Using MK1 tanks for NERVs

[NachoAyala]

Hello! Sorry if this has been asked before, it's my first post, but I've googled and got nothing.

I have a doubt. I've seen some tutorials, walkthoughs and videos where people, players much smarter and more experienced than me, are using tanks which can load oxidizer and liquid fuel for its NERVs propelled spaceships.

It was puzzling me, since NERVs do not use oxidizer. So I made some calculations.

A Jumbo loaded only with liquid fuel (yeahhh, I know you can deplete the oxidizer before the flight ) weights around 16 tones and can carry 2880 units of liquid fuel. We sould need 7,2 MK1 to carry that amount of fuel. It is a silly result, since a tank can't be divided, but let's use that figure...

7,2 MKs X 2,25, the weight of a MK1=16,2 tons. So, we might have an answer. To have the same amount of fuel we with MK1 might need 16,2 tons. 200 kilos more.

But what if we add a "light" TR18A to every tank? Then, with a weight of 2.30 * 7,2, we can have a "thing" that weighting 16.56 tons can carry the same amount of fuel than a JUMBO, but the capability of decoupling its useless parts during the journey. (we should add a TVR21-60 to make it couplable to the lander, at the top of the injection part with the MK1s and the NERV) 16.56+0.175=16.735.

Is there a point where this capability of losing parts make this cluster MK1s design more efficient than a simple JUMBO configuration?

Edited January 15, 2017 by NachoAyala
typos

[bewing]

Welcome to the forums.

And the answer is yes, certainly. Staging away empty fuel tanks always helps a lot with increasing the deltaV of your rocketships.

However, it depends on whether you ever want to refuel the thing, or use it as a tanker for refueling other ships after you are done using it for its initial purpose. There is also a question of part count. Sometimes you want to keep the number of parts on your ship as low as you reasonably can.

 

 

[AeroGav]

You could get the Configurable Containers mod, which allows you to change the ratios of LF and Ox stored by the stock tanks to store whatever you want - making a liquid fuel only jumbo.    Or you could use mk3 parts, since they have a huge mk3 liquid fuel tank which is part count efficient, and a mk3 engine mount which has three 1.25m attach nodes and a single 2.5m node in the centre.  So that'd be a 4 nervs,  or you could put a tri coupler on the 2.5m mount if you want even more engines.   Radially attach boosters based on Kerbodyne parts to get the thing in space.

 

If you're sticking with mk1, bear in mind you can put decouplers on the string of tanks to punch off the empties, which helps make them more competitive with larger stuff.

[NachoAyala]

2 minutes ago, bewing said:

Welcome to the forums.

And the answer is yes, certainly. Staging away empty fuel tanks always helps a lot with increasing the deltaV of your rocketships.

However, it depends on whether you ever want to refuel the thing, or use it as a tanker for refueling other ships after you are done using it for its initial purpose. There is also a question of part count. Sometimes you want to keep the number of parts on your ship as low as you reasonably can.

 

 

Yup, I had in mind the part count as a possible answer to my initial question. But I tested my "Cluster MK1s" design and it did not create lags or something... also, the ship was stable and worked fine. Soooo, thanks... my initial thinking was correct. Now I'll have to pick paper and pencil up and see when this extra weight becomes worthy...

A side question... I did try this design to reach Moho. I did fail, I used the "windows" approach, and lost like 3,000 DVs to match the inclination at the middle of the trip. Too much, slowing down to orbit MOHO killed my possibility to land. I ran out of fuel and had to come back using the lander. I managed to orbit Kerbin with... 0,3 seconds of remaining fuel. Poor Bill was soooOOOOOO close of flybying Kerbin and get lost in space. My closest Apollo XIII experience.

But what I noticed during my trip was that, during transfering my fuel to the lower MK1s to the top ones (the ones which had the NERVs attached) the DV started to slowly contdown. That is a MechJeb error, right??? It was the same weight, the same engines, the same everything but a slight balance change. dV shouldn't be affected, right?

Also... I have a doubt. 1,000 units of fuel. which set up has more delta V? one with two engines or one with one? Or are they the same? Of course, the more engines, the more thrust, but more comsumption. But the more thrust, the less ignition time. So, I am wondering if there is a formula or something I could use to know which set up is more dV efficient.

[bewing]

9 minutes ago, NachoAyala said:

But what I noticed during my trip was that, during transfering my fuel to the lower MK1s to the top ones (the ones which had the NERVs attached) the DV started to slowly contdown. That is a MechJeb error, right??? It was the same weight, the same engines, the same everything but a slight balance change. dV shouldn't be affected, right?

