Spaceships - Ramscoops and Fuel Tanks
 

Looking at a few things in more detail last night I read the details of Ramscoops and noticed the 1800mps required for usage and thought it seemed quite high, not many engines have a high enough delta-V to reach this sensibly.
However, a little number crunching showed it was quite reasonable for those engines specified in the ramscoop entry!
Since I had done the calculations I thought I would share them.

First, the multiple fuel tanks delta-v increase gives 19 fuel tanks a factor of 3, and they will therefore provide a total of 57 times an engines delta-v, so an engine needs to provide 32mps per tank to reach 1800 (1824). I'm not exactly sure of the use of 19 fuel tanks, since with the engine the spaceship is full!?
With 18 fuel tanks (enough for the engine and ramscoop, but nothing else) we get a total multipler of 45, which means an engine needs 40mps to reach 1800.

This rules out a Fusion Torch at TL10 which has a delta-V of 15. (With a multi-stage spaceship it still needs 6 booster stages (5 if the final spaceship also does a fair bit of boosting itself) to reach 1800mps!)

The following table lists Engine, TL, delta-V of the engine, and the number of fuel tanks required to reach at least 1800mps.

18 fuel tanks, and even 16 fuel tanks seem excessive for a single stage Spaceship under most circumstances.

In most cases the sensible limit is going to be 13 fuel tanks and an engine (core systems in Centre and Rear sections) with an Upper Stage Spaceship which has the ramscoop and its own engine! This gives a factor of 20.8, so an engine needs 87mps to reach 1800 (1810).

Spaceships with those engines requiring 11 or 9 fuel tanks will likely use a 'booster' stage to reach the 1800mps, and then separate the Upper Stage with the ramscoop and its own engine.
Spaceships with those engines which require more than 13 fuel tanks will probably want 2x 'booster' stages, or the Upper Stage Spaceship will need a fair number of fuel tanks itself.

I hope these calculations are of use to others!

Si

Re: Spaceships - Ramscoops and Fuel Tanks
 

I know you posted this a very long time ago, but nobody had thanked you yet and I wanted to let you know this was a big help to me.

I am trying to design a reaction engine powered spacecraft, preferably TL10, with a ramscoop, as an early interstellar explorer on a first contact type mission. My problem has been working out how such a ship could have enough reaction mass for a return journey, or for multiple accelerations to ramscoop speed and back.

It seems to me that a spaceship must have 1800mps worth of ΔV to accelerate to ramscoop speed, and then 1800mps worth of ΔV to decelerate. If I assume that it can use the ramscoop to refill fuel tanks after the ship has reached 1800mps, then that means the same engines which powered the acceleration can power the deceleration. The problem is then that the ship is out of fuel, and does not have enough reaction mass to accelerate again for a return journey.

The spaceship around whose mission I am planning to build a campaign must be able to:

• Accelerate to 1800mps in order to achieve ramscoop speed, using only TL10 reaction drives (limited superscience probably OK), although if this is impossible I might have to go to a higher TL "prototype"
• While the crew is in stasis of some description, accelerate gradually using the infinite fuel from the ramscoop
• Decelerate as it nears its destination
• Accelerate again after the completion of the crew's mission for a return trip to Earth, reaching ramscoop speed again

The deceleration is necessary because the crew must be able to leave the ship in a smaller lander craft, land on an alien planet, and then return to the mothership. This can't be achieved if the mothership is still moving near c after hundreds of years of continual acceleration!

For this my ship would need reaction mass to achieve at least 3600mps, I believe, without using boosters (since a spaceship cannot use the ramscoop of its upper stage).

Re: Spaceships - Ramscoops and Fuel Tanks
 

Very useful calculations, yes, and thanks for doing them, but you did not specify in the TL column whether each drive is realistic, superscience or limited superscience.

Re: Spaceships - Ramscoops and Fuel Tanks
 

I don't think it is possible to use a ramscoop to fill reaction mass tanks. All that you "pick up" must be used immediately.

