• 0
FinalFan

Is it bad for CoM=CoL on my spaceplane?

Question

TL;DR:  is this generally considered a "don't do this" type of thing for spaceplanes, or just a tradeoff?

 

Hi, I've been refining a Mk3 "do-everything" spaceplane for a while now (all science, ISRU, amphibious, etc.) and I'm wondering if I'm making a mistake with the balance.  The default CoM is very stable and just a tiny tiny bit forward of the CoL, but I understand that although these being very close makes the plane more maneuverable, it also makes it more vulnerable to loss of control.  My question is:  is this generally considered a "don't do this" type of thing for spaceplanes, or just a tradeoff? 

—Liftoff/landing:  CoM and CoL being on top of each other is good here, right? 
—Ascent:  CoM being somewhat ahead of CoL makes the plane more stable.  But if it seems pretty controllable then is there a reason to worry? 
—Reentry:  CoM being somewhat ahead of CoL makes the plane recover from aggressive reentry profiles.  Is it reasonable to say "I expect to have some fuel left to create this condition for reentry" or is that considered a bad design?  (It needs 10% fuel to ensure recovery from radial out position)

Basically, I like the idea of maximum agility (for a Mk3 spaceplane) but I don't want to overlook a fatal flaw. 

Edited by FinalFan
Clarified, for posterity, what the question was

Share this post


Link to post
Share on other sites

7 answers to this question

Recommended Posts

  • 0
9 hours ago, FinalFan said:

With SAS off, it has a slight to very slight nose-up effect above 300 m/s.

Meh!

I had the problem with my first spaceplane designs that they flew fine at first, but when I switched to rocket propulsion and pitched up (ca. 10 deg, so significant but not excessive) then they flipped out of control. Took me a while to figure out the problem.

9 hours ago, FinalFan said:

But I was wondering if I was contradicting any community-identified "best practice" with my placement of the CoM.

Well, I'd say that "best practice" is what @bewing said: try it out. ;) Shifting around fuel to adjust aircraft trim is definitely not a crime but good practice. (Even in RL.)

9 hours ago, FinalFan said:

If it's just known and identified tradeoffs like "better for low and slow (e.g. landing and takeoff), but you have to be a lot more careful on ascent path" then that's fine. 

That's about it. Maybe together with "just because it's stable low & slow, doesn't mean it has to be stable high & fast". (Which - as I said - took me a while to figure out.)

9 hours ago, FinalFan said:

What kind of rules of thumb can I use to guess at my center of drag?

I don't really know. I guess it is essentially in the geometrical center, but there are some parts around with a surprisingly high drag, so that can be off.

Edited by AHHans
fixed typo

Share this post


Link to post
Share on other sites
  • 1

I build most of my (space)planes with CoM practically on CoL, at first. Just a smidge behind, and maybe two smidges above. That last one helps with making the plane tend to keep it's belly pointing down or into the reentry path when holding radial out. Keeping CoM and CoL practically on top of each other, in combination with sufficient wing surface and AoI helps making planes take to the air by themselves, a trait I value highly because of my very frequent flight testing during design (repeatability) and as a way to keep usage instructions for my designs as simple as possible (craft sharing).

It is however often adjusted once I start flight testing, because a) CoL is often not accurately visualized in the stock game, especially when you use AoI on wings - which you should, b) CoL is not the same a CoP and that difference matters, c) CoD can also play a significant role in certain flight conditions which can overwhelm other forces. Generally though, it stays pretty close to where it started, and tweaks are mostly done by ensuring there is proper control authority.

So tl;dr: I don't see anything inherently wrong with it, it tends to work for me and generally results in fuel-efficient and easy to fly and land spaceplanes. Carry on, I'd say.

Share this post


Link to post
Share on other sites
  • 0

The only thing that you can really do is to fly it through all of these possibilities and see if it works in the real world. The CoM/CoL thing is a handy rule-of-thumb to get your plane into approximately the configuration that it will fly. However, the Center of Pressure is actually much more important than the CoL, and there is no way to actually visualize CoP in the game. So in the end, it's really a guessing game. To finalize your design, there is no other choice at all -- you absolutely have to just try it and see.

 

Share this post


Link to post
Share on other sites
  • 0

Well, you always want the CoM in front of the Center of Pressure (CoP), if it is not then you'll have to actively fight against the tendency of your craft to flip. (Which can be done with enough control authority.)

@bewing already said most of it. One additional issue is that you should keep in mind that the CoP moves when you go through different flight conditions: at low speeds in dense air it is dominated by lift, so close to the CoL. At high (hypersonic) speeds in thin air it is dominated by drag, so it will be close to the center of drag.

If you have a craft with lots of high-mass but low(-ish) drag components at the tail - like rocket engines, the nuclear engines in particular - then the center of drag for the fuselage will be quite a bit forward of CoM of the fuselage. So if you design such a craft to be just stable at liftoff, then it will probably become quite unstable during your ascent to orbit. If it keeps controllable will then depend on how much control authority you have and on your ascent profile.

Share this post


Link to post
Share on other sites
  • 0
1 hour ago, AHHans said:

Well, you always want the CoM in front of the Center of Pressure (CoP), if it is not then you'll have to actively fight against the tendency of your craft to flip. (Which can be done with enough control authority.)

@bewing already said most of it. One additional issue is that you should keep in mind that the CoP moves when you go through different flight conditions: at low speeds in dense air it is dominated by lift, so close to the CoL. At high (hypersonic) speeds in thin air it is dominated by drag, so it will be close to the center of drag.

If you have a craft with lots of high-mass but low(-ish) drag components at the tail - like rocket engines, the nuclear engines in particular - then the center of drag for the fuselage will be quite a bit forward of CoM of the fuselage. So if you design such a craft to be just stable at liftoff, then it will probably become quite unstable during your ascent to orbit. If it keeps controllable will then depend on how much control authority you have and on your ascent profile.

Thanks for the advice.  With SAS off, it has a slight to very slight nose-up effect above 300 m/s.  Basically, I'm satisfied with its controllability in normal flight.  But I was wondering if I was contradicting any community-identified "best practice" with my placement of the CoM.  There could be a situation I haven't considered where it's horrible, you know?  [edit:  Or relying on fuel placement to be stable during aggressive reentry could be considered a crime against kerbalkind.]  If it's just known and identified tradeoffs like "better for low and slow (e.g. landing and takeoff), but you have to be a lot more careful on ascent path" then that's fine. 

What kind of rules of thumb can I use to guess at my center of drag?  For instance, the CoM is about 60% of the way towards the back from the front; is that relevant? 

Edited by FinalFan
see edit

Share this post


Link to post
Share on other sites
  • 0

Having the CoM ahead of the CoL helps on recovering from stalls.

You stall on a butt heavy airplane, you are toast no matter the altitude. With the CoM above the CoL, the stall will make the aircraft's nose to go down, and if you have enough altitude, the situation is salvable by itself - the vessel will come naturally to an attitude where recovering is possible.

You will need a heavy nose, also, to keep her stable on reeentry, otherwise she will flip - the heaviest end of the spaceplane is what Newton will try to face into the plasma stream, and it's usually the engine end.

Share this post


Link to post
Share on other sites
  • 0

CG = CL isn't bad, in fact it offers the highest degree of controllability (in a GA sense at least). If your design starts off draining aft tanks first you can actually afford to keep a default aft CG to assist in takeoff. Once you start needing more stability then you can shift fuel forwards, etc, etc.

Share this post


Link to post
Share on other sites
This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.