Right.

Quote

Also... I have a doubt. 1,000 units of fuel. which set up has more delta V? one with two engines or one with one? Or are they the same? Of course, the more engines, the more thrust, but more comsumption. But the more thrust, the less ignition time. So, I am wondering if there is a formula or something I could use to know which set up is more dV efficient.

Always one. Thrust doesn't matter for delta V. Ignition time doesn't matter. All that matters is the total mass of the rocket. And a rocket with 2 engines has more mass than a rocket with one engine.

Rocket equation for calculating delta V: https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

 

Edited January 15, 2017 by bewing

[NachoAyala]

Thanks for the replies. I will analyse them in front of my computer asap. One more quick question... I have in mind creating a super light comeback pod from Moho to Kerbin... With ion engines. Makes sense? I am afraid of not being able to "brake" it and make a flyby...

Edited January 15, 2017 by NachoAyala
Typos

[Benjamin Kerman]

19 minutes ago, NachoAyala said:

Thanks for the replies. I will analyse them un front of my computer asap. One more quick question... I have mind creating a super light comeback pod from Moho to Kerbin... With ion engines. Makes sense? I am afraid of not being able to "brake" it and make a flyby...

Yes, ion engines are HIGHLY efficient. You just need to make sure to have a large amount of electric charge and solar panels. Coming in from interplanetary speeds, I would highly recommend a heat shield, but aerobraking in the upper atmosphere combined with burning of the engines could also work. The Wikipedia entry that @bewing mentioned was confusing to me (as I am only a freshman in high school, but taking a look at the Cheat Sheet on the wiki and the Advanced rocket design tutorial helped a lot. If you need, I can help clear up some of the meanings of the different numbers for you. 

 

[bewing]

Kerbin has a very large SOI -- 160 Mm across. If you have enough deltaV to brake, then there is always enough time to stop at Kerbin with ion engines. And it's not difficult to have enough ions to have lots of deltaV. So yes, it makes sense.

[NachoAyala]

17 hours ago, Benjamin Kerman said:

Yes, ion engines are HIGHLY efficient. You just need to make sure to have a large amount of electric charge and solar panels. Coming in from interplanetary speeds, I would highly recommend a heat shield, but aerobraking in the upper atmosphere combined with burning of the engines could also work. The Wikipedia entry that @bewing mentioned was confusing to me (as I am only a freshman in high school, but taking a look at the Cheat Sheet on the wiki and the Advanced rocket design tutorial helped a lot. If you need, I can help clear up some of the meanings of the different numbers for you. 

 

Thanks for the replies. I read somewhere that "Ion engines have so little thrust that they only work for probes"... But I had the idea that they could work for light manned probes as well.

Of course, we are getting a bittle "unrealistic" here, because having someone alone in a half the size of telephone cabin pod for 1 year would kill anyone, even with a Kerbintendo Gamekerbol with him. But I will leave "life support" mods for later. Benjamin, I love what you're doing in the Spanish forums, I also like to imagine parallel realities, document it and create logos and imagine companies... I will show you my "Duna society" and "KIASA" logos as soon as I can. Hey... any idea on how to include images as a signature?

Thanks.

Edited January 16, 2017 by NachoAyala
typos

[Benjamin Kerman]

10 minutes ago, NachoAyala said:

Benjamin, I love what you're doing in the Spaish forums

Wat? I didn't know I was on the Spanish forums!?! 

[NachoAyala]

52 minutes ago, Benjamin Kerman said:

Wat? I didn't know I was on the Spanish forums!?! 

Argh... Yup, I goofed. But it was definitelly you the one who is making that diary that I love...

[Benjamin Kerman]

17 minutes ago, NachoAyala said:

Argh... Yup, I goofed. But it was definitelly you the one who is making that diary that I love...

Could you post a link to the diary? I just joined the forums about a month ago!

[NachoAyala]

1 hour ago, Benjamin Kerman said:

Could you post a link to the diary? I just joined the forums about a month ago!

This is the guy... He was using your avatar yesterday. Now I realise it's a generic one... Now I feel silly... And dumb.

[Benjamin Kerman]

5 minutes ago, NachoAyala said:

This is the guy... He was using your avatar yesterday. Now I realise it's a generic one... Now I feel silly... And dumb.

Man, it's all kool! And if you are playing Kerbal Space Program, you are certainly NOT dumb!!! As Gene Kerman says: "It's not like it's rocket science... oh."  

[Aegolius13]

20 hours ago, NachoAyala said:

So, I am wondering if there is a formula or something I could use to know which set up is more dV efficient.