Re: Spaceships - Ramscoops and Fuel Tanks
 

Note that you can use a single magsail to decelerate from any velocity to zero and also use it to accelerate up to 375 mps relative to the local star. It takes a while though. Edit: I get 9 years to decelerate from 1800 mps to zero and 2 years to accelerate to 375 mps.

So technically you only need delta-V to get from 375 to 1800 mps in one direction.

Re: Spaceships - Ramscoops and Fuel Tanks
 

Since this thread has been bumped (and thanks for the comments), here is an updated table to include the Superscience tag (^) on the TL and a second Fuel Tanks column for acceleration from 375mps.

I don't know whether a Magsail would provide sufficient total thrust to reach 375mps, it would travel a considerable distance in the time taken to accelerate from rest, and may well be too far from the star to generate the thrust, definitely useful for the deceleration!. I haven't done the maths, so we'll assume its possible and I'll add the column!

The following table lists Engine, TL, delta-V of the engine, and the number of fuel tanks required to reach at least 1800mps, from Zero (A) and from 375mps (B)

Re: Spaceships - Ramscoops and Fuel Tanks
 

I can't make a craft that can do a round trip of 2x1800mps in a reasonable time and be able to land and take off from a planet. So this is one possible solution. A two stage design.

It detaches from the ramscoop mother ship, decelerates to the local solar system, refuels from an ice asteroid and drops its shuttle off at a planet. On completion of the shuttles mission it reattaches. The whole craft then boosts back to the mother ship.

YoYo class Round trip booster - TL12 Main Stage

The main stage is purely for doing the 3600 mps round trip to the circling Ramscoop mother ship. It uses a TL12 fusion rocket main drive, which allows it to reach 1800 mps in a mind-numbing 20 months. Though the ship is totally automated and the crew normally spend the journey in hibernation, only being woken up on final approach to the most Earth-like planet it has detected.

The scientific sensor scans the system during its long approach. Once the crew are awakened there is a goldmine of information to analyze and catalogue.

Habitats and control room are in the upper stage craft. The Magsail is for backup in case the main drive fails or it misses rendezvous with the mother ship, though it also provides minor acc/deceleration assistance. The refinery allows the craft to refuel by cracking water from ice-asteroids.

Note only the delta-V is TL12, the other components are all TL9 or 10.

Front
1-6 Upper Stage

Central
1 Armor (Nanocomposite)
2-3 Fuel Tank (630 mps each)
4 Scientific Sensor Array
5! Refinery
6! Magsail (0.001G)
0 Fusion Reactor (Derated 1 Power Point)

Rear Module
1 Armor (Nanocomposite)
2 Fusion Rocket (0.005 G)
3-core Fuel Tank (630 mps each)


TL 12, dST/HP 150, Hnd/SR -4/5, HT 13, Move 0.006G/3780 mps, LWt. 10000, Load 0, SM+10, Occ 0/5SV, dDR 0/50/50, $506M

Options: Total Automation.
100 yards long


Albatross class Upper Stage – Atmospheric Shuttle

The atmosphere shuttle detaches from the booster when in low orbit and makes a hot atmospheric entry. It then just stays in flight with its air-ram engines and takes samples as it is too risky to land. It carries a pair of small landers in the hangar bay if a suitable and interesting place to land is found. Once it has completed its mission it boosts back to orbit.

Front
1 Armor (Advanced Laminate)
2 Habitat (4 cabins with total life support, 8 hibernation capsules, 1 biology lab, 1 chemistry lab, 2 minifacs, 2 automed sickbay, 10 tons steerage cargo)
3 Open Space
4 Hangar Bay (2 SM+5 landers, probes and storage area)
5-6 Fuel Tank (0.63 mps each)
0 Control Room

Central
1 Armor (Advanced Laminate)
2-6 Fuel Tank (0.63 mps each)

Rear
1 Armor (Advanced Laminate)
2-5 Nuclear Thermal Air Ram (2G)
6-core Fuel Tank (0.63 mps each)


TL 9, dST/HP 100, Hnd/SR 3/6, HT 12, Move 2G/5.67 mps (+1 mps air speed), LWt.3000, Load 151, SM+9 Occ 10 ASV, dDR 15/15/15, Range 0, $384.7M

Options: Total automation, streamlined and winged.
Top air speed 3500 mph. 70 yards long.

