to the planet, and between 0° and 45° a little bit

farer away from the planet.

- when you reach an altitude of 45km switch to map-view and adjust the

pitch to keep the apoapsis about 40-60 seconds away from you

- when the apoapsis reaches 75km, stop the engines

- arriving at the apoapsis burn prograde until the periapsis reaches

75km

Tangent 12:

- Lock Gimbal of nuclear engines (Action Group 9) until they are below

the center of mass.

- Do not wait too long until the liftoff, because the rocket stands a

bit instable and will crash.

ADDONS

If you use this set of rockets with MechJeb 2.0.9, the following

settings were tested successfully with all rockets:

- orbit: 75km, inclination: 0°, Turn start altitude: 10km, Turn end

altitude: 70km, Final flight path angle: 0°, Turn shape: 40%

KNOWN ISSUES

The margin for steering errors is quite small, so keep a steady hand

during the gravity turn and don't overshoot the 75km lko too much.

Tangent 12 needs gimbaling for nuclear engines switched off and a fast

start.

CHANGES

0.1 - Initial Release

0.2 - Added Tangent 12 with a payload fraction of 16,70%

0.3 - Updated all rockets to KSP 0.22:

- the payload fraction dropped a bit

- Reworked Tangent 3 and 6 to be handled by MechJeb without

problems

FUTURE

Tangent 1 forces a very difficult design because the nuclear engine

must be placed in the center for symmetry reasons. Only made it work

during initial tests with a payload fraction < 15% which I am not

happy with. So don't expect many rockets with an odd roman numeral in

the Tangent series - actually Tangent 3 will stay most likely the only

odd numbered rocket in this series.

Tangent 48 will probably never be finished because the part count

exceeds 500 without the first propellant stage and struts. Lag is no

fun!

Tangent 4 and 24 seem to be interesting next targets.

Rework Tangent 6 to replace root piece by 1x1 plate.

A Version of Tangent 18 with an additional engine-free stage would be

nice.

Edited January 25, 2014 by mhoram
Update to V1.0

[mhoram]

There is a new ship in the series: the Tangent XII

Payload capacity: 119 Ton

Payload fraction: 16.70%

Nuclear engines: 12

[mhoram]

Just updated the series to KSP 0.22.

It seems like the physics has changed a bit in this update. I had to redo each rocket and lost a bit of payload fraction in this process.

[GusTurbo]

This is a very creative way of reaching those kinds of mass fractions. Your lifters look like nothing I've seen before on this forum.

[Rune]

I don't want to rain on your parade (I actually like people trying to come up with new ways of doing things) but... isn't this just the same as a parallel staged rocket with crossfeed? In other words, the infamous asparagus staging. All engines lit at all times, and you drop them as you stop needing them. Well, call it asparagus with a late-burning core stage that makes orbit with full fuel (I.e: the core is the payload).

And another point: LV-N's sea level isp may be horrible, but each ounce of thrust at liftoff decreases your gravity losses in general by decreasing the time to hit terminal speed. Besides, before you get to 5000m, those 200s isp have turned into 400, and then it's more efficient than aerospikes.

Rune. Raining on your parade pretty hard in the end. Sorry!

[NKL]

Asparagus need a tug to do the orbital maneuver. Or the tug engine isn't participating in the launch! I've tried back in the day to do such thing (a push-push thingy with only engine on top to "pull" the payload) but i never managed to avoid the rocket to tear into two parts.

Goodjob here sir!

[mhoram]

I don't want to rain on your parade (I actually like people trying to come up with new ways of doing things) but... isn't this just the same as a parallel staged rocket with crossfeed? In other words, the infamous asparagus staging. All engines lit at all times, and you drop them as you stop needing them. Well, call it asparagus with a late-burning core stage that makes orbit with full fuel (I.e: the core is the payload).

Yes indeed it is - with a focus on payload fraction optimization.

And another point: LV-N's sea level isp may be horrible, but each ounce of thrust at liftoff decreases your gravity losses in general by decreasing the time to hit terminal speed. Besides, before you get to 5000m, those 200s isp have turned into 400, and then it's more efficient than aerospikes.

Usually I started the LV-N's at an altitude where it's isp equals the average of the other engines. But thanks to your hint I tried to start them at launchtime and the rockets perform better. Never had the idea to test this before... Another detail to tweak ;-)

[Rune]

Asparagus need a tug to do the orbital maneuver. Or the tug engine isn't participating in the launch! I've tried back in the day to do such thing (a push-push thingy with only engine on top to "pull" the payload) but i never managed to avoid the rocket to tear into two parts.

Goodjob here sir!

Actually, if your tug is the center stage of the parallel bundle, no it doesn't. Though for aesthetics reasons, then it usually looks more like a heavy interplanetary nuclear stage in my designs, and then because I like puller designs for the long IP burns, it often turns out to have the engines on top and docking ports on the bottom... But, you know, same difference.

Usually I started the LV-N's at an altitude where it's isp equals the average of the other engines. But thanks to your hint I tried to start them at launchtime and the rockets perform better. Never had the idea to test this before... Another detail to tweak ;-)

Glad to be of help! It's the reason I always try to nitpick a little, because when I'm on the receiving end, I often walk away with a new trick.

Rune. Good job BTW!

[labtopia]

these are the best lifters i've found, i use them on everything!