As @bewing says, fewer engines of a given type will always give you a higher calculated delta-v.  But in terms of effective delta-v, that may not always be the case.  If your TWR is extremely low, you may have to long burns, including far away from your maneuver node.  This decreases the efficiency of your burn (e.g., from lost Oberth effect) and can lead to inaccuracies, which require extra burns to correct.  Also, if you're taking off or landing, the faster you can those maneuvers the less delta-v you'll have to spend fighting gravity drag.  

 

2 hours ago, NachoAyala said:

Thanks for the replies. I read somewhere that "Ion engines have so little thrust that they only work for probes"... But I had the idea that they could work for light manned probes as well.

 

Yep, they can work for somewhat heavier ships, provided you are patient and do a little advance planning to work around their limitations.  In particular, they are great for Moho and low-sun activities due to the huge delta-v requirements and ample energy available from sunlight.  

Your idea of using ions to get back from Moho to Kerbin is a great one.  Burning ions when leaving from Kerbin is a pain, since solar energy is weak.  But for the Moho-Kerbin ejection burn, you'll have plenty of energy.  And with a heat shield, you shouldn't need to do much, if any, engine braking.  Since ion upper stages are so light, it makes some sense to use a nuclear or chemical rocket for the first half of a Moho mission, and an ion for the second half.  

One note on that Moho-Kerbin ejection burn: if you're in a normal west-to-east orbit, you'll be burning on the night side of Moho, which will not let you keep your batteries topped up.  One possible solution is to get into a retrograde (east-to-west) orbit around Moho.  The planet revolves so slowly that it will not make much of a difference if you intend to land.  

[NachoAyala]

25 minutes ago, Benjamin Kerman said:

Man, it's all kool! And if you are playing Kerbal Space Program, you are certainly NOT dumb!!! As Gene Kerman says: "It's not like it's rocket science... oh."  

Man, I love Kerbal. I mean, to do something you need: A) The knowleadge. B) The skill. I KNOW I will never be a FIFA 17 or WoW God, because there are people who are so skilled that it will be impossible to beat them or, even worse, have fun compeating with them. The worst example is SW: Battlefront. I virtually survive for around just 10 seconds everytime I spawn. It's silly and annoying. With Kerbal, 95% is the "know how" and the rest is skill. I love that. When I started I did know nothing about fuels, engines, periapsis, apoapsis or how to use the navball. Now I can use it to make perfect dockings, and I know how to kill horizontal velocity before landings. I've landed on Mun, Minmus and Duna, and flybyed Moho and Eve... I know that someday this (landing in all celestial bodies and come back) will happen. It is just a matter of go on working, studying, and having fun with it.

And a mature, helpfull, comprehensive and kind community helps. A lot. Thanks for that.

Edited January 16, 2017 by NachoAyala
typos

[Benjamin Kerman]

I totally agree. When the mods see a newbie in the forums, they always say hello. When someone doesn't know how to post a pic, they tell them, even though there have to be over 100 threads saying how. The community is just so great!

[NachoAyala]

13 minutes ago, Aegolius13 said:

As @bewing says, fewer engines of a given type will always give you a higher calculated delta-v.  But in terms of effective delta-v, that may not always be the case.  If your TWR is extremely low, you may have to long burns, including far away from your maneuver node.  This decreases the efficiency of your burn (e.g., from lost Oberth effect) and can lead to inaccuracies, which require extra burns to correct.  Also, if you're taking off or landing, the faster you can those maneuvers the less delta-v you'll have to spend fighting gravity drag.  

 

Yep, they can work for somewhat heavier ships, provided you are patient and do a little advance planning to work around their limitations.  In particular, they are great for Moho and low-sun activities due to the huge delta-v requirements and ample energy available from sunlight.  

Your idea of using ions to get back from Moho to Kerbin is a great one.  Burning ions when leaving from Kerbin is a pain, since solar energy is weak.  But for the Moho-Kerbin ejection burn, you'll have plenty of energy.  And with a heat shield, you shouldn't need to do much, if any, engine braking.  Since ion upper stages are so light, it makes some sense to use a nuclear or chemical rocket for the first half of a Moho mission, and an ion for the second half.  

One note on that Moho-Kerbin ejection burn: if you're in a normal west-to-east orbit, you'll be burning on the night side of Moho, which will not let you keep your batteries topped up.  One possible solution is to get into a retrograde (east-to-west) orbit around Moho.  The planet revolves so slowly that it will not make much of a difference if you intend to land.  

That is A)Encouraging (When you told me that Ion engines make sense to come back) and B)Usefull. (The "do the burn on the light side" thingie). Many thanks.