EDIT: Design Notes:

The ramscoop mother ship is just assumed to be there. When I did the design I was of the opinion then that once you had incurred the expense of getting a ramship up to ram speeds you would keep it at ramspeed or higher. So it needed a smaller in-system explorer that could refuel itself using local resources.

Things driving the design were:
• No superscience – I prefer hard science settings
• A low-risk best-fit approach to each part of the design – each stage is optimised for one purpose
• A ship that could refuel itself in-system from easily available resources like ice asteroids – so hydrogen or possibly water
• Capable of carrying people – so hibernation capsules – makes it a possible PC ship (I was thinking this made it sort of a similar setting to the Alien movies with harder science)
• Capable of entering atmosphere and getting back up to escape velocity
• Capable of landing and taking samples
• Landing on a planet is incredibly risky with no landing field – if you damage the lander you are marooned or worse – the atmospheric shuttle doesn’t dare land - so it has to carry the landers – more than one to be safe.

The craft sizes are all dictated by the design system working upwards from the smallest craft. The hanger bay carried SM5 landers dictate an SM9 atmospheric shuttle which dictates an SM10 delivery craft. And implies at least an SM11 ramscoop ship.

Re: Spaceships - Ramscoops and Fuel Tanks
 

All of the eligible drive systems are either full superscience or realistic, with none being in-between?

Re: Spaceships - Ramscoops and Fuel Tanks
 

I know it's not canon, but could one use the ramscoop as a slow break when it's not powering the engine? That is, use it to fill the tanks and slow the craft at the same time. Once the tanks are full, the craft can pull a 180 and finish slowing itself down. It would have to collect more fuel in the system that it entered, of course.

Re: Spaceships - Ramscoops and Fuel Tanks
 

There are only two limited superscience items in the whole of spaceships and they are both torch drives (fusion and AM plasma).

Is there a good symbol for limited superscience?

Re: Spaceships - Ramscoops and Fuel Tanks
 

I would suggest a '. It's kind of like a ^ but only half as much. Or something like that.

Re: Spaceships - Ramscoops and Fuel Tanks
 

It might not be RAW, but it ought to work. Ramscoops are very efficient brakes. In fact, that's why they ought to be considered superscience as drives.

Re: Spaceships - Ramscoops and Fuel Tanks
 

Plus the problem of the Local Bubble.

Re: Spaceships - Ramscoops and Fuel Tanks
 

http://www.sjgames.com/pyramid/login...e.html?id=6603
Designer's Notes: GURPS Spaceships

Answers your question perfectly.

Re: Spaceships - Ramscoops and Fuel Tanks
 

And for the Pyramid deprived.
http://www.sjgames.com/pyramid/sample.html?id=6603

Re: Spaceships - Ramscoops and Fuel Tanks
 

Cool article. I'm disappointed by the implication that those rules are mostly not going to make it into future books in the series.

Re: Spaceships - Ramscoops and Fuel Tanks
 

As far as I know, that ought to work.

Re: Spaceships - Ramscoops and Fuel Tanks
 

Yes indeed. In fact, if you use the ramscoop to fill the tanks it ought to slow you down whether you want to or not.

Re: Spaceships - Ramscoops and Fuel Tanks
 

The drives are as tagged in GURPS Spaceships.

Exactly. There is no such Tag (that I'm aware of) in Spaceships or any GURPS Tech rules.

Re: Spaceships - Ramscoops and Fuel Tanks
 

I think I favor tilde ~ representing wobbly science as oppose to ^ representing broken in half science

Re: Spaceships - Ramscoops and Fuel Tanks
 

I've started using '' , double apostrophes.