[Spricigo]

1 hour ago, Aegolius13 said:

... If your TWR is extremely low, you may have to long burns, including far away from your maneuver node.  This decreases the efficiency of your burn (e.g., from lost Oberth effect) and can lead to inaccuracies, which require extra burns to correct. ...

 Not only Oberth, but also cosine loses will be higher. Since your burn will be longer you will have more deviation between direction of thrust and direction of movement.

Anyways, just a small nitpick because I find a easier to grasp example then Oberth.

 

[Aegolius13]

45 minutes ago, Spricigo said:

 Not only Oberth, but also cosine loses will be higher. Since your burn will be longer you will have more deviation between direction of thrust and direction of movement.

Anyways, just a small nitpick because I find a easier to grasp example then Oberth.

 

Good point.  Oddly, I tend to have trouble visualizing the cosine loss aspect, so Oberth makes more intuitive sense to me.  I guess the cosine thing is akin to how it's cheaper to change your orbit via prograde and retrograde maneuvers than via radial maneuvers.

 

 

[FleshJeb]

23 hours ago, Aegolius13 said:

Good point.  Oddly, I tend to have trouble visualizing the cosine loss aspect...

 

 

Does this grossly-simplified model help?

COS(36deg) = 0.81

COS(16deg) = 0.96

 

A maneuver node in KSP represents the results of an instantaneous burn at that point. (In this case, it's a pure prograde burn.) For shorter burns, this is close enough.

The in-game blue navball marker points parallel to the node. (My yellow arrows.) At the beginning and ends of that burn, there's a significant radial component. This keeps the Argument of Periapsis relatively stable. (At the cost of some efficiency.) You could lock your burn to your current prograde and possibly be more efficient, but it would be much harder to figure out what angle your apoapsis would end up at. Also, I believe the Oberth Effect would diminish more rapidly, because your altitude would climb faster.

 

[AeroGav]

What's a good TWR for interplanetary transfers then? My Jool-5 spaceplane has about 0.3 on its NERVs

I guess I can always split the ejection burn over several orbits.

You can actually get this kind of TWR on a Ion ship too,  I used editor extensions to enable surface attach and stick a large number of them on the bottom of a 2.5m rocket.  It does make for a crazy part count though, would be even worse in stock since there's no mulit mount adapters for size 0.  Also the TWR of the bare engine is only about 40% of the NERV (itself a real porker), so an even greater dry mass penalty for achieving high TWR than with NERVs.  At some point that's going to negate the modest improvement in delta V you get going from nerv to ion

[bewing]

5 hours ago, AeroGav said:

What's a good TWR for interplanetary transfers then?

It's a good enough TWR if you are accelerating through your entire burn, and your Ap ends up pointing in the correct direction. If your TWR is really low, you will find that your velocity starts falling as you move significantly past your Pe.

[tseitsei89]

On 16.1.2017 at 0:48 AM, NachoAyala said:

Thanks for the replies. I will analyse them in front of my computer asap. One more quick question... I have in mind creating a super light comeback pod from Moho to Kerbin... With ion engines. Makes sense? I am afraid of not being able to "brake" it and make a flyby...

One thing/trick I didnt see mentioned regarding this. If you are low on dv and coming back to kerbin (lets say from Moho for example) and dont have enough left to get a stable kerbin orbit, you can just do a flyby/gravity assist and set it up so that it kicks up your sun periapsis so that you get some kind of a resonant orbit. What that means is that the orbital period of your ship is some fraction*kerbin orbital period. Here we could for example choose a resonant orbit of 3/4 which means that kerbin will perform 4 complete orbits in the same time our ship completes 3 orbits. That means way you will have a new encounter after 3 kerbin orbits around the sun so after 3 kerbin years and your relative velocity to kerbin will be much less... 

The altitude of your target pe can be quite easily calculated from the keplers third law and in this case it would be around 8.76 * 10^9m.

It takes a longer time but you can save dv by using the first kerbin encounter as a gravity assist.

[Sharpy]

16 hours ago, AeroGav said:

What's a good TWR for interplanetary transfers then? My Jool-5 spaceplane has about 0.3 on its NERVs

 

0.3 is decent for orbit of some 200-300km. The lower the orbit the higher TWR you need for a precise burn. Go to a good 1000km for a burn of less than 0.1. It's a matter of error introduced by the orbit curvature into the result trajectory.

My preferred technique is to overengineer the launch stage and use it in the initial departure burn phase. That way I'm getting a huge TWR early on when it matters most. As your orbit "straightens out" TWR ceases to be so critical.